comp.periphs.scsi FAQ part 1 of 2 - Tripod.commontel4.tripod.com/new/doc/scsi.doc · Web viewWhat are the pros and cons regarding SCSI vs IDE/ATA ? 9 Pros of IDE/ATA: 9 Cons of IDE/ATA:

DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003

SCSI FAQSCSI FAQ................................................................................................1

What is SCSI?......................................................................................3What do all these SCSI buzzwords mean?...........................................3

Host adapter:...................................................................................3Terminators (passive):.....................................................................3Terminators (active)........................................................................3Single ended:...................................................................................3Differential:......................................................................................4Apple kludge:...................................................................................4Asynchronyous SCSI:.......................................................................4Synchronous SCSI............................................................................4Fast SCSI:.........................................................................................4RAID:................................................................................................4Ultra SCSI:........................................................................................4Wide SCSI:........................................................................................5

What is the history of SCSI (What is SASI)?.........................................5Can I access SASI drive with SCSI controller?.....................................6How should I lay out my SCSI bus? What should I avoid?...................6Where do I put the terminators?.........................................................7Where Should I place the SCSI adapter on the SCSI bus?...................8What is a SCSI terminator? Why do I need them?...............................8Is the spacing of connectors on a SCSI cable important?....................8How long can my SCSI bus be?...........................................................9What are the pros and cons regarding SCSI vs IDE/ATA ?...................9

Pros of IDE/ATA:...............................................................................9Cons of IDE/ATA:..............................................................................9Pros of SCSI:...................................................................................10Cons of SCSI:..................................................................................10

Can I have both IDE/ATA drives and SCSI in the same system?........10Is it possible for two computers to access the same SCSI disks?......10Is it possible for two computers to access the same SCSI tape?.......11What is the problem with the Adaptec 1542C and external cables?.11What is the difference between the Adaptec 1542A and 1542B?.....12What are the differences between the Adaptec 1542B and the.......121542C?..............................................................................................12What are the differences between the 1542C and the 1542CF?.......12What kinds of Optical Drives are available?......................................13

Magneto-Optical.............................................................................13Sony MiniDisc.................................................................................13Panasonic phase-change double-function (PD)..............................14

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WORM and CD-R............................................................................14Where can I find SCSI info on the Web?.........................................17

What is FAST SCSI?...........................................................................17How can I check a passive SCSI terminator?.....................................17Can someone explain to me the difference between 'normal'..........19SCSI and differential SCSI?................................................................19What are the pinouts for differential SCSI?.......................................19How can I tell if I have a single ended or a differential drive?...........20What are the pinouts for SCSI connectors?.......................................20

SCSI Connector Pinouts (single-ended)..........................................21"Centronics" 50 male (use pin numbers for MINI-MICRO)..............23IBM's "Not really SCSI" connectors:...............................................24Macintosh Plus SCSI Connector Pinouts.........................................25

What is the difference between SCSI-1 and SCSI-2?.........................26SCSI-1_versus_SCSI-2.....................................................................26How is SCSI-2 different from SCSI-1?.............................................26What's New in SCSI-2.....................................................................28The Inquiry command....................................................................33

What is the difference between SCSI-2 and SCSI-3?.........................35Are SCSI-3 hard drives and/or controllers available yet?..................35

What is the difference between SCSI-3 and Fast & Wide SCSI-2?..35After perusing the latest issue of Computer Shopper, I came away with the impression that companies are calling F&W SCSI-2 HD's SCSI-3. Is this an incorrect assumption, or is F&W SCSI-2 known as SCSI-3?Is this really mostly marketing hype?...................................36Is SYNCHRONOUS faster than ASYNCHRONOUS?..............................36Is the NCR 53C90 Faster than spec?.................................................36What is FPT (Termination)?...............................................................38What is Active Termination?.............................................................39Why Is Active Termination Better?....................................................40How can I tell whether an unmarked terminator is active or passive?..........................................................................................................40What is Plug and Play SCSI?..............................................................40My SCSI bus works, but is not reliable. What should I look at?.........42Where can I find information about programming using the ASPI....43interface from DOS and Windows?....................................................43How do I replace Macintosh internal HD and terminate the SCSI chain properly?...........................................................................................43Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?....................................................................................43Can I connect a SCSI-3 disk to my SCSI-1 host adapter?..................44Can I connect a WIDE device to my narrow SCSI host adapter?.......44Can I connect a narrow device to my WIDE SCSI host adapter?.......44

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How does device ID numbering work with WIDE vs NARROW devices?..........................................................................................................45What is spindle-sync and why would I want it?.................................46What are the general steps I need to do to install a SCSI disk to be used with Windows?..........................................................................46

What is SCSI?

SCSI stands for Small Computer Systems Interface. It's a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands. The SCSI standard can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and fast).SCSI2 is the most recent version of the SCSI command specification and allows for scanners, hard disk drives, CD-ROM players, tapes [and many other devices] to connect.

What do all these SCSI buzzwords mean?

Host adapter:The card that connects your computer to the SCSI-bus. Usually called SCSI- controller by marketing droids.

Terminators (passive):A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus. Each terminated signal is connected by:

* 220 Ohm to +5 volt (TERMPWR)* 330 Ohm to ground.

The 18 signals that are terminated are:I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).

Terminators (active)Rather than passive terminators that use TERMPWR which may not be exactly +5v, active terminators use a voltage regulator.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Single ended:"Normal" electrical signals. Uses open collector to the SCSI bus, [usually] survives wrong cable insertion. DIFFSENSE signal is used to detect connection of wrong type devices. The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a device to the main-cable. Most devices are single ended.

Differential:Uses two wires to drive one signal.Max. cable length of 25 meters. Electrically incompatible with single ended devices! SCSI-1 and upwards.

Apple kludge:The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector.

Asynchronyous SCSI:A way of sending data over the SCSI-bus. The initiator sends a command or data over the bus and then waits until it receives a reply (e.g. an ACKnowledge). All commands are send asynchronously over the 8 bit part of the SCSI-bus.

Synchronous SCSI.Rather then waiting for an ACK, devices that both support synchronous SCSI can send multiple bytes over the bus in the folowing way: send data1 : send data2 : ... : send data3 (max outstanding bytes): wait : wait : response1 : reponse2: ...This improves throughput, especially if you use long cables. (The time that a signal travels from one end of the cable to the other end of the cable IS relevant.)

Fast SCSI: Fast SCSI allows faster timing on the bus. ( 10MHz instead of 5MHz ) On a 8 bit SCSI-bus this increases the *theoretical* maximum speed from 5MB/s to 10MB/s. I know of no single drive that reaches these speeds.

RAID:A Redundant Array of Independant Disks is a set of drives connected to a special dual ported SCSI adapter that allows certain types of access optimization. A RAID 0 array stripes the data accross multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives for increased data integrity. A RAID 5 array uses extra drives in a distributed manner to store parity information that can be used to apply data correction and recover any data in the event of any individual disk failure. This provides high reliability.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Ultra SCSI:Allows up to 20MHz signals on the bus.

Wide SCSI:Uses an extra cable (or 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus. Note that no *single* drive reaches these speeds, but groups of several drives can.

What is the history of SCSI (What is SASI)?

1979 The disk drive manufacturer Shugart begin working on a new drive interface with logical rather then physical adressing. It used 6 byte commands. Shugart Associates Systems Interface (20 pages long) made public. A few SASI drives are developed 1980 Attempt to make SASI an ANSI standard failed. 1981 Shugart and NCR request an ANSI committee be formed for SASI 1982 ANSI committee X3T9.2 is formed.SCSI adds the ATN signal to the bus and creates the message protocol. 1983 Development of SCSI drives and ST-506 to SCSI bridges begins. 1985 CCS (Common Command Set) used in most disk drives. Only disk and tape commands were adequately specified. 1986 Work begins on SCSI-2. 1986 SCSI-1 becomes official as ANSI X3.131-1986 (yes, after the work had begun on SCSI-2)6 and 10 byte commands.

SCSI-2 specifies CDROM commands. 1988 Production of SCSI-2 devices begins. 1993 Work begins on SCSI-3. 1994 SCSI-2 becomes official as X3.131-1994. SCSI-2 is backwards compatible with SCSI-1 and adds the following:*Fast SCSI-2. Optional bus speed of 10MHz instead of 5MHz.*Wide Optional 16 or 32 bit cable instead of 8 bits.*more commands defined, many optional (I'm not going to type the entire list here)*broader support for non-disk devices (tape.CDROM,Scanners....)

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003SCSI-2 devices can talk to the host adaptor on their own inititive.(e.g. to set in which mode they shoud operate, FAST or not, wide, extra wide or normal ...) This can confuse some older SCSI-1 HA. 1995 Production of drives that have some SCSI-3 enhancements. Ultra SCSI: Bus speed of 20MHz? 1996: SCSI-3 proposals include:-Support for graphical commands.-Fibre channel protocol (fibre channel)-Serial packet protocol (IEEE P1394)-SCSI-3 general packet protocol (almost all serial interfaces) and of course the old SCSI-2 commands and more.-Low Voltage Differential Parallel interface-CD-R command set and algorithms Future(after 1996): SCSI-3 becomes officialSCSI becomes a more network-like environment where devices can be physically distributed and shared more easily.

Can I access SASI drive with SCSI controller?

Well, the answer is a definite maybe, but very unlikely. Old low performance SCSI adapters and drivers that use only a minimal subset of the SCSI commands may work with SASI devices that happen to support the INQUIRY command. Newer adapters and drivers expect to be able to use messages and will get very upset with a SASI device that doesn't understand them.In reality, there is no practical reason to do this. Any SASI device is so obsolete that is has no real value in a system being used in 1990 or later.

How should I lay out my SCSI bus? What should I avoid?

One confusing thing about SCSI is what the SCSI bus is supposed to look like, and how devices should be placed on the bus. The SCSI bus MUST run continuously from one device to another, like this: DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D Where device A, B, C, and D can either be internal or external devices. The devices on the SCSI bus should have at least 4 to 6 inches of cable between devices. This is to satisfy the SCSI-2 requirement that "stubs" be placed at least .1 meters apart. Some devices that have a lot of internal wiring between the connector and the SCSI chip can look like a "stub" or bus discontinuity. The reason for all these requirements is that a SCSI bus is really 18 "transmission lines" in the wave theory sense. A pulse propagating along it will "reflect" from any part of the transmission line that is different from the rest of it. These relections add and subtract in odd

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003combinations and cause the original pulse to be distorted and corrupted. The terminators "absorb" the energy from the pulses and prevent relections from the ends of the bus. They do this because they (hopefully) have the same impedance as the rest of the transmission line. The SCSI bus must not have any "Y" shape cabling. For example, setting up a cable that looks like this is NOT allowed: DEVICE B \ \ \ >------------- DEVICE C ----------- DEVICE D / / / DEVICE A

Where do I put the terminators?

