Modes of Data Transmission

8/14/2019 Modes of Data Transmission

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Modes of data Transmission


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Parallel data transfer:

Many bits (usually 4 or 8) are sent at a time over many wires in the cable.

- Faster

- Limited to small distances

- Data skew: The difference in arrival time of bits transmitted at the

same time.

Serial data transfer:

Sending one bit at a time over one wire through the serial port is known as

serial transfer.

- Slower

- Cheaper


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To interface a microcomputer with serial data lines, the data must be

converted to and from serial form.

A parallel-in-serial-out shift register and a serial-in-parallel-out shiftregister can be used to conversion.


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Terms in serial data transmission

Simplex

Transmit data only in one direction Eg: radio station, earthquake sensor

Half-duplex

Communication can take place in either direction between twosystems, but can only occur in one direction at a time.

Eg: walky-talky, push to talk (PTT) devices

Full-duplex

Full-duplex means that each system can send and receive data at thesame time.

Eg: phone conversation.


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Serial transmission

Asynchronous

-Each byte is encoded for transmission with Start and stop bits

- No need for sender and receiver synchronization

Synchronous

-Sender and receiver must synchronizeDone in hardware using phase locked loops (PLLs)

-Block of data can be sent

-More efficient and less overhead than asynchronous transmission

-Expensive


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Fig. Bit format used for sending asynchronous serial data

D0 D3D1 D4D2 D6D5START PARITY STOP STOP

ALWAYS LOW ALWAYS HIGH

ONE CHARAC TER


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Format of asynchronous transmission (One byte of async data)


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Pin Configuration of the 8251 USART


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16 SYNDET/BD - Refer Data Sheet

17 CTS - Clear To Send data (Active Low)

18 TXEMPTY - Transmitter Register Empty

19 TXD - Transmitter data (Output)

20 CLK - Clock

21 RESET - Reset

22 DSR - Data Set Ready

23 RTS - Request To Send data

24 DTR - Data Terminal Ready

25 RXC - Receiver Clock (Active Low)

26 Vcc --- + 5v Supply


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Block diagram of the 8251 USART

D0 to D7 (l/O terminal)


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D0 to D7 (l/O terminal)

This is bidirectional data bus which receives control words and transmits data from the

CPU and sends status words and received data to CPU.

RESET (Input terminal)

The device waits for the writing of "mode instruction. The min. reset width is six clock inputs during the operating status of CLK.

CLK (Input terminal)

CLK signal is independent of RXC or TXC.

The frequency of CLK must be greater than 30 times the RXC and TXC atSynchronous mode and Asynchronous "x1" mode It must be greater than 5 times at Asynchronous "x16" and "x64" mode.

WR (Input terminal)

It receives a signal for writing transmit data and control words from the CPUinto the 8251.

RD (Input terminal)

It receives a signal for reading receive data and status words from the 8251.


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C/D (Input terminal)

It selects data or command words and status words when the 8251 isaccessed by the CPU.

CS (Input terminal)

TXD (output terminal)

This is an output terminal for transmitting data from which serial-converted data is sent out.

TXRDY (output terminal)

This is an output terminal which indicates that the 8251is ready to accepta transmitted data character.

TxRDY=1 when holding buffer is empty


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TXEMPTY (Output terminal)

This is an output terminal which indicates that the 8251 has transmitted all the characters

and had no data character.

TxEMPTY=1 when both buffers are empty.

In "synchronous mode," the terminal is at high level, if transmit data characters are no

longer remaining and sync characters are automatically transmitted.

TXC (Input terminal)

1. This is a clock input signal which determines the transfer speed of transmitted data.

2. In "synchronous mode," the baud rate will be the same as the frequency of TXC.

3. In "asynchronous mode", it is possible to select the baud rate factor by mode

instruction.

4. It can be 1, 1/16 or 1/64 the TXC. The falling edge of TXC sifts the serial data out

of the 8251.

RXD (input terminal)

This is a terminal which receives serial data.


