CN Serial Ports

8/7/2019 CN Serial Ports

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Chapter Goals

� Understand serial port terminology.

� Understand serial port capabilities.

� Understand serial port signals.

� Understand serial port conf iguration

� Understand serial port management.

� Understand modem connections and setup.

� Disabling serial port services.

� Understand new-technology serial ports.



Serial Ports� Serial Ports

± Serial ports are universal I/O ports. They are available

on almost every computer.± Serial ports are slow, but they are f lexible.

± Serial ports can be used f or terminal connections, printer

connections, network connections, modem connections,

keyboar ds, mice, and other input/output devices.




± Serial ports use standar d pin-outs f or the connectors.

The most common of these standar d is known as RS-232.

� Proposed by the Electronic Industries Association (EIA).

� RS stands f or Recommended Standar d.

� RS-232 def ines the pin-outs of the connectors and the

voltage levels allowed on each pin of a serial portconnection. RS-232 is f unctionally identical to the CCITTV.24/V.28 standar ds.

� RS-232 def ines two types of signals; Data signals, and f low-control signals.




� The RS-232 standar d def ines how the f low control signals

interact to manage the f low of data across a connection.

�The

f ull

f u

nctio

nality o

f the R

S-232 sta

ndar

dis rarely supporte

d,as several of the signals are not required f or simple

communications connections.

� Traditional RS-232 uses 22 gauge stranded, twisted-pair cable.

� RS-232 signal voltages are def ined to be +/- 12 volts DC.

�R

S-232 co

nnectio

ns termi

nate i

na 25 pi

n D

-conn

ector (alsoknown as a DB-25). The computer has the receptacle, while the

cable has a plug.

� Accor ding to the standar d, RS-232 will drive a 50 f oot cable at

9600 baud.



Serial Ports� Newer standar ds have emerged since RS-232 became the

standar d.

± RS-422/RS-423 use lower signaling voltages (+/- 5

volts DC) , and can drive longer lines.

± RS-422 is electrically compatible with CCITT

recommendation V.11.

± RS-423 is electrically compatible with CCITT

recommendation V.10.

± RS-449 is the standar d which def ines the pin-outs and connector characteristics f or the RS-422/RS-423

standar ds.

± MOST RS-422/RS-423 implementations will connect

directly to RS-232 devices and f unction properly.



Serial Ports

� RS-422 uses diff erential line drivers, and can drive

several kilometers of cable.

� RS-423 uses single-ended line drivers and can drive

up to a kilometer of cable.

� All of these standar ds use the same signals, so we will

f ocus most of our discussion on the RS-232

standar ds.



Serial Ports� RS-232 def ines two cabling conf igurations: DTE (Data

Terminal Equipment) and DCE (Data Communications

Equipment).

±These cabling conf igurations determine which

signals a device expects on what pins.

±Most computers, printers, and terminals are

D

TE equipmen

t.±Modems are DCE equipment.

±Cables f or terminals connections are diff erent

f rom the cables used f or modem connections!





Serial Ports� Although 25 pins are def ined, many devices only use

a small subset (4 to 9 pins). These devices require a

sof tware generated carrier signal.

4-pin onne tion

1 ------- 1 Frame Ground

2 ------- 2 Transmit Data

3 ------- 3 Receive Data

7 ------- 7 Signal Ground



Serial Ports

� Devices which provide f or limited modem control use

9 pin connections.

9- i c cti

DB9 DB25

1 ------- 20 Data ermi al ea y

2 ------- 2 ra smit Data

3 ------- 3 eceive Data

4 ------- 8 Data Carrier Detect

5 ------- 7 ig al ro6 ------- 6 Data et ea y

7 ------- 5 Clear o e

8 ------- 4 eq est o e



Serial Ports� Har dwired connections generally use a straight-

through connection.

Straight-Through Conne tion

1 ------- 1 Frame Ground

2 ------- 2 Transmit Data

3 ------- 3 Receive Data

4 ------- 4 Re uest To Send5 ------- 5 lear To Send

6 ------- 6 Data Set Ready

7 ------- 7 Signal Ground

8 ------- 8 Data arrier Detect

20 ----- 20 Data Terminal Ready



Serial Ports� Some devices require a null-modem cable. This cable

f ools the device at each end on the link into thinking it

is receiving the proper mo

dem co

ntrol sig

nals, while i

n f act each device is generating it¶s own modem control

signals.N ll cti

1 ------- 1 rame ro

2 ------- 3 ra smit Data

3 ------- 2 eceive Data

4 ------- 5 eq est o e

5 ------- 4 Clear o e

6,8 ---- 20 Co ectDSR/DCD to DTR

7 ------- 7 Sig al ro

20 ----- 6,8 Co ectDTR to DSR/DCD



Serial Ports� Some manuf acturers have come out with alter nate

connectors f or use with RS-232 connections: Apple

uses a Mini-Din 8 connector, PC¶s use DB-9

connectors, and some patch panels and other serial

equipment use telephone style RJ-45 connectors.