Termination must be present at two and ONLY two positions on the SCSI bus, at the beginning of the SCSI bus, and at the end of the SCSI bus. There MUST be no more than two, and no less than two, terminators on the bus. Termination must occur within 4 inches (.1 meter) of the ends of the SCSI bus. The following ARE acceptable: +------------+----------+-----------+-----------+---------+ | | | | | |DEVICE A Unconnected Unconnected DEVICE B DEVICE C Adapter Terminated Terminated +------------+----------+-----------+-----------+---------+ | | | | | |DEVICE A Unconnected DEVICE B Unconnected Adapter DEVICE C Terminated Terminated

+------------+----------+-----------+-----------+---------+ | | | | | |Adapter DEVICE A DEVICE B Unconnected Unconnected DEVICE CTerminated Terminated The following ARE NOT allowed: +------------+----------+-----------+-------------------+ | | | | |DEVICE A DEVICE B Adapter Unconnected Unconnected Terminated Terminated +------------+----------+-----------+-----------+

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 | | | | |Termination DEVICE A DEVICE B DEVICE C Adapter Terminated

Where Should I place the SCSI adapter on the SCSI bus? The placement of the SCSI adapter card can be on the end, at the beginning, or somewhere in the middle of the SCSI bus. Quite frankly, placement of the controller card isn't special. The adapter card is just another device on the SCSI bus. As long as the rules above and in other sections of this FAQ are followed, there should be no problem placing the adapter card anywhere on the SCSI bus. However, if you place the adapter card somewhere in the middle of the SCSI bus, you must be sure to disable termination on the adapter card. As noted previously, a SCSI device is only allowed to have terminations if it's at the end of the bus. Only two terminators are allowed to terminate the SCSI bus, one at each end. One last note: It doesn't make any difference where each SCSI ID is placed along the bus. It only matters that no two devices have the same ID. Don't forget that the adapter has an ID too. (Usually ID 7).

What is a SCSI terminator? Why do I need them?

A SCSI bus is a transmission line. To prevent reflections from the ends of the bus, you need a device which makes the transmission line appear to be of infinite length. This is done by attaching resistors which have the same resistance as the characteristic impedance of the transmission line to the ends of the bus. Also, since SCSI line drivers are open-collector (which can only pull a signal low), a pull-up resistor is needed to pull the signal high when it's not asserted. If the ends of the bus are not terminated, the signal pulses will reflect off these open ends and travel back along the bus in the other direction. The resulting adding and cancelling of signal amplitudes distorts and destroys the SCSI signals. There are two basic types of terminators, active and passive.

Is the spacing of connectors on a SCSI cable important?

The ANSI SCSI spec's say that "stubs" on a SCSI bus must not be any more than .1 meters (4 in.) long. In the most recent spec's there are also guidelines that say you shouldn't place "stubs" any closer than .3 meters (12 in.) apart. Since each device attached acts as a "stub", you really shouldn't place connectors any closer than this. This gets to be more important as your bus performance goes up. i.e. with Fast20 it is very important, but with SCSI-1 it doesn't really matter much. Since Fast20 also limits your overall bus length to 1.5 meters (for single ended) this also means you shouldn't really connect more than 5 devices for best reliability.

How long can my SCSI bus be?

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The SCSI length limits are based on the speed of the fastest device attached to the bus. Here's a table which shows the limits: Speed of FASTEST device Max. single-ended bus length Max. HV Diff. bus len.5 MHz (SCSI1 synch.) 6 meters 25 meters10 MHz (SCSI2 FAST) 3 meters(not rec.) 25 meters 20 MHz (Ultra or Fast20) 1.5 meters(not rec.) ?40 MHz (Ultra2 or Fast40) Only differential connections allowed

These limits assume the use of good quality cable which maintains its characteristic impedance between 90 and 130 Ohms and the use of active terminators at each end of the bus.Notice that I used the term MHz to specify speed since MB/sec. changes with the bus width. When Low Voltage Differential(LVD) devices are available, this will allow lengths between the single-ended numbers and the HV Diff. numbers. Note: Bus width doesn't change the maximum allowable length.The bus width is independent of bus length or speed. The above table assumes that you know the max. speed of your devices (usually by looking in the manuals). Some software (like Adaptec EZ-SCSI) provides a driver status monitor which will tell you what mode the devices are actually in. This is important since any synchronous speed must be negotiated by either the device or the adapter. The speed actually used will be the least common denominator between the two.For example, if a Fast20 disk is attached to a 'SCSI2" host adapter that only goes up to Fast10, the device will only run at 10 MHz.

What are the pros and cons regarding SCSI vs IDE/ATA ?

Pros of IDE/ATA:Inexpensive due to high volume of production. Supported directly by system

BIOS in most cases Less overhead per command

Cons of IDE/ATA:Very limited device attachment (two drives including CDROMs) Only supports disk, CDROM (and limited support for tape) Single threaded (commands do not overlap even with a second drive) CPU is tied up transferring all data IDE/ATA and ATAPI evolved as one kludge on top of another (so compatibility is not always good) Cannot handle scatter/gather operations well

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Pros of SCSI:Flexible device attachment (up to 7 or 15 devices per SCSI bus) Support for almost any peripheral type (disk, tape, CDROM, scanner etc) All commands can overlap with commands on other devices Usually uses DMA to transfer data (which frees CPU for other tasks) Interface and protocol is carefully specified by ANSI. Largest, highest performance devices are available in SCSI before IDE Most adapters can do scatter/gather DMA which is a necessity in virtual memory systems (Like Unix, NT) (Win 95 ?)

Cons of SCSI:Generally more expensive than IDE/ATA Slightly more complicated to install than IDE/ATA

Can I have both IDE/ATA drives and SCSI in the same system?

The short answer is YES. There are a few issues to consider however. The main issue is which device will be used for booting the system. Under MSDOS, The system BIOS determined this completely. A couple third party BIOSes (like MRBIOS) allowed the user to choose the boot source, but most conventional BIOSes just booted from the IDE if it as present. If no IDE was present then the standard option card BIOS scan would find the SCSI card's BIOS and use it to boot.Under Windows 95 and Windows NT, there are more options. Since the motherboard BIOS is used to load the boot sector that will still happen according to the same rules as under MSDOS described above. After the boot sector is loaded, the O/S's device drivers take over and those can be unloaded or drive letters re-ordered via the O/S configuration tools.

Is it possible for two computers to access the same SCSI disks? Yes, two (or more) systems can be on the same SCSI bus as SCSI disk and tape drives. As long as the SCSI requirements are met - cable lengths, termination and type - the devices can share the SCSI bus.Each host adapter needs to have a unique ID just as the devices do. Some adapters don't let you set this. ]The question should be - Are there any O/S' that will allow the sharing of file systems? It would not make sense for two hosts to go about treating shared disks as if they each owned the device. Data would be destroyed pretty quickly.[Editor(GF): CDROM drives can be shared pretty easily because they are by definition READ-ONLY] Disks can be best shared by having two (or more) partitions on a disk. Each host "owning" its own file system.[Editor(GF): You also need to watch out for host adapters that reset the bus when booting. Some adapters let you control this. ] The above discussion refers primarily to PCs. There are high end systems that do allow sharing SCSI devices. Usually, this is to allow fault tolerance. Two systems are connected to the same set of SCSI storage devices and when one of them fails, the other takes control.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003AIX with HACMP, Digital UNIX, and Digital VMS are examples of systems that allow this.

Is it possible for two computers to access the same SCSI tape?

Yes, this is not usually as problematic as sharing disks as long as the operator is sensible about what is attempted.Some things you need to watch out for:Both host's device drivers must use RESERVE/RELEASE commands to lock access. This locks the drive for access by only one system, the conflicting host gets BUSY status until the currently accessing host sends a RELEASE cmd. The adapter on both hosts have unique IDs. Good and common grounding of both systems and the devices. SCSI length limits are not violated. Make sure both hosts select the same data transfer mode (synch or asynch). Both hosts can be told which disks and other devices to access and not to attempt to access the ones owned by the other host. Neither host adapter resets the SCSI bus.

What is the problem with the Adaptec 1542C and external cables?

Several articles lately have cited the importance of SCSI-2-compliant cables when cabling SCSI bus subsystems. Perhaps the most accurate and technically detailed one was published in Computer Technology Review in March ‘93 (Volume XIII, No. 3. PP. 6). In short, it explains the double-clocking mechanism that can occur due to cables whose impedance falls below the 90-Ohm SCSI-2 spec. Steep edge speeds on the REQ and ACK lines of the SCSI bus exacerbate the problem, but non-compliant cables are the root cause. Both LAN TIMES in the US (5/24/93, page 115) and CT Magazine in Germany (7/93, page 18) cite this cable problem.

In an extensive survey of cables available in the US and Europe, we found that more than half of the cables available have single-ended impedances in the 65 to 80 Ohm range -- below the 90 to 132 Ohms specified in the SCSI-2 spec. It seems that some (not all) cable vendors do not understand the specification, describing their cables as SCSI-2 compliant when they are not. A common misconception is that SCSI-2 means a high-density connector. In fact, there are several connector options. I have published a technical bulletin that summarizes the critical requirements (TB 001, April 1993). An artifact of its faster design left the AHA-1540C with faster edge-speeds than its predecessor, the AHA-1540B. As I have said, this can exacerbate the effect of bad cables. This explains why some users could get their AHA-1540B to work when an early AHA-1540C might not.Essentially, the 1540B was more forgiving than the early 1540Cs. Good cables fixed the problem, but unfortunately for the user, good cables are hard to find.After surveying the cable market and many of our customers, we decided that bad cables were going to be here for a while, and we had to make the 1540C as forgiving as the 1540B was. At the end of April '93 we made a change to the AHA-1540C that involved using a passive filter to reduce the slew rate of the ACK line, the signal that the host adapter drives during normal data transfers. Extensive testing with many

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003intentionally illegal configurations confirms that we succeeded. Prior to release, we tested the AHA-1540C with over 200 peripherals, systems and demanding software programs with no failures. Then, a second team retested the AHA-1540C across a wild combination of temperatures, humidities and other stresses. This testing gives me confidence that the AHA-1540 line continues to serve as the gold standard for SCSI compatibility.

What is the difference between the Adaptec 1542A and 1542B?

The AHA-1542A is obsolete and no longer supported by Adaptec. They stopped providing firmware upgrades at some level prior to the equivalence to the 3.10 level of the AHA-1542B firmware. I am not sure just where though. The present latest AHA-1542B firmware is version 3.20, and supports drives up to 8GB under MS-DOS.

What are the differences between the Adaptec 1542B and the

1542C? The 1542C is an an updated model which replaces the 1542B. The 1542C features jumperless setup, having only 8 DIP switches. All other configuration options are set using the 1542C's built-in BIOS configuration utility. Configurable features not found on the 1542B are: Ability to enable/disable sync negotiation on a per-ID basis (the 1542B could only do it for all ID's on the SCSI bus) Ability to send "start unit" commands on a per-ID basis BIOS works with alternate I/O port settings on the adapter. Ability to boot from ID's other than 0 Software-selectable termination Software-selectable geometry translation Additional DMA speeds of 3.3 and 10 MB/sec Additionally, the 1542C uses a Z80 CPU and 8Kb buffer instead of an 8085 and 2Kb buffer as on the 1542B.

What are the differences between the 1542C and the 1542CF?

