Path: utzoo!attcan!uunet!husc6!rutgers!ucsd!ucbvax!unisoft!gethen!isaac
From: isaac@gethen.UUCP (Isaac Rabinovitch)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Maximum Length of a PC Serial Cable?
Summary: RS232?  Which RS232?
Message-ID: <1114@gethen.UUCP>
Date: 24 Jul 88 20:03:16 GMT
References: <1986@silver.bacs.indiana.edu>
Organization: There's Unix there in Oakland
Lines: 30

In article <1986@silver.bacs.indiana.edu>, regoli@silver.bacs.indiana.edu (michael regoli) writes:
> before i go crazy trying to do something that can't be done, does
> anyone know a safe limit on the length of a serial cable?  i'd like
> to run a serial cable approximately 30 feet @ 9600 bps to drive a
> laser.

Several others have given good practical answers, but I want to chime in
with a more general answer.  It's hard to answer questions like this
because (a) Not one of the RS232 interfaces completely complies with the
standard (usually for good reason); (b) Many RS232 uses are not for the
purpose originally intended (connection to a modem at less than 50');
and (c) there are a lot of hardware variations, though PCclones seem to
be almost standard these days.

At first glance, I would have said, Yes, of course, you're connecting a
device less than 50' from the host, even poor hardware should be able to
handle it.  But does "standard" hardware always go up to 9600 baud?
Can't remember.  As too often happens in PC Land, a simple, clear,
general question has no general answer.

I'd give odds of about 100 to 1 that the setup you describe will work,
especially if (as others suggested) you use shielded cable -- you should
probably use shielded cable anyway.  But if there *is* any doubt, I'd use
a dumb terminal or terminal emulator to test the interface before doing
something more sophisticated.

The odds are not nearly as good at longer distances and higher speed.
Most hardware makers seem to make RS232 that can easily go several
hundred fee and faster than 1 mega-bps -- but which ones?  I think the
only good general answer is to take it one small step at a time.