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RXRDY (Output terminal)

This is a terminal which indicates that the 8251 contains a character that is ready to READ

(by the CPU).

RxRDY=1 when a character has been shifted into the receiver buffer.

RXC (Input terminal)

This is a clock input signal which determines the transfer speed of received data.

SYNDET/BD (Input or output terminal)

1. This is a terminal whose function changes according to mode.

2. In "synchronous mode," it is at high level, if sync characters are received andsynchronized.

3. In "asynchronous mode," this is an output terminal which generates "high level outpu

upon the detection of a "break" character if receiver data contains a "low-level" space

between the stop bits of two continuous characters.


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DSR (Input terminal) : Data Set Ready

This is an input signal for MODEM interface. The input status of the terminal can berecognized by the CPU reading status words.

DTR (Output terminal) : Data Terminal Ready

This is an output signal for MODEM interface. It is possible to set the status of DTRby a command.

CTS (Input terminal) : Clear To Send data

This is an input signal for MODEM interface which is used for controlling atransmit circuit. CTS=0 then terminal is ready to transmit data.

RTS (Output terminal): Request To Send data

This is an output signal for MODEM interface. It is possible to set the status RTS bya command.


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The 8251 functional configuration is programmed by software. Operation between the

8251 and a CPU is executed by program control. Table below shows the operation

between a CPU and the device.


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Interfacing 8251 with 8086


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RS 232 I t f


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RS-232 Interface:

RS-232 stands for Recommend Standard number 232 and was given by

the EIA (Electronic Industry Association), Bell Laboratories in the year

1969.

The purpose of the standard was to provide an Interface between Data

Terminal Equipment (DTE) and Data communication Equipment

(DCE) employing serial data transfer.

Data terminal equipment (DTE) is an end instrument that converts

user information into signals or reconverts received signals. A DTE

device communicates with DCE. The DTE/DCE classification was

introduced by IBM

The DTE device is the terminal (or a computer emulating a terminal),

and the DCE is a modem


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The RS-232 standard requires a modem to be

connected between the receiving and transmitting ends.

This interface is useful for point-to-point

communication at slow speeds. For example, port

COM1 in a PC can be used for a mouse, port COM2for a modem, etc.

RS 232 was designed for communication of local

devices, and supports one transmitter and one receiver.


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RS232 on DB9 (9-pin D-type connector)

Male RS232 DB9

Signals


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Commonly-used signals are:

Transmitted Data (TxD) Data sent from DTE to DCE.

Received Data (RxD) Data sent from DCE to DTE.

Request To Send (RTS) Asserted (set to logic 0, positive voltage) by DTE to prepare DCE to receive data.

Ready To Receive (RTR) Asserted by DTE to indicate to DCE that DTE is ready to receive data.

Clear To Send (CTS) Asserted by DCE to acknowledge RTS and allow DTE to transmit

Data Terminal Ready (DTR) Asserted by DTE to indicate that it is ready to be connected. If the DCE is a modem, this may "wake up" the modem, bringing it out of a

power saving mode.

Data Set Ready (DSR)

Asserted by DCE to indicate the DCE is powered on and is ready to receive commands or data for transmission from the DTE.

Data Carrier Detect (DCD) Asserted by DCE when a connection has been established with remote equipment.

Ring Indicator (RI) Asserted by DCE when it detects a ring signal from the telephone line

Li it ti f RS 232


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Limitations of RS-232

RS-232 has some serious shortcomings as an electrical interface.

Firstly, the interface pre-supposes a common ground between the DTE andDCE.

-- This is a reasonable assumption where a short cable connects a DTE and DCEin the same room, but with longer lines and connections between devicesthat may be on different electrical busses, this may not be true. We have

seen some spectacular electrical events causes by "uncommon grounds".

Secondly, a signal on a single line is impossible to screen effectively for noise.

-- By screening the entire cable one can reduce the influence of outside noise,but internally generated noise remains a problem.