The 1542CF includes all of the 1542C features, and adds "Fast" SCSI operation, providing SCSI data rates of up to 10MB/sec (compared with an upper limit of 5MB/sec on the 1542C). This is unrelated to the host DMA rate. It also has a software configurable address for the floppy controller and a "self-healing" fuse for termination power.

What kinds of Optical Drives are available?

As magnetic recording approaches the current engineering limit, more and more attention is paid to optical storage solutions. Optical storage has good points going for it -- immunity to stray magnetic field, potential for higher storage capacity per unit area, and relatively low media cost.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 Although CD-ROM and CD-R are also optical storage units, they are not rewritable -- that puts them out as either secondary storage or primary backup storage for most of us. There is an upcoming sub-format called CD-E ("E" for erasable) that is suppose to become available in late 1996, but I haven't seen much news or even definite rumors. With the advent of DVD, the CD-E may only be a temporary stepping stone to recordable DVDs. Currently, the most popular magnetic storage format is magneto-optical (MO) format. It was the only popular rewritable optical storage disc technology before Panasonic's phase-change double-function (PD) format came out in 1995.

Magneto-OpticalAs the name implies, MO uses both magnetic and optical technology to store data on the disc. The disc itself is rare earth metal substrate. When data is to be written, the particular spot is first heated by the laser to the Curie point, and the magnetic field is generated while the spot cools. By varying the magnetic field angle, the substrate is polarized in certain way that it will reflect the laser beam differently depending on the magnetic field angle when the particular spot was cooling down. MO comes in many sizes and capacities. Consumers were first exposed To MO in Steve Jobs' NeXT computer in the mid-1980s. Although 5.25" had A slow start due to initial high cost, it has been evolving quite nicely.The more popular ISO capacities for 5.25" MO are 2.4GB/2.6GB,1.2GB/1.3GB, and the 600MB/650MB. In 3.5" form, MO is available in540MB/640MB, 230MB, and the 128MB. There are also some 12" MO, 14" MO, and other odd sizes in odd capacities. But they are limited to niche markets. Sony MiniDisc

Data Derived from the Mini-Disc (MD) audio format Sony introduced, MD-Data is to MD as CD-ROM is to digital audio compact disc (CD-DA). MD-Data (and digital audio MD) is based on the same magneto-optical technology, which explains the high-cost of the consumer MD audio units. MD-Data is the smallest of the MO family. With 2.5" form factor, it can store 140MB of uncompressed data. Current MD-Data drives are rather slow at 150KB/sec sustained transfer rate, but Sharp is hoping to change that.

Sharp will (hopefully) ship a 300KB/sec by the end of 1996, with a second generation of MD-Data available by sometime in 1997. The current schedule from Sharp indicates the second generation MD-Data will be able to store up to 700MB with 600KB/sec transfer rate. The most important technical advancement MD-Data brought for MO in general is the one-pass recording. Prior to 5.25" 2.4GB/2.6GB MO and 3.5" 540MB/640MB MO, almost all MO used two passes to write data onto the disc

-- one pass to erase the whole track, and a second pass to write the updated data. MD's one pass recording, called light intensity modulation, direct over-write (LIM-

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003DOW, ISO 14517) will be in almost all the future MO formats until another better technology comes along. Just like CD, MD-Data comes in various flavors -- rewritable, write-once, and read-only cartridges. There is also a hybrid disc for MD and MD-Data that is part read-only, and part rewritable.

Panasonic phase-change double-function (PD) In around mid-'95, Panasonic released a proprietary optical storage format called phase-change double-function (PD) drive. The PD uses substrate that will reflect the light differently when heated to different temperatures. Write-once-read-multiple (WORM) drives were actually the first phase-change formats, but PD is the first *reversible* (that is, re-writable) phase-change format. Current PD stores 650MB per PD cartridge. Currently, PD's only advantage over its MO brethren is the PD drive's ability to read regular CD-DA and CD-ROMs. The PD rewritable cartridge is not usable in regular CD-ROM drives.

WORM and CD-RBoth write-once-read-multiple (WORM) and compact disc recordable (CD-R) are both write-once formats -- once you have written the data to the disc, the data cannot be changed. Put another way, the disc media can only be used once. For long term archival of data that need not be changed, it makes sense -- as CD-R media price is unbeatable [As of mid 1996, 650 MB CD-R media sells for $6 to $8 each or about 1 cent per MB!] . Current CD-R offers maximum of 650MB per disc. WORM was the first popular format for optical storage, before being eclipsed by MO. WORM is still used by big companies and the government for archival purposes since it has the characteristic of not being able to be altered without damaging the media (good audit trail).The new WORM formats being introduced are tending to be more proprietary. There is rarely any interchangability between different vendor's drives and media. During the WORM to MO transition, a curious format called continuous composite write-once (CCW) appeared. CCW cartridges function as WORM cartridges, writable using the installed base of WORM drives. But put it into MO drive, CCW cartridges becomes rewritable. Simply put, CCW is MO in WORM's clothing. Many of today's 5.25" MO drives still have the capability to read CCW cartridges. The futureAlmost all the formats mentioned above have future plans -- usually an "improved version" with faster and more storage capacity. The 5.25" MO camp is shooting for the 4.8GB/5.2GB range, with faster sustained transfer rate in writing data. 3.5" may double their 650MB soon by using both sides of the disc. PD may also double the storage space by using both sides of the disc. But currently it's doubtful as DVD has pretty much been finalized. It'll be interesting to see how Panasonic will interpret the PD in the DVD marketplace (DVD-PD?). DVD-RAM is rumored to use phase-change technology.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 The same goes for CD-E, the latecomer of the bunch. If the CD-E is truly playable in ordinary CD-ROM (and audio CD player), it'll probably become the optical storage standard in all but the high-capacity, high-end/server market.

Format Phys. Standard Size

Capacity perDisk

Bytes perSector

# of sideper side

note

MO 1p 2.5" 140MB 2048/2336 single Sony MD-Data MO 2p 3.5" 128MB 512 single 128MB

ISO/IEC10090, ECMA 154

MO 2p 3.5" 230MB 512 single 230MB ISO/IEC 13963, ECMA 201

MO 1p 3.5" 540MB 512 single 540MB DIS(ISO/IEC) 15041

MO 2p 5.25" 1GB 512 dual 463MB ISO

MO 1p 5.25" 2.4GB 512 dual 2.298GB DIS(ISO/IEC) 14517

2.6GB 1024 dual 1.3GB

MO 2p 5.25" 1.5GB 4096 dual 750MB Panasonic

MO 1p 5.25" 4.6GB 1024 dual 2.3GB Pinnacle Micro "Apex"

MO 12" 8GB NikonMO 12" 3.2GB SonyMO 14" 6.8GB 1024 dual 3.4GB KodakSystem 2000 10.2GB

14.8GB 1024 1024

Dualdual

5.1GB7.4GB

WORM 5.25" 2.6GB DIS(ISO/IEC) 15486

WORM 5.25" 650MB single 650MB ISO/IEC 9171 Format A

WORM 5.25" 470MBWORM 12" 15GB SonyPD 1p 5.25" 650MB 4096 single 650MB

Panasonic 1p -- one-pass write

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/20032p -- two-pass write Standards for storage are set by many organizations. International Standards Organization (ISO), European Computer Manufacturers Association (ECMA), Deutsche Institut fur Normung (DIN), Japanese Industrial Standards Committee (JISC), and American National Standards Institute (ANSI) set the main optical disc storage standards. The ISO standards take precedence over all other standards. In the above table, the heading defines one standard -- e.g. 5.25" MO 1.2GB/1.3GB has both ISO 13549 and ECMA 184 listed for it. IT IS NOT THAT 1.2GB FOLLOWS ISO 13549 AND 1.3GB FOLLOWS ECMA 184. Of CD standards...Funny as it seems, CD is actually considered as proprietary a format made by Sony and Phillips. The physical format for derivatives like CD-ROM and CD-R are "written in mutual agreement" in form of Red Book, Yellow Book, Orange Book, etc. Of bytes/sector and usability...As many of you might notice (especiallly on 5.25" MOs), there are different sized sectors. Many O/Ses assume one sector to contain 512 bytes. If you buy any of the media that use different than 512 byte/sector, you will need a software driver of some sort to use the media. In optical media, the sectors are "hard sectored" at factory -- in other words, you cannot change the number of sectors by reformatting (low-level formatting) them. Take the 5.25" 1.2GB/1.3GB MO for example again. The 1.3GB media is sectored at 1024 bytes per sector. So the 1.3GB media has total of 637,041 sectors (per side) on it. If you do not use a software driver and your operating system does not properly recognize it, the 1.3GB media will become a 650MB cartridge (~325MB per side)!!The safest bet is to use the 512 bytes/sector media. That should make the drive and media usable on most operating systems. Addendum: (11/15/96)Sony and Phillips have just announed finalization of compact disc re-writable (CD-RW), together with HP, Matsushita, etc. Long story short, the CD-RW uses phase-change media -- same as Panasonic proprietary PD format. Not only that, it also stores 650MB like PD. And also like the PD, the CD-RW media cannot be read in regular CD and CD-ROM drives (surprise!)!! So, the good news is that CD-RW is here. The bad news is that it's as proprietary as Panasonic's PD in compatibility with current installed base of CD and CD-ROM players.

Where can I find SCSI info on the Web?

Try some of these: http://www.delec.com/Tech_Links/SCSIGuide/ http://www.quantum.com/src/

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What is FAST SCSI?

There are 2 handshaking modes on the SCSI bus, used for transferring data:ASYNCHRONOUS and SYNCHRONOUS.

ASYNCHRONOUS is a classic Req/Ack handshake.SYNCHRONOUS is "sort of" Req/Ack, only it allows you to issue multiple Req's before receiving Ack's. What this means in practice is that SYNCHRONOUS transfers are approx 3 times faster than ASYNCHRONOUS. SCSI1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec and synchronous transfers at up to 5.0 Mbytes/Sec. SCSI2 had some of the timing margins "shaved" in order that faster handshaking could occur. The result is that asynchronous transfers can run at up to 3.0 Mbytes/Sec and synchronous transfers at up to 10.0 Mbytes/Sec. The term "FAST" is generally applied to a SCSI device which can do syncrhonous transfers at speeds in excess of 5.0 Mbytes/Sec. This term can only be applied to SCSI2 devices since SCSI1 didn't have the timing margins that allow for FAST transfers.

How can I check a passive SCSI terminator?With an Ohmmeter:The terminator contains 18 220-ohm resistors from signals to TERMPWR, and 18 330-ohm resistors from those signals to GROUND. I've drawn that below:

TERMPWR --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | | | | | | | | | | | | | | | | | R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 | | | | | | | | | | | | | | | | | |sig o o o o o o o o o o o o o o o o o o | | | | | | | | | | | | | | | | | | R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 | | | | | | | | | | | | | | | | | | --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+GROUND

R1 = 220 Ohms, R2 = 330 Ohms When you measure from any one signal to termpower, you aren't measuring that resistor in isolation, you are measuring that resistor IN PARALLEL with the combination of the corresponding 330 ohm resistor plus 17 220+330 ohm resistor pairs in series.

