Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ucsd!rutgers!att!lzaz!mtunb!dmt
From: dmt@mtunb.ATT.COM (Dave Tutelman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Maximum Length of a PC Serial Cable?
Summary: Yes, RS232 does exist!
Message-ID: <1265@mtunb.ATT.COM>
Date: 19 Jul 88 12:47:45 GMT
References: <1986@silver.bacs.indiana.edu> <1804@akgua.ATT.COM>
Reply-To: dmt@mtunb.UUCP (Dave Tutelman)
Organization: AT&T Information Systems - Lincroft, NJ
Lines: 50

I guess I'm getting curmudgeonly in my old age, but there seems
a lot of guessing posted on the net in the name of being helpful.
(Yes, I do it myself sometimes, when constrained by home/office
spacial displacement  :-)
This discussion got to me because EVERYBODY knows the reference where
the answer can be found but NOBODY looks it up, preferring to post
admittedly vague recollections.

Here's what the standard actually says:

RS232 -	Speed to 20 kb/s, length to 15 m (50 ft).
	Section 3.1 says "longer cables are permissible, provided that
	the resulting load capacitance ... does not exceed 2500 pF."
	Usually you can get away with it, but it ain't guaranteed.
	The foreword to RS232-C (1981) points to RS449 for higher speeds.
	Since 449 can interwork with 232, and since some serial ports
	have upgraded their drivers, it's worth looking at RS449.

RS449 -	supports 2 physical interfaces:
		RS422 for balanced circuits.
		RS423 for unbalanced circuits.
	It is possible to interwork RS423 and RS232 interfaces,
	provided the proper lead transposition.  The actual speeds
	and distances in the spec for unbalanced operation are:
	Speed to 20 kb/s, length to 60 m (200 ft).
	However, explanatory note 6.10 says that, with proper wave shaping,
	you can get to 60 m at a speed of 60 kb/s.

RS423 -	This standard goes beyond the detail in RS 449, giving
	curves of distance vs. speed for various wave shaping.

This is probably more than you really wanted to know, and in some sense
less; you wanted to know about YOUR serial interface.  Here's how:
   -	Look at the driver and terminator chips on the leads.  Get the
	part numbers, and look them up in the catalog.
   -	If the catalog says they are compatible with RS449 or RS 423,
	you can get 200 feet safely.  If not, you can get 50 feet safely.
   -	In either event, you can exceed these distances if you use
	low-capacitance cable and connectors, or are generally not
	too unlucky in your environment.
   -	If you try the above, and get flaky operation, well the
	standard didn't guarantee it.  HOWEVER, lower speed and
	longer distance sometimes work.  (See RS423 if you want
	the exact details.)

+---------------------------------------------------------------+
|    Dave Tutelman						|
|    Physical - AT&T  -  Lincroft, NJ				|
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