-- As the baud rate and line length increase, the effect of capacitance betweenthe cables introduces serious crosstalk until a point is reached where the dataitself is unreadable.

TTL t RS 232C i


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TTL to RS 232C conversion

USART (8251) is not directly compatible with RS-232 signal

levels.

The TTL to RS232 Serial Adapter is used to connect TTL

(Transistor-Transistor Logic) level signals to an RS-232

interface.

The TTL side is a 9-pin female connector, and the RS-232 side is

a 9-pin male connector.

The TTL side has a voltage suppression network designed to

protect against ESD (Electro Static Discharge) and EFT

(Electrical Fast Transient).


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TTL to RS232 Serial Adapter

RS232C TTL i


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RS232C to TTL conversion


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High Speed serial communication


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RS-422:

A high-speed system similar to RS-232 but with differential signaling

RS-423:

A high-speed system similar to RS-422 but with unbalanced signaling

RS-449:

A functional and mechanical interface that used RS-422 and RS-423signals - it never caught on like RS-232 and was withdrawn by the EIA

RS-485:

A descendant of RS-422 that can be used as a bus in multidrop

configurations

MIL-STD-188:

A system like RS-232 but with better impedance and rise time control

High-Speed serial communication


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TIA-574:

standardizes the 9-pin D-subminiature connector pinout for use with

EIA-232 electrical signaling, as originated on the IBM PC/AT

SpaceWire

high-speed serial system designed for use on board spacecraft

High-Speed serial communication standards


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High Speed serial communication standards

IEEE 1394 interface:

The IEEE 1394 interface is a serial bus interface standard for high-speedcommunications and isochronous real-time data transfer, frequently used by personalcomputers, as well as in digital audio, digital video, automotive, and aeronauticsapplications.

The ISOCHRONOUS (ISOC) format for data transmission is a procedure or protocolin which each information CHARACTER or BYTE is individually synchronized orFRAMED by the use ofStart and Stop Elements (bits).

This interface is also known by the brand names ofFireWire (Apple Inc.), i.LINK(Sony), and Lynx (Texas Instruments).

IEEE 1394 has been adopted as the High-Definition Audio-Video Network Alliance(HANA) standard connection interface for A/V (audio/visual) componentcommunication and control.

FireWire is also availablein wireless, fiber optic, and coaxial versions using theisochronous protocols

Nearly all digital camcorders have included a four-circuit 1394 interface

Standards and versions


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Standards and versions

FireWire 400 (IEEE 1394 -1995):

A 6-circuit FireWire 400 alpha connector

The original release of IEEE 1394-1995 specified what is now known asFireWire 400.

It can transfer data between devices at 100, 200, or 400 Mbit/s half-duplexdata rates (the actual transfer rates are 98.304, 196.608, and 393.216 Mbit/s,

These different transfer modes are commonly referred to as S100, S200,and S400.

Cable length is limited to 4.5 metres (14.8 ft), although up to 16 cables canbe daisy chained using active repeaters.


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FireWire 800 (IEEE 1394b-2002)


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FireWire 800 (IEEE 1394b 2002)

A 9-circuit beta connector.

IEEE 1394b-2002 introduced FireWire 800(Apple's name for the 9-circuit "S800

bilingual" version of the IEEE 1394bstandard)

Data transfer rate is 786.432 Mbit/s full-duplex via a new encoding scheme termed

beta mode.

It is backwards compatible to the slower ratesand 6-circuit alpha connectors of FireWire400.

A 9-circuitbeta connector


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FireWire S1600 and S3200

In December 2007, the 1394 Trade Associationannounced that products will be available before the endof 2008 using the S1600 and S3200 modes that, for themost part, had already been defined in 1394b and wasfurther clarified in IEEE Std. 1394-2008.

The 1.6 Gbit/s and 3.2 Gbit/s devices use the same 9-circuit beta connectors as the existing FireWire 800 andwill be fully compatible with existing S400 and S800devices.

It will compete with the forthcoming USB


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FireWire S800T (IEEE 1394c-2006)

IEEE 1394c-2006 was published on June 8 2007.