I've redrawn the schematic to make this easier to see:

TERMPWR

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 /+---+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+| | | | | | | | | | | | | | | | | || R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1| | | | | | | | | | | | | | | | | || o o o o o o o o o o o o o o o o o| | | | | | | | | | | | | | | | | || R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2| | | | | | | | | | | | | | | | | || --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+| / GROUNDR1 || || R2| / o <--------- 17 other pairs in parallel ----------> sig

We're trying to measure that one resistor from a signal to TERMPWR, but there's a ton of other stuff in parallel. The resistance of that "stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a parallel combination of 17 550 ohm resistors). The general formula for the equivalent of two resistances in parallel is r1*r2/(r1+r2).Whipping out my trusty spreadsheet, I find that the "stuff" has a resistance of about 362 ohms, and that, in parallel with 220 ohms is about 137 ohms.

Can someone explain to me the difference between 'normal'

SCSI and differential SCSI?

"Normal" SCSI is also called "Single-ended" SCSI. For each signal that needs to be sent across the bus, there exists a wire to carry it. With differential SCSI, for each signal that needs to be sent across the bus, there exists a pair of wires to carry it. The first in this pair carries the same type of signal the single-ended SCSI carries. The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.

What are the pinouts for differential SCSI?

Differential SCSI Connector Pinouts---------------------------------------- ----------------------------------------| SCSI | | MINI | | | SCSI | | MINI | || SIGNAL | DD-50P | MICRO | DD-50SA | | SIGNAL | DD-50P | MICRO | DD-50SA |---------------------------------------- ----------------------------------------| -GND | 2 | 26 | 34 | | (open) | 1 | 1

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003| 1 || -DB(0) | 4 | 27 | 2 | | +DB(0) | 3 | 2 | 18 || -DB(1) | 6 | 28 | 19 | | +DB(1) | 5 | 3 | 35 || -DB(2) | 8 | 29 | 36 | | +DB(2) | 7 | 4 | 3 || -DB(3) | 10 | 30 | 4 | | +DB(3) | 9 | 5 | 20 || -DB(4) | 12 | 31 | 21 | | +DB(4) | 11 | 6 | 37 || -DB(5) | 14 | 32 | 38 | | +DB(5) | 13 | 7 | 5 || -DB(6) | 16 | 33 | 6 | | +DB(6) | 15 | 8 | 22 || -DB(7) | 18 | 34 | 23 | | +DB(7) | 17 | 9 | 39 || -DB(P) | 20 | 35 | 40 | | +DB(P) | 19 | 10 | 7 || GND | 22 | 36 | 8 | |DIFSENS | 21 | 11 | 24 || GND | 24 | 37 | 25 | | GND | 23 | 12 | 41 ||TERMPWR | 26 | 38 | 42 | |TERMPWR | 25 | 13 | 9 || GND | 28 | 39 | 10 | | GND | 27 | 14 | 26 || -ATN | 30 | 40 | 27 | | +ATN | 29 | 15 | 43 || GND | 32 | 41 | 44 | | GND | 31 | 16 | 11 || -BSY | 34 | 42 | 12 | | +BSY | 33 | 17 | 28 || -ACK | 36 | 43 | 29 | | +ACK | 35 | 18 | 45 || -RST | 38 | 44 | 46 | | +RST | 37 | 19 | 13 || -MSG | 40 | 45 | 14 | | +MSG | 39 | 20 | 30 || -SEL | 42 | 46 | 31 | | +SEL | 41 | 21 | 47 || -C/D | 44 | 47 | 48 | | +C/D | 43 | 22 | 15 || -REQ | 46 | 48 | 16 | | +REQ | 45 | 23 | 32 || -I/O | 48 | 49 | 33 | | +I/O | 47 | 24 | 49 || GND | 50 | 50 | 50 | | GND | 49 | 25 | 17 |---------------------------------------- ----------------------------------------

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Please note that I can only verify the DD-50P connector. The Mini Micro and DD-50SA pinout above is a pin for pin mapping from the SCSI pinout in this FAQ.

How can I tell if I have a single ended or a differential drive?

Most times the model number of the drive will end with "D". Use an ohm meter to check the resistance between pins 21 & 22. On a single ended system, they should both be tied together and tied to GND.On the differential drive, they should be open or have a significant resistance between them. Differential drives are less common than single-ended ones, because they are mainly used only where longer cable runs are necessary, and they are not generally used in PCs, but state of the art drives are available with differential interfaces. Generally only the higher performance drives have a differential option because of the added cost.

What are the pinouts for SCSI connectors?

The connector families described by the drawings have standard pin numberings which are described the same way by all vendors that I have encountered. The SCSI-2 specification identifies the standard numbering, using that convention. It happened to be documented by AMP, but all the vendors use the same convention. The following diagrams have the outline drawings of connector sockets at the bottom. This is really for reference only, because the connector sockets and plugs are both specified as to their numbering and usually are labeled. There are some minor problems in naming the microconnector conductor pairs, which I have corrected in the enclosed diagram. All the conductor pairs of the Mini-Micro (High Density) connector are in fact passed through on the cables. SCSI-2 defines the RSR (Reserved) lines as may be ground or may be open, but they are still passed through the cable. Most present standard SCSI devices will ground those lines.

SCSI Connector Pinouts (single-ended)

------------------------------------ ------------------------------------- | SCSI | | MINI | | | SCSI | | MINI | || SIGNAL| DD-50P | MICRO | DD-50SA | | SIGNAL | DD-50P | MICRO | DD-50SA |------------------------------------ -------------------------------------| -DB(0)| 2 | 26 | 34 | | GND | 1 | 1 | 1 || -DB(1)| 4 | 27 | 2 | | GND | 3 | 2 | 18

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003|| -DB(2)| 6 | 28 | 19 | | GND | 5 | 3 | 35 || -DB(3)| 8 | 29 | 36 | | GND | 7 | 4 | 3 || -DB(4)| 10 | 30 | 4 | | GND | 9 | 5 | 20 || -DB(5)| 12 | 31 | 21 | | GND | 11 | 6 | 37 | | -DB(6)| 14 | 32 | 38 | | GND | 13 | 7 | 5 || -DB(7)| 16 | 33 | 6 | | GND | 15 | 8 | 22 || -DB(P)| 18 | 34 | 23 | | GND | 17 | 9 | 39 || GND | 20 | 35 | 40 | | GND | 19 | 10 | 7 || GND | 22 | 36 | 8 | | GND | 21 | 11 | 24 || RSR | 24 | 37 | 25 | | RSR | 23 | 12 | 41 ||TERMPWR| 26 | 38 | 42 | | OPEN | 25 | 13 | 9 || RSR | 28 | 39 | 10 | | RSR | 27 | 14 | 26 || GND | 30 | 40 | 27 | | GND | 29 | 15 | 43 || -ATN | 32 | 41 | 44 | | GND | 31 | 16 | 11 || GND | 34 | 42 | 12 | | GND | 33 | 17 | 28 || BSY | 36 | 43 | 29 | | GND | 35 | 18 | 45 || -ACK | 38 | 44 | 46 | | GND | 37 | 19 | 13 || -RST | 40 | 45 | 14 | | GND | 39 | 20 | 30 || -MSG | 42 | 46 | 31 | | GND | 41 | 21 | 47 || -SEL | 44 | 47 | 48 | | GND | 43 | 22 | 15 || -C/D | 46 | 48 | 16 | | GND | 45 | 23 | 32 || -REQ | 48 | 49 | 33 | | GND | 47 | 24 | 49 || -I/O | 50 | 50 | 50 | | GND | 49 | 25 | 17 |------------------------------------ -------------------------------------* NC = NOT CONNECTEDCONNECTOR TYPES: DD-50SA ________________________ MINI-MICRO

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 DD-50P | ------------------- | ______________________ ______ ______ |17 \. . . . . . . . . /1 | | _________________ |49| . . . . . .| 1 |33 \. . . . . . . . /18 | | 1\ - - - - - - - /25 |50| . . . . . .|2 |50 \. . . . . . . / 34 | | 26\- - - - - - -/50 | ------------- | ------------- | | ------------- | ------------------------- ----------------------ribbon cable Old style Sun SCSI "SCSI-2" male male __________________ ( 1 25 ) \ ++++++++++++++ / \ 26 50/ --------------

"Centronics" 50 male (use pin numbers for MINI-MICRO) (VIEWED FROM FACE OF CONNECTOR - USE VENDOR NUMBERING SYSTEM AS SPECIFIED) 16 bit Wide SCSI-3 "P" (Primary) Connector pinout (single-ended)-------------------- --------------------| SCSI | HIGH DEN | | SCSI | HIGH DEN || SIGNAL | 68 PIN | | SIGNAL | 68 PIN |-------------------- --------------------| GND | 1 | | -DB(12)| 35 || GND | 2 | | -DB(13)| 36 | | GND | 3 | | -DB(14)| 37 | | GND | 4 | | -DB(15)| 38 | | GND | 5 | | -DB(P1)| 39 | | GND | 6 | | -DB(0) | 40 | | GND | 7 | | -DB(1) | 41 | | GND | 8 | | -DB(2) | 42 | | GND | 9 | | -DB(3) | 43 | | GND | 10 | | -DB(4) | 44 | | GND | 11 | | -DB(5) | 45 | | GND | 12 | | -DB(6) | 46 | | GND | 13 | | -DB(7) | 47 | | GND | 14 | | -DB(P) | 48 | | GND | 15 | | GND | 49 | | GND | 16 | | GND | 50 | |TERMPWR | 17 | |TERMPWR | 51 | |TERMPWR | 18 | |TERMPWR | 52 || RSRVD | 19 | | RSRVD | 53 |

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003| GND | 20 | | GND | 54 | | GND | 21 | | -ATN | 55 | | GND | 22 | | GND | 56 | | GND | 23 | | BSY | 57 | | GND | 24 | | -ACK | 58 | | GND | 25 | | -RST | 59 | | GND | 26 | | -MSG | 60 | | GND | 27 | | -SEL | 61 | | GND | 28 | | -C/D | 62 | | GND | 29 | | -REQ | 63 | | GND | 30 | | -I/O | 64 | | GND | 31 | | -DB(8) | 65 | | GND | 32 | | -DB(9) | 66 | | GND | 33 | | -DB(10)| 67 | | GND | 34 | | -DB(11)| 68 | --------------------- --------------------- ____________________________| _______________________ || 1\ - - - - - - - - - - /34 || 35\- - - - - - - - - -/68 || ------------------- | ----------------------------"WIDE SCSI-3 P"male ---------------------------------------------------------------------------

IBM's "Not really SCSI" connectors: Note that this connector is NON-COMPLIANT WITH ANY SCSI STANDARD! 60 pin Burndy connector as used on IBM RS/6000 systems: Pin Signal Pin Signal--- ------ --- -----1 Gnd 31 Gnd2 -DB(0) 32 -ATN3 Gnd 33 Gnd4 -DB(1) 34 Gnd5 Gnd 35 Gnd6 -DB(2) 36 -BSY7 Gnd 37 Gnd8 -DB(3) 38 -ACK9 Gnd 39 Gnd10 -DB(4) 40 -RST11 Gnd 41 Gnd12 -DB(5) 42 -MSG13 Gnd 43 Gnd14 -DB(6) 44 -SEL15 Gnd 45 Gnd

