COMPUTER AND NETWORK
SYSTEM ADMINISTRATION
Summer 1996 - Lesson 11
Serial Devices
A. RS-232 standard
0. Synchronous versus asynchronous serial devices
- Sync: there is a reference signal (timing pulse)
- bisync modems used in older SNA network, for example
- Async: self-timing:
- start "bit"
- data bits (8 bits w/ no parity, 7 with parity that
can be even or odd)
- stop "bits" (1, 1.5, 2)
- Due to async overhead on a per-byte basis, it is actually
slower than doing it synchronously.
1. Intro
- Electronic Industries Association (EIA) standard
> originally described standard for computer to modem connection
> used for sync & async connections
> has been extended to terminals, modems, printers, other computers
- RS-232 consists of up to 25 wires
> 2 are used for data tranmission
> the others are control lines
- the "standard" is not very standard ("RS" = "recommended standard", after all :)
> you will frequently refer to vendors wiring diagrams
> may be building cables if you are connecting
equipment from several different vendors
2. Important pin assignments (using DB25 connector)
(full list is in Table 8.1, page 111)
2 - transmit data
3 - receive data
4 - RTS (ready to send)
5 - CTS (clear to send)
6 - DSR (data set read)
7 - signal ground
8 - DCD (data carrier detect)
20 - DTR (data terminal ready)
- grounding
> pin 1 is frame ground
> if used, should make cable more resistant
to electrical fluctuations
> must be connected to the device frame
> pin 7 is signal ground
> this is the reference voltage from which the other
signals measure their voltage
- voltages
> a pin is "asserted" when a voltage greater than
+ or - 3 volts is present
> data lines
< -3 volts is a binary one
> +3 volts is a binary zero
> control lines
< -3 volts is off
> +3 volts is on
- DTE and DCE
> DTE - piece of equipment is a terminal device ("Data Terminal Equipment")
> DCE - piece of equipment is a computer device ("Data Communications Equipment")
> not always intuitively obvious
> to determine whether a device is DTE or DCE
check the voltage on pins 2 and 3
> the transmitter should always have a negative ("one")
voltage even when idle
> pin 2 negative - DTE
> pin 3 negative - DCE
3. minimum configuration
- for some applications only 2, 3, and 7 are needed
- a console vt100 terminal is an example
4. carrier detection (DCD = Data Carrier Detect)
- DCD is needed only if you are not sure if the device
is connected (modem)
- if DCD is expected this is called hard carrier detection
> a console terminal can be counted on to stay connected
- if DCD is not expected this is called soft carrier
detection
> you may (rarely!) have to tell UNIX not to expect DCD
- "ttysoftcar -y tty..."
5. Flow control
- keeps one device from overflowing the other devices
buffers
- can a 9600 baud computer serial port outpace a local
vt100 terminal?
> not usually, so 2, 3, and 7 are still OK for console
terminal
- what about printers? modem? PPP or SLIP connections?
> yes
- the CTS and RTS signals are used to implement hardware
flow control
- without them you can implement software flow control
using control special characters on the data lines
(CTL-S and CTL-Q; aka XON and XOFF) -- potential problem:
using two data byte values for flow control
6. example session between a modem and computer
DEVICE SIGNAL meaning
-----------------------------------------------------
Computer DTR+ I want to call out, are you ready?
Modem DSR+ I'm turned on. Go ahead and dial.
Modem DCD+ I've got your party!
Computer RTS+ Can I send data now?
Modem CTS+ Sure, go ahead.
Computer TxD... Data sent out.
Modem ...RxD Data received.
Modem CTS- Hold on for a moment!
Modem CTS+ I'm OK again. Go ahead.
...
... repeat transmission and handshaking
...
Computer DTR- I'm done. Please hang up.
Modem DCD- Whatever you say.
7. null modem cables
- used to connect two DCE or two DTE devices together
w/out requiring two modems in between
- full null modem cable crosses:
2 and 3
4 and 5
8 and 20
(Exhibit B, page 111)
B. Types of connectors and other parts
DB-25
DIN-8
DB-9
RJ-45
C. Yost system
- use DB-25, RJ-45 converter
- make two types DTE and DCE
- permanently connect them to the devices
- use 8-wire ribbon cable to connect any serial devices!
(Details in Table 8.4, page 115)
D. /etc/ttytab - BSD, SunOS
upsilon:/etc/ttytab
#
# @(#)ttytab 1.6 89/12/18 SMI
#
# name getty type status comments
#
console "/usr/etc/getty std.9600" vt100 on local secure
ttya "/usr/etc/getty std.9600" unknown off local secure
ttyb "/usr/etc/getty std.9600" unknown off local secure
tty00 "/usr/etc/getty std.9600" unknown off local secure
1. name field
- device name
2. getty
- command to be executed (doesn't have to be getty - could be
a uucp process, for instance)
- /etc/gettytab file contains some common port configurations
to use as paramters to getty (std.9600, for example)
3. status field
- on and off: specify whether init should execute the command given
in the second field
- secure: in addition to on allows "root" to login on this line
- if the console is not marked "secure," the system prompts for the
root password before coming up in single-user mode
- local: in addition to on indicates the modem control signals for
this line, such as Carrier Detect, will be ignored (soft mode)
#
# Printcap file for upsilon (drives pclab)
#
pclab:lp=/dev/ttyb:sd=/usr/spool/pclab:br#9600:sb:sh:fs#06320:\
^^^^^^^^^ ^^^^^^^
:lf=/usr/adm/lpd-errs:mc#1:mx#50:sc:tr=\f:if=/usr/pclab/pf:
4. note that printer runs off of serial port b
> no change in ttytab (terminal control)
> baud rate and other serial configuration is specified in
/etc/printcap
E. /etc/inittab - SysV, Linux
1. inittab defines actions that occur at certain run levels
c1:12345:respawn:/sbin/agetty 38400 tty1
c2:12345:off:/sbin/agetty 38400 tty2
- init reads inittab
calls agetty for port tty1, agetty calls login
does not call agetty for port tty2
F. Terminal control
- a rich area
- look at "man stty"; esp. the term "canonical" (authoritative)
- "reset" (BSD), "stty sane" are useful; may need to use
CONTROL-J (linefeed) to actually terminate the command string
if the terminal is insane :)
G. Modems
- modem = "modulator de-modulator"
- another rich area; only touched on here
- baud (transitions per second) != bps (bits per second)
- Table 8.8 page page 128
- Missing: "V.fast" *vs* "V.34"; not equal!
- present/near future: 33.6K bps, ISDN (128K bps,
ADSL [Asymmetric Digital Subscriber Line] (2-6 Mbps), cable modems (2-10 Mbps)
Here's an excellent source of latest & greatest high speed gear!
H. Debugging a serial line
1. init not reset
TRY: "kill -1 1" after editing /etc/ttytab or /etc/inittab
2. soft carrier detection not enabled
TRY: ttysoftcar -a /dev/tty?
TRY: local flag in /etc/ttytab
3. wrong cable pin configuration
TRY: breakout box to check several wiring
TRY: finding cable configuration chart in your device
manual, it may be non-standard
4. bad cable
TRY: another cable
TRY: looking at pin length, is one shorter than the
others?
5. nothing appears to go out serial port
TRY: cat > /dev/ttya
6. wrong baud rate
SYMPTOM: garbage characters emanating from connected device