It provided a major technical improvement, namely new portspecification that provides 800 Mbit/s over the same RJ45

connectors with IEEE 802.3 (Ethernet) devices.

Though the potential for a combined Ethernet and FireWireRJ45 port is intriguing, as of November 2008, there are no

products or chipsets which include this capability.

Comparison to USB


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Comparison to USB

Firewire (which started development in 1986) had implementationspredating USB.

However USB reached industry standardization (1994) before the IEEE-1394-1995 specification was released (1995).

At this time USB 1.0 had a signaling speed of 12 and 1.5 Mbit/s (comparedto 400 Mbit/s of IEEE-1394a (FireWire 400)) but cheaperimplementations. USB 2.0 with (480 Mbit/s) signal rate was made availablein computers early 2001.

FireWire 800 is substantially faster than Hi-Speed USB, both in theory

and in practice.

Alternative uses for IEEE 1394

1. Aircraft

2. Automobiles

USB-Universal Serial Bus


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USB is intended to replace many varieties of serial and parallelports.

USB can connect computer peripherals such as mice

keyboards PDAs

gamepads

joysticks

scanners digital cameras printers personal media players flash drives external hard drives.

As of 2008, there are about 2 billion USB devices sold per year,and about 6 billion total sold to date.

The design of USB is standardized by the USB Implementers Forum


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(USB-IF), an industry standards body incorporating leading

companies from the computer and electronics industries.

Year created: January 1996

Created by: Intel, Compaq, Microsoft, Digital Equipment

Corporation, IBM, Northern Telecom

A USB Series A plug, the most

common USB plug

USB communication takes the form of packets


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USB communication takes the form of packets.

The original USB 1.0 specification had a data transfer

rate of12 Mbit/s.

The USB 2.0 specification was released in April 2000and was standardized by the USB-IF at the end of

2001. Data transfer rate of480 Mbit/s

The USB 3.0 specification was released on November12, 2008 by the USB 3.0 Promoter Group. It'smaximum transfer rate is up to 10 times faster thanthe USB 2.0 release. It has been dubbed the SuperSpeed USB.


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USB extension cord

Different types of USB connectorsfrom left to right

8-pin AGOX

Mini-B plug Type B plug

Type A receptacle

Type A plug

A USB system has an asymmetric design consisting of a host a


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A USB system has an asymmetric design, consisting of a host, amultitude of downstream USB ports, and multiple peripheral devicesconnected in a tiered-star topology.

Additional USB hubs may be included in the tiers, allowing branchinginto a tree structure with up to five tier levels.

A USB host may have multiple host controllers and each host

controller may provide one or more USB ports.

Up to 127 devices, including the hub devices, may be connected to asingle host controller.

USB devices are linked in series through hubs. A physical USB devicemay consist of several logical sub-devices that are referred to as device

functions.

A single device may provide several functions, forexample a webcam (video device function) with a built in


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example, a webcam (video device function) with a built-inmicrophone (audio device function). Such a device iscalled a compound device in which each logical device is

assigned a distinctive address by the host and all logicaldevices are connected to a built-in hub to which thephysical USB wire is connected.

USB device communication is based onpipes (logical

channels).

Pipes are connections from the host controller to a logicalentity on the device named an endpoint.

USB endpoints actually reside on the connected device: thechannels to the host are referred to as pipes.

USB mass-storage


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USB mass storage

USB implements connections to storage devices using aset of standards called the USB mass storage device

class (referred to as MSC or UMS).

This was initially intended for traditional magnetic andoptical drives, but has been extended to support a widevariety of devices, particularly flash drives.

This generality is because many systems can becontrolled with the familiar idiom of file manipulationwithin directories (the process of making a novel devicelook like a familiar device is also known as extension).

Though most newer computers are capable of bootingoff USB mass storage devices.

USB interface port: the drive appears to the user muchlike an internal drive.

Documents

Modes of Data Transmission