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/200316 -DB(7) 46 -C/D17 Gnd 47 Gnd18 -DB(P) 48 -REQ19 Gnd 49 Gnd20 Gnd 50 -I/O21 Gnd 51 Gnd22 Gnd 52 Reserved23 Gnd 53 Reserved24 Gnd 54 Reserved25 N/C 55 Reserved26 TERMPWR 56 Reserved27 Gnd 57 Reserved28 Gnd 58 Reserved29 Gnd 59 Reserved30 Gnd 60 Reserved

Macintosh Plus SCSI Connector Pinouts Note that this connector is NON COMPLIANT WITH ANY SCSI STANDARD!The grounding is insufficient and does not allow for proper twisted-pair transmission line implementation. It is recommended that a short adapter cable be used to convert to the more common Centronics style 50 pin connection, rather than extend the 25 pin connection any further than necessary. The Macintosh Plus used a NCR 5380 SCSI chip controlled by the MC68000 processor.___________________| SCSI DB-25S || SIGNAL pin(s) |+------------------+ DB-25S (female)| -DB(0) | 8 | _____________________________| -DB(1) | 21 | 13\ o o o o o o o o o o o o o /1| -DB(2) | 22 | 25\ o o o o o o o o o o o o /14| -DB(3) | 10 | ------------------------ | -DB(4) | 23 | View from rear of computer.| -DB(5) | 11 || -DB(6) | 12 || -DB(7) | 13 || -DB(P) | 20 || GND | 7,9,14 || GND |16,18,24 || -ATN | 17 || BSY | 6 || -ACK | 5 || -RST | 4 || -MSG | 2 || -SEL | 19 || -C/D | 15 || -REQ | 1 || -I/O | 3 |+------------------+

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Pin 25 is NOT CONNECTED in the Mac Plus implementation. Newer Macs connect TERMPWR to pin 25, but are otherwise the same.

What is the difference between SCSI-1 and SCSI-2?

SCSI-1_versus_SCSI-2 In 1985, when the first SCSI standard was being finalized as an American National Standard, the X3T9.2 Task Group was approached by a group of manufacturers. The group wanted to increase the mandatory requirements of SCSI and to define further features for direct-access devices. Rather than delay the SCSI standard, X3T9.2 formed an ad hoc group to develop a working paper that was eventually called the Common Command Set (CCS). Many products were designed to this working paper. In parallel with the development of the CCS working paper, X3T9.2 sought permission to begin working on an enhanced SCSI standard, to be called SCSI-2. SCSI-2 would include the results of the CCS working paper, caching commands, performance enhancement features, and whatever else X3T9.2 deemed worthwhile. While SCSI-2 was to go beyond the original SCSI standard (now referred to as SCSI-1), it was to retain a high degree of compatibility with SCSI-1 devices.

How is SCSI-2 different from SCSI-1? 1. Several options were removed from SCSI-1: a. Single initiator option was removed.b. Non-arbitrating Systems option was removed.c. Non-extended sense data option was removed.d. Reservation queuing option was removed.e. The read-only device command set was replaced by the CD-ROM command set.f. The alternative 1 shielded connector was dropped. 2. There are several new low-level requirements in SCSI-2: a. Parity must be implemented.b. Initiators must provide TERMPWR -- Targets may provide TERMPWR.c. The arbitration delay was extended to 2.4 us from 2.2 us.d. Message support is now required. 3. Many options significantly enhancing SCSI were added:

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 a. Wide SCSI (up to 32 bits wide using a second cable)b. Fast SCSI (synchronous data transfers of up to 10 Mega-transfers per second -- up to 40 MegaBytes per second when combined with wide SCSI)c. Command queuing (up to 256 commands per initiator on each logical unit)d. High-density connector alternatives were added for both shielded and non- shielded connectors.e. Improved termination for single-ended buses (Alternative 2)f. Asynchronous event notificationg. Extended contingent allegianceh. Terminate I/O Process messaging for time-critical process termination. 4. New command sets were added to SCSI-2 including: a. CD-ROM (replaces read-only devices)b. Scanner devicesc. Optical memory devices (provides for write-once, read-only, and erasable media)d. Medium changer devicese. Communications devices 5. All command sets were enhanced: a. Device Models were addedb. Extended sense was expanded to add:+ Additional sense codes+ Additional sense code qualifiers+ Field replaceable unit code+ Sense key specific bytes c. INQUIRY DATA was expanded to add:+ An implemented options byte+ Vendor identification field+ Product identification field+ Product revision level field+ Vital product data (more extensive product reporting) d. The MODE SELECT and MODE SENSE commands were paged for all device typese. The following commands were added for all device types: + CHANGE DEFINITION+ LOG SELECT+ LOG SENSE+ READ BUFFER+ WRITE BUFFER f. The COPY command definition was expanded to include information on how to handle inexact block sizes and to include an image copy option.g. The direct-access device command set was enhanced as follows: + The FORMAT UNIT command provides more control over defect management+ Cache management was added:- LOCK/UNLOCK CACHE command

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003- PREFETCH command- SYNCHRONIZE CACHE command- Force unit access bit- Disable page out bit + Several new commands were added:- READ DEFECT DATA- READ LONG- WRITE LONG- WRITE SAME + The sequential-access device command set was enhanced as follows: - Partitioned media concept was added:* LOCATE command* READ POSITION command - Several mode pages were added- Buffered mode 2 was added- An immediate bit was added to the WRITE FILEMARKS command + The printer device command set was enhanced as follows:- Several mode pages defined:* Disconnect/reconnect* Parallel printer* Serial printer* Printer options + The write-once (optical) device command set was enhanced by:- Several new commands were added:* MEDIUM SCAN* READ UPDATED BLOCK* UPDATE BLOCK - Twelve-byte command descriptor blocks were defined for several commands to accommodate larger transfer lengths.

What's New in SCSI-2 Scuzzy is the pronunciation and SCSI (Small Computer System Interface) is the acronym, for the best known and most widely used ANSI (American National Standards Institute) interface. Despite use of the term "Small" in its name, everyone has to agree that Scuzzy is large - in use, in market impact, in influence, and unfortunately, in documentation. The standards effort that began with a 20-page specification in 1980 has grown to a 600 page extravaganza of technical information. Even before ANSI (American National Standards Institute) published the first run of SCSI as a standards document in 1986, ASC (Accredited Standards Committee) X3T9.2 was hard at work on SCSI-2.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 No technical rationale can be offered as to why SCSI-1 ended and SCSI-2 began, or as to why SCSI-2 ended and SCSI-3 began. The justification is much more simple - you have to stop sometime and get a standard printed. Popular interfaces never stop evolving, adapting, and expanding to meet more uses than originally envisaged. Interfaces even live far beyond their technological lifespan. SMD (Storage Module Drive) has been called technically obsolete for 5 years but every year there are more megabytes shipped on the SMD interface than the year before. This will probably continue for another year or so before the high point is reached, and it will at least a decade before SMD is considered to be insignificant. If SCSI enhancements are cut off at an arbitrary point, what initiates the decision? Impatience is as good an answer as any. The committee and the market get sick of promises that the revision process will "end soon," and assert pressure to "do it now." The SCSI-3 effort is actively under way right now, and the workload of the committee seems to be no less than it was a year ago. What is pleasant, is that the political pressures have eased. There is a major difference between the standards for SCSI in 1986 and SCSI-2 in 1990. The stated goal of compatibility between manufacturers had not been achieved in SCSI in 1986 due to a proliferation of undocumented "features." Each implementation was different enough that new software drivers had to be written for each device. OEMs defined variations in hardware that required custom development programs and unique microcode. Out of this diversity arose a cry for commonality that turned into CCS (Common Command Set), and became so popular that it took on an identity of its own. CCS defined the data structures of Mode Select and Mode Sense commands,defect management on the Format command, and error recovery procedures. CCS succeeded because the goals were limited, the objectives clear and the time was right. CCS was the beginning of SCSI-2, but it was only for disks. Tape and optical disks suffered from diversity, and so it was that the first working group efforts on SCSI-2 were focused on tapes and optical disks. However, opening up a new standards effort is like lifting the lid on Pandora's Box - it's hard to stay focused on a single task. SCSI-2 went far beyond extending and consolidating CCS for multiple device types. SCSI-2 represents three years of creative thought by some of the best minds in the business. Many of the new features will be useful only in advanced systems; a few will find their way into the average user's system. Some may never appear in any useful form and will atrophy, as did some original SCSI features like Extended Identify. Before beginning coverage of "what's new in SCSI-2," it might be well to list some of the things that aren't new. The silicon chips designed for SCSI are still usable. No new features were introduced which obsolete chips. The cause of silicon obsolescence has been rapid market shifts in integrating functions to provide higher performance. Similarly, initiators which were designed properly, according to SCSI in 1986, will successfully support SCSI-2 peripherals. However, it should be pointed out that not all

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003the initiators sold over the last few years behaved according to the standard, and they can be "blown away "by SCSI-2 targets. The 1986 standard allows either initiators or targets to begin negotiation for synchronous transfers, and requires that both initiators and targets properly handle the sequence. A surprisingly large percentage of SCSI initiators will fail if the target begins negotiation. This has not been as much of a problem to date as it will become in the future, and you know as well as I do, that these non-compliant initiators are going to blame the SCSI-2 targets for being "incompatible." Quirks in the 1986 standard, like 4 bytes being transferred on Request Sense, even if the requested length was zero have been corrected in SCSI-2. Initiators which relied on this quirk instead of requesting 4 bytes will get into trouble with a SCSI-2 target. A sincere effort has been made to ensure that a 1986-compliant initiator does not fail or have problems with a SCSI-2 target. If problems occur, look for a non-compliant initiator before you blame the SCSI-2 standard. After that little lecture, let us turn to the features you will find in SCSI-2 which include:

Wide SCSI:

SCSI may now transfer data at bus widths of 16 and 32 bits. Commands, status, messages and arbitration are still 8 bits, and the B-Cable has 68 pins for data bits. Cabling was a confusing issue in the closing days of SCSI-2, because the first project of SCSI-3 was the definition of a 16-bit wide P-Cable which supported 16-bit arbitration as well as 16-bit data transfers. Although SCSI-2 does not contain a definition of the P-Cable, it is quite possible that within the year, the P-Cable will be most popular non-SCSI-2 feature on SCSI-2 products. The market responds to what it wants, not the arbitrary cutoffs of standards committees.

Fast SCSI: A 10 MHz transfer rate for SCSI came out of a joint effort with the IPI (Intelligent Peripheral Interface) committee in ASC X3T9.3. Fast SCSI achieves 10 Megabytes/second on the A-Cable and with wider data paths of 16- and 32-bits can rise to 20 Megabytes/second and even 40 Megabytes/second. However, by the time the market starts demanding 40 Megabytes/second it is likely that the effort to serialize the physical interface for SCSI-3 will attract high-performance SCSI users to the Fiber Channel.

