Subject: Re: Null modem snafu?
To: Todd Vierling <email@example.com>
From: Phil Knaack <firstname.lastname@example.org>
Date: 08/13/1996 00:18:57
Todd Vierling <email@example.com> writes:
>In attempting to attach a 9600bps serial terminal with null modem to a
>serial port (16550 based com1), I get what appears to be a bunch of $FF
>chars. I can't find my serial analyzer, but that looks like the getty is
>constantly sending a BREAK to the line. I've fiddled with various null
>modem setups with no luck (in fact, a 1-1 7-7 2-3 3-2 cable causes getty to
>exit, restart, exit, restart so fast that init puts it on hold).
I'm confused; is this a 25-pin or 9-pin serial cable you're
referring to? pin 7 is ground on 25-pin, put pin 1 is unassigned. If you're
talking 9-pin, RTS does not connect to RTS.
(Serial port pins, DB9 or DB25 male)
9-pin 25-pin assignment
1 8 DCD (Data Carrier Detect) (input)
2 3 RX (Receive Data) (input)
3 2 TX (Transmit Data) (output)
4 20 DTR (Data Terminal Ready) (output)
5 7 GND (Signal Ground)
6 6 DSR (Data Set Ready) (input)
7 4 RTS (Request To Send) (output)
8 5 CTS (Clear To Send) (intput)
9 22 RI (Ring Indicator) (input)
You no doubt need to set some flags in /etc/ttys. For my PPP router
with serial console it looks like this:
tty00 "/usr/libexec/getty std.19200" vt100 on secure local softcar
(my bootblocks run over serial port at 19.2k instead of the 9600 baud
default, and CONSPEED=19200 in my kernel config, etc etc)
To make the BIOS and the BIOS-using-bootblocks happy, the computer's
DTR is connected to its own DCD, and its own RTS to its own CTS. Then
only pins 2,3,5 (9-pin connector) need to go to the terminal. Normally
you would want a (assuming both ends 9-pin) 2-3 3-2 5-5 1-4 4-1 7-8 8-7
cable. (RX <--> TX, GND <--> GND, DTR <--> DCD, RTS <--> CTS).
If you don't connect DCD, you need the "softcar" flag above. If you
don't care about DTR, add "local". If you want to use hardware flow
control, and have connected RTS <--> CTS, add "rtscts" above. ("rtscts"
is off by default.)
Hope this answers your question.
Phillip F Knaack
Database Programmer, Information Development for Extension Audiences (IDEA)
Iowa State University Extension