Preventing serial cable interference

[George Robbins]

In article <33 at octel.UUCP> mike at octel.UUCP (Michael D. Crawford) writes:
> We have a great deal of trouble here with our Sun serial ports either hanging up
> (meaning rebooting fixes them) or being blown (meaning we need to replace the
> board).
> 
> What I would like to know is if there is some kind of isolator I could make
> or buy that would overcome this problem, at least some kind of terminator
> I could plug in the other end of the cables when they are not being used.
> 
> I would greatly appreciate helpful suggestions of any sort, as Sun does not
> seem to know anything better to tell me.

You can play various games with resistors to pull the idle/disconnected
lines further away from the threshold and to reduce the "zap" effect when
things are plugged/unplugged or switched.

The better solution, especially if your cables are real long, is to invest
in some "line drivers", which are in effect cheapo modems that convert the
RS232 signals into "analog" signals that can be tranmitted over two pairs
of wire.  Depending on distance you can reuse your serial cabling or just
just cheap telephone type twisted pair station cable.

An assortment of flavors/prices are available, either from the DP supply
catalog (Inmac, Black Box, etc) or modem manufacturers (Gandalf, etc).
Just make sure that the one you pick supports asynchronous terminals, and
if it is supposed to be "self powered", that the Sun's and terminals put
out appropriate voltages on whichever pins the the things draw power from.

Do test a pair to make sure their behaviour when the remote end is "powered
off" is one of appropriate silence and not random noise.  8-)

-- 
George Robbins - now working for,	uucp: {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr at uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)