A word of caution. At this time the fast parameters cannot be met by the Single Ended electrical class, and is only suitable for Differential. One of the goals in SCSI-3 is to identify the improvements needed to achieve 10 MHz operation with Single Ended components.

Termination: The Single Ended electrical class depends on very tight termination tolerances, but the passive 132 ohm termination defined in 1986 is mismatched with the cable

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003impedance (typically below 100 ohms). Although not a problem at low speeds when only a few devices are connected, reflections can cause errors when transfer rates increase and/or more devices are added. In SCSI-2, an active terminator has been defined which lowers termination to 110 ohms and is a major boost to system integrity. Bus Arbitration, Parity and the Identify Message were options of SCSI, but are required in SCSI-2. All but the earliest and most primitive SCSI implementations had these features anyway, so SCSI-2 only legitimizes the de facto market choices. The Identify message has been enhanced to allow the target to execute processes, so that commands can be issued to the target and not just the LUNs.

Connectors: The tab and receptacle microconnectors chosen for SCSI-2 are available from several sources. A smaller connector was seen as essential for the shrinking form factor of disk drives and other peripherals. This selection was one of the most argued over and contentious decisions made during SCSI-2 development.

Rotational Position Locking: A rose by any other name, this feature defines synchronized spindles, so than an initiator can manage disk targets which have their spindles locked in a known relative position to each other. Synchronized disks do not all have to be at Index, they can be set to an offset in time relative to the master drive. By arraying banks of synchronized disks, faster transfer rates can be achieved.

Contingent Allegiance: This existed in SCSI-1, even though it was not defined, and is required to prevent the corruption of error sense data. Targets in the Contingent Allegiance state reject all commands from other initiators until the error status is cleared by the initiator that received the Check Condition when the error occurred. Deferred errors were a problem in the original SCSI but were not described. A deferred error occurs in buffered systems when the target advises Good Status when it accepts written data into a buffer. Some time later, if anything goes wrong when the buffer contents are being written to the media, you have a deferred error.

Extended Contingent Allegiance (ECA): This extends the utility of the Contingent Allegiance state for an indefinite period during which the initiator that received the error can perform advanced recovery algorithms.

Asynchronous Event Notification (AEN):This function compensates for a deficiency in the original SCSI which did not permit a target to advise the initiator of asynchronous events such as a cartridge being loaded into a tape drive.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Mandatory Messages: The list of mandated messages has grown: +----------------------+--------------------------+-------------------+| Both | Target | Initiator |+----------------------+--------------------------+-------------------|| Identify | Abort | Disconnect || | | || Message Reject | No Operation | Restore Pointer || | | || Message Parity Error | Bus Device Reset | Save Data Pointer || | | || | Initiator Detected Error | |+----------------------+--------------------------+-------------------+ Optional messages have been added to negotiate wide transfers and Tags to support command queueing. A last-minute inclusion in SCSI-2 was the ability to Terminate I/O and receive the residue information in Check Condition status (so that only the incomplete part of the command need be re-started by the initiator). Command Queueing: In SCSI-1, initiators were limited to one command per LUN e.g. a disk drive. Now up to 256 commands can be outstanding to one LUN. The target is allowed to re-sequence the order of command execution to optimize seek motions. Queued commands require Tag messages which follow the Identify.

Disk Cacheing: Two control bits are used in the CDB (Command descriptor Block) to control whether the cache is accessed on a Read or Write command, and some commands have been added to control pre-fetching and locking of data into the cache. Users do not have to change their software to take advantage of cacheing, however, as the Mode Select/Mode Sense Cache page allows parameters to be set which optimize the algorithms used in the target to maximize cache performance. Here is another area in which improvements have already been proposed in SCSI-3, and will turn up in SCSI-2 products shipping later this year.

Sense Keys and Sense Codes have been formalized and extended. A subscript byte to the Sense Code has been added to provide specifics on the type of error being reported. Although of little value to error recovery, the additional information about error causes is useful to the engineer who has to analyze failures in the field, and can be used by host systems as input to prognostic analysis to anticipate fault conditions.

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Commands:Many old commands have been reworked and several new commands have been added.

Pages:Some method had to be found to pass parameters between host and target, and the technique used is known as pages. The concept was introduced in CCS and has been expanded mightily in SCSI-2. A number of new Common Commands have been added, and the opcode space for 10-byte CDBs has been doubled.

Change DefinitionAllows a SCSI-2 initiator to instruct a SCSI-2 target to stop executing according to the 1986 standard, and provide advanced SCSI-2 features. Most SCSI-2 targets will power on and operate according to the 1986 standard (so that there is no risk of "disturbing" the installed initiators), and will only begin operating in SCSI-2 mode, offering access to the advanced SCSI-2 capabilities, after being instructed to do so by the initiator using the Change Definition command.

The Mode Select and Mode Sense pages which describe parameters for operation have been greatly expanded, from practically nothing in 1986 to hundreds of items in SCSI-2. Whenever you hear of something being described as powerful and flexible tool, think complicated. Integrators are advised to be judicious in their selection of the pages they decide to support.

The Inquiry commandnow provides all sorts of interesting data about the target and its LUNs. Some of this is fixed by the standard, but the main benefit may be in the Vendor Unique data segregated into the special designation of Vital Product Data, which can be used by integrators as a tool to manage the system environment.

Select Log and Sense Log have been added so that the initiator can gather both historical (e.g. all Check Conditions) and statistical (e.g. number of soft errors requiring ECC) data from the target.

Diagnostic capabilitieshave been extended on the Read/Write Buffer and Read/Write Long commands. The ways in which the target can manage bad blocks in the user data space have been defined further and regulated to reduce inconsistencies in the 1986 standard. A

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003companion capability to Read Defect Data permits the initiator to use a standard method to be advised of drive defect lists. A new group of 12-byte command blocks Has been defined for all optical devices to support the large volume sizes and potentially large transfer lengths. The Erase command has been added for rewritable optical disks so that areas on the media can be pre-erased for subsequent recording. Write Once disks need Media Scan, so that the user can find blank areas on the media. o New command sets have been added for Scanners, Medium Changers, and CDROMs. All of this technical detail can get boring, so how about some "goodies" in SCSI-2 which benefit the common man and help the struggling engineer? First, and probably the best feature in SCSI-2 is that the document has been alphabetized. No longer do you have to embark on a hunt for the Read command because you cannot remember the opcode. In the 1986 standard, everything was in numeric sequence, and the only engineers who could find things easily were the microprogrammers who had memorized all the message and opcode tables. Now, ordinary people can find the Read command because it is in alphabetic sequence. This reorganization may sound like a small matter but it wasn't, it required a considerable amount of effort on the part of the SCSI-2 editors. It was well worth it. Another boon is the introduction for each device class of models which describe the device class characteristics. The tape model was the most needed, because various tape devices use the same acronym but with different meanings or different acronyms for the same meaning. The SCSI-2 tape model defines the terms used by SCSI-2, and how they correspond to the acronyms of the different tapes. For example, on a 9-track reel, End of Tape is a warning, and there is sufficient media beyond the reflective spot to record more data and a trailer. Not so on a 1/4" tape cartridge. End of Tape means out of media and no more data can be written. This sort of difference in terms causes nightmares for standardization efforts.

What is the difference between SCSI-2 and SCSI-3?

Are SCSI-3 hard drives and/or controllers available yet? Allegedly, Previous postings have said "I heard that SCSI-3 has been standardized," but I haven't seen anything firm about it. I've seen controllers advertised by JDR Microdevices and some cheap clones; the Quantum "Empire" drives are also advertised as SCSI-3 by some mail order vendors. Seagate and IBM call their fastest drives (probably comparable in speed to the Quantums, if not faster) "Wide SCSI-2."That's a misnomer. See below.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 What is the difference between SCSI-3 and Fast & Wide SCSI-2? Wide SCSI-2 required two cables to do 16 bit wide transfers. SCSI-3 defined a single cable, single REQ/ACK 16 bit, WIDE transfer. The reason you are hearing 16-bit single cable being called SCSI-3 is that they CAN. The fact that single cable 16-bit has been around for a while just shows you how much the standardization process lags behind the real world. SCSI-3 is really a family of standards. SCSI was broken up from a single document into different layers and command sets. This was done to allow for different physical transport layers (like fibre channel and SSA) to be defined, and to allow for smaller "bite-sized" projects that maybe get done a little faster ;-) The family includes the following members with TLAs: - SCSI-3 Parallel Interface (SPI): Defines the mechanical, timing, phases, and electrical parameters of the parallel cable we all know and love. Some of the electrical and cable parameters are tightened/improved over SCSI-2. - SCSI-3 Interlock Protocol (SIP): Defines the messages and how the phases are invoked. No real change from SCSI-2, except for some new messages. - SCSI-3 Architectural Model (SAM): In a nutshell, defines a common set of functions and services and definitions for how a physical transport properly gets commands, data, and status exchanged between two devices, complete with error handling and queueing. - SCSI-3 Primary Commands (SPC): All of the commands executed by any and all SCSI devices, like REQUEST SENSE and INQUIRY, etc. - SCSI-3 Block Commands (SBC): Disk commands. - SCSI-3 Stream Commands (SBC): Tape commands. - SCSI-3 Controller Commands (SCC): RAID box commands. - SCSI-3 Multimedia Commands (MMC): For CDROMS etc. - SCSI-3 Fibre Channel Protocol (FCP): SCSI commands over gigabit Fibre Channel. - SCSI-3 Serial Bus Protocol (SBP): SCSI commands over IEEE 1394 High Speed Serial Bus (Apple's "Firewire"). - SCSI-3 Serial Storage Protocol (SSP): SCSI commands over SSA.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003After perusing the latest issue of Computer Shopper, I came away with the impression that companies are calling F&W SCSI-2 HD's SCSI-3. Is this an incorrect assumption, or is F&W SCSI-2 known as SCSI-3?Is this really mostly marketing hype? Actually, there is something to that. TECHNICALLY, what is out there is often a hybrid: SCSI-3 "SPI" silicon with some other hodgepodge of SCSI-3 proposals, all mixed in with SCSI-2 stuff. An earlier posting said that the Quantum Empire ("SCSI-3") drives contain some commands from the SCSI-3 command set, and Adaptec suggested a specific setting on its 2940W controller to work properly with the drive. I understand there are some drives with proposed SCSI-3 command features. These are mostly in the MODE SELECT and in error codes, as I recall. Perhaps someone who knows more about this could elaborate? Note also that the major players (like DC Drives) don't have any "SCSI-3" stuff advertised; only JDR and some cheap clones are promoting it.Besides, Wide SCSI-2 has yet to really catch on (mostly because only a few drives are fast enough to take advantage of it). There is no "wide SCSI-2" because that would mean two cables. Single cable wide SCSI has always been SCSI-3, it just took too d*** long to get into a standard! :-)

Is SYNCHRONOUS faster than ASYNCHRONOUS?