Serial Ports

� Working With Serial Devices

± When dealing with serial devices there are several complicating

f actors:� You must set the communication speed (baud rate) of the device

and the serial port.

� You must set the correct f orm of parity checking on the device

and the serial port.

� You must set the number of stop bits to be used on the deviceand the serial port.

� You must set the wor d length on the device and the serial port.



Serial Ports� Terminal Setup (Sof tware)

± The built-in terminal devices on a UNIX workstation are

usually /dev/ttya (/dev/term/a) and /dev/ttyb (/dev/term/b).

� The modem control equivalents of ttya and ttyb are /dev/cua0

/dev/cua1.

± or any serial connection you must set up certain system

parameters:

� You must tell the system to liste

n f or logi

ns o

na termi

nal port.

� Dial-up modems listen f or logins, but they must also monitor

modem control signals to control the status of the link.

� Dial-out modems require setup inf ormation in /et /remote so the

tip and u commands know how to talk to the modem.




± Most terminals are used as login devices by the users. The

sequence of events to start up a login session are:� User enters login name at the getty prompt.

� Getty executes login

� login requests a passwor d, and conf irms it against /etc/passwd.

� login prints the message of the day f rom /et /motd.

� login runs a shell and sets the TERM variable.

� The shell executes the appropriate startup f iles f or the user.

� The shell prints the UNIX prompt and waits f or input.



Serial Ports� Managing Terminals

� When the user logs out, init takes over and starts a new getty

process.

� Getty reads conf iguration inf ormation f rom /et /ttytab (BSD) or

/et /inittab (ATT)

� /etc/ttytab contains entries which specif y the terminal port, the

program to run on that port, the terminal type connected to the

port, {on|off } and possibly [se ure].

± ttya ³/usr /et /getty std. 600´ wyse50on

� The getty program checks /etc/gettytab to determine what the

³std.9600´ entry means, and sets the line disciplines

appropriately.

± 2|std. 600| 600-baud::sp# 600:




± In the ATT model, getty uses the /etc/inittab f ile todetermine how to set the serial port.

± The /etc/inittab f ile consists of entries with the f ormatid:run-levels:a tion:pro ess.

± A typical entry might be:

� 11:234:respawn:/et /getty tty11 600

± The getty program consults the /etc/gettydefs f ile todetermine how to set the port disciplines:� 9600# B9600 HUPCL # B9600 SA E IXA Y HUPCL #login:

#4800

± The f ormat of the entries is: label#initf lags#f inalf lags#prompt#next






� The SA is not a single program; it is a suite of programs and

background processes which control other programs which do all

of the work.

± The top-level SA program is the Service AccessController (SAC).

» The SAC is initialized at system boot. It is started by the

scripts in the directory /et /init.d.

» The SAC initializes the system environment by

interpreting the /et /saf /_saf onfig script. This script

allows the system administrator to customize the serial

port parameters f or the machine.






» Solaris implements two port monitors; ttymon,

and listen.

» ttymon is the program which monitors the serialports, and directs data to/f rom the ports to the

correct destination.

» The ttymon command uses the def initions in

the /etc/ttydefsf ile to set li

ne

discipli

nes a

nd baud rate f or each port.

» Once the line disciplines are set, the ttymon

hands control of the port over to the login

process.



Serial Ports




± The sacadm command is used to administer the SAC.

» sacadm allows the system administrator to add/remove

port monitors, start/stop port mo

nitors, e

nable/

disableport monitors, install/replace conf iguration scripts,

install/replace port monitor conf iguration scripts, and print

port monitor inf ormation.

» sacadm accepts input f rom stdin, and uses this input to

control the SAC.

» Non-privileged users may use sacadm to request the

status of print jobs, port monitors, and system

conf iguration scripts.






± The ttyadm command is used to f ormat ttymon

administrative inf ormation f or interpretation by the

sacadm and pmadm commands.» The ttyadm command does not work with listen

monitors.




± Listen is the port monitor program which monitors the

network f or service requests f rom other hosts.

± Listen monitors may also manage some networkservices which are not controlled by the inet f acility.

» or example the network printer services are controlled

by the listen port monitor.

» The sacadm, pmadm, and nlsadmin commands are used

to conf igure/administer the listen port monitor.» We will discuss the listen port monitor in more detail in

the section on printers, and again in the networking

section.