Yes, the asynchronous transfer option waits for each byte to be transferred before it is acknowledged. With synchronous protocol, the device sending the data is allowed to get ahead of the device receiving the data by a number of bytes (called the offset). The offset is negotiated between the initiator and the target some time prior to the transfer beginning. The synchronous protocol is considerably more efficient and therefore faster than asynchronous.

Is the NCR 53C90 Faster than spec?

I've seen a few comments about our 54C90 being faster than spec. While I doubt the author was really complaining (I got twice as much as I paid for - sure makes me mad ;) I'd like to explain the situation. Along the way, I'll also show that asynchronous is faster on short cables, while synchronous is faster on long cables. The cross-over point occurs somewhere around six feet--assuming that you have our 53C90 family devices at both ends of the cable. The reason has to do with the propagation delay of the cable; the turn around time of the silicon; and the interlocked nature of the asynchronous handshake. 1) We have measured propagation delays from various cables and found an average of 1.7 nanoseconds per foot, which is roughly 5.25 ns per meter.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 2) The turn-around time is the amount of time the SCSI chip takes to change an output in response to an input. If REQ is an input then ACK is an output. Or if ACK is an input then REQ is an output. Typical turn-around time for the 53C90 is 40 nanoseconds. 3) The asynchronous transfer uses an interlocked handshake where a device cannot do the next thing until it receives positive acknowledgment that the other device received the last thing. First REQ goes true /* driven by Target */ then ACK is permitted to go true /* driven by Initiator */then REQ is permitted to go false then ACK is permitted to go false Thus we have four "edges" propagating down the cable plus 4 turn-around delays. Asynchronous transfer requires 55 ns setup and no hold time (paragraph in 5.1.5.1 in SCSI-1 or SCSI-2) which gives an upper speed limit around 18 MB/s. A detailed analysis (assuming 53C90 family) shows that the setup time subtracts out. This is mostly because we are running at one-third the max rate, but also because setup for the next byte can begin anytime after ACK is received true or REQ is received false, depending on who is receiving. You can either take my word for it or draw the waveforms yourself. Thus, the asynchronous transfer reduces to: (4 * 1.7 * 1) + (4 * 40ns) = 167 ns /* 1 foot cable */= 6 MB/s (4 * 5.25 * 6) + (4 * 40ns) = 286 ns /* 6 meter cable */= 3.5 MB/s (4 * 5.25 * 25) + (4 * 40ns) = 685 ns /* 25 meter cable */= 1.5 MB/s note: cables longer than 6 meters require external differential transceivers which add delay and degrade the performance even more than indicated here. Our simulations say that under very best conditions (fast silicon, low temperature, high voltage, zero length cable) we can expect more than 8 MB/s asynchronously. In the lab, I routinely measure 5 MB/s on 8 foot cables. So, if you were writing the data manual for this, how would YOU spec it? The framers of the SCSI spec threw in synchronous mode to boost the performance on long cables. In synchronous mode, the sending device is permitted to send the next byte without receiving acknowledgment that the receiver actually received the last byte. Kind of a ship and pray method. The acknowledgment is required to come back sometime, but we just don't have to wait for it (handwave the offset stuff and the ending boundary conditions). In this mode any external transceivers add a time shift, but not a delay. So if you negotiate for 5 MB/s, you get 5MB/s regardless how long the cable is and regardless whether you are single-ended or differential. But you can't go faster than 5.5 MB/s, except in SCSI-2. Synchronous mode does have a hold time (unlike asynch) but again, setup and hold times subtract out. In SCSI-1 synchronous mode, the speed limit comes from the combined ASSERTION PERIOD + NEGATION PERIOD which is 90ns + 90ns = 180ns = 5.5 MB/s. Our 53C90 family doesn't quite hit the max, but we do guarentee 5.0 MB/s. In SCSI-2, anything above

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/20035.0 MB/s is considered to be FAST. Here the maximum transfer rate is explicitly limited to 100 ns or 10MB/s; you don't have to read between the lines to deduce it. Interesting tid-bit: given a SCSI-2 FAST period of 100 ns and a cable delay of 131 ns on a 25 meter cable, you can actually stack 1.31 bytes in the 8-bit cable. In FAST and WIDE SCSI you can stack 5.24 bytes in this copper FIFO. Hummm...

What is FPT (Termination)?

FPT stands for Forced Perfect Termination. FPT is actually really simple, I wish I had thought of it. What it does is use diode clamps to eliminate over and undershoot. The "trick" is that instead of clamping to +5 and GND they clamp to the output of two regulated voltages. This allows the clamping diodes to turn on earlier and is therefore better at eliminating overshoot and undershoot. The block diagram for a FPTed signal is below. The resistor value is probably in the 110 Ohm range. The actual output voltages of the regulators may not be exaclty as I have shown them but ideally they are matched to the diode characteristics so that conduction occurs when the signal voltage is greater than 3.0 V or less than 0.2 V. +-----------*--- TERMPWR | | ____|___ || | || Vreg 1 |---------------------------------* 2.8 V|________| | | | | | ----- | | --- | \ - | / term resistor | \ (110 Ohms) | / ____|___ | | | | | Vreg 2 |-*--------* 2.4 V | |________| | | | | --+-- | | / \ | +------+ /___\ | | | | | | | terminated | *----------*------------- signal | | | | | --+-- | / \ | /___\ Both diodes are fast silicon | | switching diodes (.6 V drop) ___|____ |

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 | | | | Vreg 3 |----------* 0.8 Volts |________| The diagram shows the circuit for terminating one signal. In a complete FPT there would be 36 diodes and 18 110 Ohm resistors plus the regulator chips.Using the values shown, transients would be clamped at 0.2V and 3.0V.

What is Active Termination?

An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than using resistor voltage dividers. This is a passive terminator: TERMPWR ------/\/\/\/------+------/\/\/\/----- GND | | SCSI signal Notice that the termination voltage varies with the voltage on the TERMPWR line. One voltage divider (two resistors) is used for each SCSI signal. An active terminator looks more like this (supply filter caps omitted):

2.85 Volt Regulator +-----------+ +2.85V 110 OhmsTERMPWR -----| in out |------+------/\/\/\/-------SCSI signal | gnd | | +-----------+ | | +------/\/\/\/-------SCSI signal | |GND ---------------+ | +------/\/\/\/-------SCSI signal | etc. Assuming that the TERMPWR voltage doesn't drop below the desired termination voltage (plus the regulator's minimum drop), the SCSI signals will always be terminated to the correct voltage level.Several vendors have started making SCSI active terminator chips, which contain the regulator and the resistors including Dallas Semiconductor, Unitrode Integrated Circuits and Motorola.

Why Is Active Termination Better?

Typical passive terminators (resistors) allow signals to fluctuate directly in relation to the TERM Power Voltage. Usually terminating resistors will suffice over short

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003distances, like 2-3 feet, but for longer distances active termination is a real advantage. Active termination provide the following advantages:- Helps reduce noise.- A logic bit can be used to effectively disconnect the termination.- Regulated termination voltage.- SCSI-2 spec. recommends active termination on both ends of the scsi bus.- Improved resistance tolerances (from 1% to about 3%)

How can I tell whether an unmarked terminator is active or passive?

If you have an Ohm-meter of one kind or another, measure the resistance from the TERMPWR pin to an adjacent GROUND pin. Reverse the probes and take another reading. If the reading is about 30.5 Ohms, with the probes both ways, you have a passive single-ended terminator. If the reading is about 45 Ohms, with the probes both ways, you have a passive differential terminator. Active terminators should read much higher and give very different readings with the probes interchanged.

What is Plug and Play SCSI?

Plug and Play is the name of a technology that lets PC hardware and attached devices work together automatically. A user can simply attach a new device ("plug it in") and begin working ("begin playing"). This should be possible even while the computer is running, without restarting it.Plug and Play technology is implemented in hardware, in operating systems such as Microsoft Windows, and in supporting software such as drivers and BIOS. With Plug and Play technology, users can easily add new capabilities to their PCs, such as sound or fax, without having to concern themselves with technical details or encountering problems. For users of mobile PCs (who are frequently changing their configurations with docking stations, intermittent network connections, etc.) Plug and Play technology will easily manage their changing hardware configuration. For all users, Plug and Play will reduce the time wasted on technical problems and increase their productivity and satisfaction with PCs. The Plug and Play technology is defined in a series of specifications

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003covering the major component pieces. There are specifications for BIOS, ISA cards, PCI, SCSI, IDE CD-ROM, PCMCIA, drivers, and Microchannel. In a nutshell, each hardware device must be able to be uniquely identified, it must state the services it provides and the resources which it requires, it must identify the driver which supports it, and finally it must allow software to configure it. The first Plug and Play compliant products are available now, as are development kits for drivers and hardware. Twenty different Plug and Play products were shown at Comdex in November 1993. Specifications:The Plug and Play specifications are now available via anonymous ftp at ftp.microsoft.com in the \drg\plug-and-play subdirectory. The files are compressed in .zip format, and are in Microsoft Word format.) Plug and Play ISA files (.\pnpisa\*) errata.zip Clarifications and corrections to pnpisa.docisolat.zip MS-DOS testing tool to isloate ISA PnP hardwarepnpdos.zip Plug and Play device driver interface specificationpnpisa.zip Hardware spec for PnP ISA enhancementvhdlzi.zip Hardware spec for PnP ISA enhancement Plug and Play SCSI files (.\scsi_ide\*): pnpscsi.zip Plug and Play SCSI specification proposalscam.zip SCAM (SCSI Comnfigured Auto-Magically) specification Plug and Play BIOS files (.\bios\*): apmv11.zip Advanced Power management spec v.1vios.zip Plug and Play BIOS specescd1.zip Spec for optional method of storing config info for PnP BIOS [email protected] alias: There is an alias, [email protected], which you can email and get on a Microsoft mailing list related to Plug and Play, where the Hardware Vendor Relations Group (HVRG) will mail out new specifications, announcements, information on workshops, Windows Hardwware Engineering Conference (WinHEC), etc... Compuserve PlugPlay forum: There is a forum on Compuserve, GO PLUGPLAY. This forum is the method for support, discussions and dialogs about Plug and Play. In addition, the forum's library contains all of the current specification. Intel Plug and Play kits:

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003 If you are interested in Intel's two Plug and Play kits, either "Plug and Play Kit for MS-DOS and Windows" or "Plug and Play BIOS Enhancements Kit", FAX your name and company information to Intel at 1.503.696.1307, and Intel will send you the information.

My SCSI bus works, but is not reliable. What should I look at?If you still have problems after you're sure that you have all the ID and termination and cable issues resolved, it's time to dig a little deeper.If you get your SCSI bus to the point where it basically works, but it isn't reliable I have found that the gremlin can be the TERMPWR voltage. With your system fully powered up, and both terminators attached, measure the TERMPWR voltage at the far end of your bus. It needs to be between 4.25 and 5.25 Volts. Many vendors start with the system's +5 VDC and add a regular silicon rectifier diode and fuse in series. Silicon rectifiers have an inherent voltage drop of .6 to 1.0 Volts depending on the current through them. Schottky barrier rectifiers are much better for this application. I always use a 1N5817 myself. If the diode on the host adapter is a 1N400x type, change it to a 1N5817. If you add up the drop across the diode and the fuse and 15 feet of ribbon cable and the connector contact resistances, many times you'll find yourself below 4.0 Volts. When using passive terminators, this can shift the signal threshold and decrease the signal to noise ratio on the bus.If you aren't able to get relief with these methods, sometimes you can solve the problem by having several devices supply TERMPWR to the bus. Sometimes the voltage is high enough, but there is too much noise on the TERMPWR line. This can cause really strange problems! If you can see more than about 200 mV of noise on TERMPWR, add a .1 uF and 10 uF capacitor from TERMPWR to one of the adjacent GROUND lines. You need to have the bus as active as you can get it when measuring the noise. I have actually seen over 1 Volt of noise in some severe cases. Another way you can help to solve TERMPWR problems is to use active terminators. These don't draw as much current from the TERMPWR source and they also have a built in regulator which can operate on lower voltage than the standard passive terminators. The regulator also tends to reduce the noise.

Where can I find information about programming using the ASPI

interface from DOS and Windows?

The Adaptec BBS has some documents about ASPI. They also have a WWW server.See the FAQ Question "How can I contact Adaptec?" for phone numbers and URL information etc. ftp://ftp.adaptec.com/pub/BBS/adaptec/

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003Dr Dobb's Journal March 1994 issue pg 154, has an article called "The Advanced SCSI Programming Interface" by Brian Sawert. Example code in C and x86 assembly language is included. The code can be obtained via anonymous ftp from: ftp.mv.com: /pub/ddj/1994.03/aspi.zip.

How do I replace Macintosh internal HD and terminate the SCSI chain properly?

The factory installed Macintosh internal HD should be terminated. Make sure the terminator/resitor-package is installed in the drive before using it. Most vendors will install the terminator for you if you tell them it is for use in Macintosh as the system disk. Manufacturers usually have toll free numbers for SCSI termination, ID, and such. If you don't already have the terminator, they may send you one for free. BTW, Macintosh SCSI chain starts at the system disk (ID=0), and ends at the control board (ID=7). ID numbers from 1-6 should be used for any other devices on the chain.

Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?

Attaching a SCSI-1 device to a system with a SCSI-2 host adapter and several SCSI-2 devices already attached will not hurt over-all performance significantly unless it doesn't handle disconnect/reconnect well. This assumes that the host adapter keeps track of protocol options separately for each target device. Some people have the idea that attaching a SCSI-1 device to a SCSI-2 bus will cause the entire bus to run at SCSI-1 speeds. This is not true.

Can I connect a SCSI-3 disk to my SCSI-1 host adapter?Questions of this nature really cannot be answered in a useful way. There are so many aspects and options to each of the SCSI standards, you need to be much more specific about what devices and adapters you're interested in connecting. Most of the time the best thing to do is just try it! Most combinations will work, but if you're considering a purchase and looking for a guarantee from "The Net", forget it. The issue is further complicated by the fact that vendors like to latch onto the latest acronyms before they even know what's involved. For example SCSI3 is not approved yet, but vendors are already saying their devices are SCSI3 compatible. Since there is no standards compliance testing organization, they can pretty much say what they want. If you buy a high end host adapter (probably called SCSI3 :-) ) from a reputable vendor, and it has enough control over the various options (like synch xfer rate 5,10,20 xfers/s and the ability to disable WIDE or FAST/Ultra negotiation), and you carefully think out what devices you connect to it (all WIDE devices nearest the host adapter end of the bus etc.), and you are careful to properly terminate not only both ends, but both halves (upper byte and lower byte) of the bus, and none of the older devices you might already have (like a Panasonic CDROM) do anything stupid (like

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003not handle the WIDE negotiation message without hanging) then it will all work fine. :-) Even though a host adapter may be called SCSI3 doesn't mean it can enable or disable each optional feature, yet this is vital for supporting older devices. To make matters worse, you won't know which older devices do some of the stupid things unless you know someone who's been bitten already. Your best bet is to look for good deals on name brand devices and adapters and before you buy, ask in comp.periphs.scsi whether anyone has tried the combination you're considering. It's also important to buy from a well known vendor with reasonable return policies. If you're looking at buying a Vendorxyz spiffydisk which claims to be SCSI-3 compatible and you have a Seagate ST-01 host adapter and you want to know if anyone else has tried this combination, then that's exactly what you should ask. In general, most SCSI devices and adapters made less than 4 years apart will probably work together, but without specific information about exactly which devices there's no assurance of it. There's also the potential for poor performance even if it does work.

Can I connect a WIDE device to my narrow SCSI host adapter?

Can I connect a narrow device to my WIDE SCSI host adapter?

Yes, you just need an appropriate adapter. Most WIDE devices use the 68 pin "P" connector so you need a 68 pin to 50 pin adapter. You do need to make sure that both the upper byte and lower byte of the bus will be properly terminated though. Some adapters provide a place for terminators, others do not. If the wiring adapter is placed right at the SCSI host adapter, you can usually configure the host adapter's on-board terminators to only terminate the high byte. You need to be clear on what type of connectors are present where you want to do the conversion. You also need to plan your bus so that all the WIDE devices will be at one end and all the narrow devices will be at the other end. Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. When using WIDE/narrow adapters this assumption is not valid.If for some reason you attach a WIDE device to a narrow bus, you must be sure to disable WIDE negotiation in the host adapter BIOS or the device will hang when it is accessed.

One further caveat is that if narrow devices are attached to a WIDE adapter, the adapter's ID must be between 0 and 7 because narrow devices would not be able to see it if the ID was any higher than 7.

How does device ID numbering work with WIDE vs NARROW devices?

Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is generally wise to reserve 0-7 for narrow devices though. SCSI-2 only specified the use of IDs 0-7 even for WIDE

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003devices, but SCSI-3 allows 0-15 for WIDE devices. All devices on one bus must have unique Ids of course. The arbitration priorities are as follows:highestID 7...ID 0ID 15...ID 8ID 23...ID 16ID 31...ID 24lowest A WIDE device that is set to ID 10 knows not to respond to selection for ID 2 because the parity bit P1 (for bits 8-15) will not be set by the initiator. During a selection of ID 10, the P parity bit (for bits 0-7) will not be set by the initiator, but the P1 bit will be.To use both WIDE and narrow devices on the same bus, the host adapter must be set to ID 7 (or less) so that the narrow devices can talk to it.

What is spindle-sync and why would I want it?

It fundamentally affects just one aspect of performance, the 'latency'. With a single drive, if you are waiting for a sector to 'arrive' round a track, you have (on average) to wait for approximately one half the rotational time of the drive for it to arrive. So you might arrive at the track just as the sector has gone by, and have to wait one whole rotation at the worse, or the sector might arrive just as you want it, and latency would be zero. This average time, is the minimum latency achievable. There are two methods of reducing this time. The first is to increase the rotational rate of the drive. This is why for certain types of application a 7200RPM drive, will still outperform a 5400RPM drive that has the same data rate off the drive. The other method is to have multiple copies of the required data on unsynchronized drives, and take whichever copy arrives first. This can be done with mirrored drives, and gives a small improvement in the latency time. However the 'down side' of multiple drives comes when we have to wait for all the data parts to arrive. So (for instance) on a striped array, if the drives are synchronized, the latency will remain the same as for the single drives with both data 'parts' arriving together. However, if the drives are unsynchronized, the 'total' latency goes up, to 33% 'worse' than the single drive, as we now have to wait for both parts to arrive. Similar 'extensions' take place with other RAID configurations, unless the drives are synchronized. Basically, in RAID arrays, the drives should be synchronized, _unless_ the total required data can be assembled from a small fraction of the drives. RAID 1, and RAID 10, are the commonest configurations where synchronization is not advised.

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003What are the general steps I need to do to install a SCSI disk to be used with Windows? This description assumes an Adaptec host adapter, but other types should involve about the same procedure. Let me start from scratch and describe one by one all necessary steps:

Prepare a bootable MS-DOS floppy (SYS A: ) containing, in addition to the system files, the FDISK.EXE and FORMAT.COM programs. (Preferably the ones that came with your Win95 distribution). Make sure there isn't anything in the AUTOEXEC.BAT or CONFIG.SYS files that could make trouble later. Better still, delete these two files. Do not insert the floppy yet. Reset your computer and enter the BIOS setup, (not the SCSI setup) and make sure that the "disk type" is set to 'none' or 'not installed' or something similar. Verify that the boot sequence is A: first. Exit and reset (If your host adapter is new, you can probably skip the next step, but if you want to avoid mysteries later, it’s not a bad idea to do it.)Enter the SCSI setup (CTRL-A) and go to the setup menu. Press F6 to restore all the default settings. Exit and reset Enter the SCSI setup (CTRL-A) and go to the utilities menu. Make sure you see your disk in the list of devices, and the name and model look OK. Select the proper disk and run the "Format" Choose Verify media" to build your confidence that the drive is really working right. If these two steps work ok, your disk and controller are fine and they are communicating correctly. If not, you have a hardware problem. (check cables?, terminators?, TERMPWR?, disk itself?)(It is not necessary to wait for the verify function to finish, although it is a good idea to do it with a new disk.)Exit and reset. Boot from the floppy this time. While the system is coming up, a message on the screen will show up saying something like "<your disk model> C: 80H BIOS Installed." This means that the SCSI controller recognized the disk, and since there is not an IDE C: disk, it installed the necessary BIOS functions to use the SCSI disk as 'C:' It does NOT mean that the C: drive is ready for DOS/Windows. If you don't get that message check that the SCSI disk is installed as device ID 0. (With newer host adapters you can use Ids other than 0) After getting the A:> prompt, run FDISK. Create a primary DOS partition. (2 GB max except for Win 95 OSR2 w/FAT32). Make that partition active. Exit and reboot from the floppy. At this point you already have a C: drive, but you can not use it because it has no file system. (Typing DIR C:, for example, will produce the error message 'Invalid media type', different from the 'Invalid drive specification' you got before) To make a file system run FORMAT C: /S /U. The /S tells the format program to copy the system files to C: at the end of the formatting. This will make C a bootable disk. (Assuming the partition was made active above )

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DisclaimerThis document is an edited version. The full version of this document can be obtain at http://www.faqs.org/faqs/scsi-faq/part1/. Edited by [email protected] http://pokemontel.halted.net 30/10/2003When the FORMAT program ends, you should be able to switch to C:, do a DIR, etc. Remove the floppy, reset and (hopefully) reboot from the hard disk. Notes for mixing IDE and SCSI disks on the same system?The IDE disk must be defined properly in the BIOS setup (disk type= number or autodetect instead of "not installed as above"). If you will only boot from the IDE disk, the SCSI disk doesn't need to be made bootable. (Some modern BIOSes let you choose to boot from SCSI even if an IDE disk is installed) The BIOS in the SCSI controller will install a maximum of two disks. If you have an IDE disk installed, the SCSI BIOS will still install the (first) SCSI disk. If you have 2 IDE disks You'll have to install SCSI drivers in the boot disk to access the SCSI disk or disks. If you have a system with 4 SCSI disks (no IDE) the controller's BIOS will install only the first two; Again you'll have to install drivers to access the rest, etc.

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