The Service Access Controller




± Admintool is another method f or managing serial devices. It is a

point and click GUI which inter f aces to the SA commands f or

you.

± Because admintool relies on the command line f orm of the

commands, and sometimes you want to do non-standar d things,

it is best to discuss how to use the command line commands.

Once we understand them, we may never use them again...but

understanding them is important!




± In or der to connect a har dwired terminal to the system we must

f irst:

� Set up the port mo

nitor so

f tware.

± As root, use the sacadm command to enable login

services on the port.

± OTE: You must use the bour ne shell to execute the

sacadm commands, or they may f ail or provide

unexpected results.� Conf igure the terminal.

� Test the setup to determine if the setup is correct.




± To conf igure the sof tware, f irst we check to see if a ttymon is

already running on the port:

� sacadm -l -t ttymon

± If no ttymon is running, set one up:

± sacadm -a -p zsmon -t ttymon -c /usr /lib/saf /ttymon -v µttyadm -V¶

� NOTE: The string zsmon is known as the Port Monitor Tag

(PMTAG)




± Check to see if a service running on the zsmon port monitor:

± pmadm -l

± If a service is running, remove it (Note, the SVCTAG isgenerally the port name such as ttya. The preceding

command will tell which SVCTAG to use here).

� pmadm -r -p (PMTAG) -s (SVCTAG)

± Set the port line disciplines and start login f or the terminal:

± pmadm -a -p zsmon -s (SVCTAG) -i root -f u -v \

µ/usr/sbin/ttyadmin -V¶ -m ³µ/usr/sbin/ttyadm -l BAUDRATE \ -p

³MESSAGE´ -d /dev/term/(PORT) -T vt100 -i \ µterminal

disabled¶ -s /usr/bin/login -S -y¶´



Serial Ports

� The stty command is a utility that allows the user, or administrator, to set

specif ic terminal port characteristics.

� Typical uses of the stty command include setting the baud rate, size of

the display, parity, and special characters (erase character, interrupt

character, redraw character, and so on).� stty also allows the user to view the current settings of these (and other)

parameters.

� Viewing Port Settings Using stty

± The invocation of stty that allows the user to view port settings ison

of :stty a or stty all or stty everything

� The second command lists several variants, because each version of

UNIX seems to have its own f lag to cause stty to display this

inf ormation.



Serial Ports� Terminal Problems and Solutions

± Most terminal problems are caused by one of :

� You f orgot to make init re-read the conf iguration f iles.

Typing kill -1 1 (as root) will cause this to happen.� Forgot to set sof t carrier mode on 3 wire connections.

� Forgot to use the cu device f or modem controlconnections.

� Wrong type of cable (wrong pinout, null-modem instead of straight through, bad cable)

� Port Monitor incorrectly conf igured.

� Wrong line discipline inf ormation at one end of the link(parity, baud rate, stop bits).

� Bad port/terminal



Serial Ports� Terminal Problems and Solutions

� Sometimes a program will wedge a terminal. Vi, and any other

screen oriented sof tware can exit non-gracef ully and cause the

terminal to be lef t in a strange state. Many times you can recover

f rom this.

± On BSD systems, type reset

± On ATT systems, type stty sane

± Sometimes the terminal will not accept a <CR>, so you have to

use <LF

> or CTRL-J af ter the comma

nds to get a

ny actio

n f romthe system.



Modems� Modem Control Signals

± Har d and Sof t carrier

� UNIX systems expect to see the Data Carrier Detect signal (pin

8) active (+5VDC) when a device is connected and ready to

communicate.

� If the har dware actually expects pin 8 to be active, then the

device is said to be using ³har d carrier´.

� Many systems employ sof tware which pretends that the DCD

signal is active. These systems are using ³sof t carrier´.

� Sof t carrier simplif ies the wiring, and is of ten adequate f or

terminal connections.

� Sof t carrier is not adequate f or modem connections, or f or many

printer connections.





Modems� Modem Control

± When a device is wired f or modem control operation, the

CTS/RTS signals def ine when each device may send data to

the other. If DCD is true, and:� a device sets it¶s Clear-To-Send signal to true, then it is signaling

the other end of the connection that it is ready to receive data.

� a device sets it¶s Request-To-Send signal to true, it is telling the

device at the other end of the connection that it is ready to send

data.

� If the Data Carrier Detect signal transitions to f alse, the carrier

has been lost, so the system should log the user out, and the

connection should be terminated.



Modems� Managing Modems

± Modems take a little more setup inf ormation in the way of

conf iguration f iles.

± The /etc/remote f ile contains inf ormation used f or dial-outconnections by the tip and cu programs. A typical entry might be:

� dial9600|9600 Baud \Hayes:dv=/dev/cul0:br#9600:cu=/dev/cul0:at=hayes:du:

� systema:pn=5551212:tc=dial9600

� monet:pn=@:tc=dial9600

± The /etc/phones f ile contains entries of the f orm:

� monet 8,,510,555-4567,,,,xxxx-xxx

� cc 5552530






� Set up the port monitor sof tware.

± As root, use the sacadm command to enable login

services on the port.± OTE: You must use the bour ne shell to execute

the sacadm commands, or they may f ail or provide

unexpected results.

± To conf igure the sof tware, f irst we check to see if attymon is already running on the port:

� sacadm -l -t ttymon

± If no ttymon is running, set one up:




± sacadm -a -p zsmon -t ttymon -c /usr /lib/saf /ttymon -v µttyadm -V¶

� NOTE: The string zsmon is known as the Port Monitor

Tag (PMTAG)± Check to see if a service running on the zsmon port

monitor:

± pmadm -l

± If a service is running, remove it (Note, the SVCTAG isgenerally the port name such as ttya. The preceding

command will tell which SVCTAG to use here).

� pmadm -r -p (PMTAG) -s (SVCTAG)

±S

et the port line

discipli

nes a

ndstart logi

n f or themodem:




± pmadm -a -p zsmon -s (SVCTAG) -i root -v \ µ/usr/sbin/ttyadmin -

V¶ -f u -m ³µ/usr/sbin/ttyadm -p \ ³MESSAGE´ -d /dev/term/(PORT)

-s /usr/bin/login -l \ BAUDRATE -b -S n -m ldterm,ttcompat¶´ -y

³dial-in/out \ on serial port´

± Conf igure the modem.

± Test the setup to determine if the setup is correct.



Modems

� Using Dial-in Passwords

± Some versions of UNIX provide a dial-in passwor d, an additionalaid f or def ending dial-in terminal access. The dial-in passwor d iscontrolled by two f iles, /etc/dialups and /etc/d_passwd .

� 1. Create /etc/dialups. This f ile contains a single entry per linewith the path of each dial-in modem line (e.g., /dev/term/a).

� 2. Create /etc/d_passwd . This file contains a single entry for eachpossible shell a dial-in modem user might use.

The following is a sample d_passwd file.

/usr/lib/uucp/uucico: passwd-goes-here:

/usr/bin/csh: passwd-goes-here:/usr/bin/ksh: passwd-goes-here:

/usr/bin/sh: passwd-goes-here:

� 3. Insert an encrypted password for each shell in place of thepasswd-goes-here strings shown previously.



PPP

� A host can be a PPP client or a PPP server.

± If your host is dialing out to another server, your host is a PPP client.

± If other hosts are dialing in to your system, your host is a PPP

server.

± Most operating systems provide tools f or implementing both client

and server sides of PPP.

� The process a user goes through to establish a PPP link to a

typical PPP server includes the f ollowing steps.

±D

ial into the server¶s mo

dem.

± Log in using a valid user name and passwor d pair.

± At the shell prompt, issue the command ppp to start PPP on the

server.

± Start PPP on the user¶s system.



PPP

� Windows Point-to-Point Protocol (PPP)

± The Microsof t PPP protocol is the Windows PPP client.

± The PPP protocol setup is part of the Network and Dialup

connections control panel.

� The selections under this wizar d are somewhat

limited; the dial-up line can be a terminal-style dial-up,

a PPP dial-up, or a SLIP dial-up line.

� The NETWORK tab of the dial-up control panel allowsyou to select between the PPP and SLIP connections.



PPP

� Linux includes the f ollowing utilities, used to conf igure the

client end of the PPP link.

± K ppp: The kppp utility is a PPP setup and dialer tool

used under the KDE desktop manager.

± WvDial : A PPP driver f or Linux. This driver was designed

with an emphasis on simplicity.

± RP3: Short f or RedHat PPP. RP3 includes a ³wizar d´

inter f ace to guide the user through the

setup/conf iguration of the PPP link.

± Linuxconf : A generalized tool f or conf iguring and

managing your Linux machine. Linuxconf includes a

utility to help conf igure PPP.



New Serial Ports

� Two new serial bus technologies have recently joined the

mainstream off erings on computer systems:

± the Universal Serial Bus (USB) and

± FireWire inter f aces.± These two inter f aces are high-speed system

interconnects that support devices beyond modems and

terminals.

± Both inter f aces allow you to add disks, tapes, and network adapters to the system via the USB/FireWire

ports.









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CN Serial Ports