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From: robertb@cs.washington.edu (Robert Bedichek)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Serial-port connections
Message-ID: <12773@june.cs.washington.edu>
Date: 13 Aug 90 00:24:45 GMT
References: <54123@iuvax.cs.indiana.edu>
Reply-To: robertb@june.cs.washington.edu (Robert Bedichek)
Organization: University of Washington, Computer Science, Seattle
Lines: 32

In article <54123@iuvax.cs.indiana.edu> bobmon@iuvax.cs.indiana.edu (RAMontante) writes:
>"How long is a serial port?" Or, more accurately...my cousin has a dumb
>device with a serial port (connected to some relays), and he'd like to
>control the device remotely from his P.C.  My question is, how far can
>the remote device be from the the P.C., and how should it be connected?
>Would a "null modem" cable be reliable over a 200-foot span?  If not,
>what's the cheapest connection?  We thought of two modems and some phone
>wire; would the phone wire need to be powered then?  (300-baud would be
>good enough; it's not a high-speed device out there.)
>
>Thanks in advance for any suggestions.

The RS-232 spec says the wires can't be longer than 50 feet and the
baud can't be greater than 9600.  Lots of people exceed these numbers
without problem.  You can probably go 200 feet w/o a problem.

Try it and see.  If you are still concerned, check the signal quality
with a scope at both ends.  If the peak-to-peak is over 10V, its fine.
If it is less, it might not be fine.

The chips used to drive RS-232 vary, some are capable of more drive
than others.

Has anyone tried connecting two modems w/o a telephone circuit?  The
modems might not work w/o the telco's line impedance.  I don't know.

You can reliably go thousands of feet with a current loop interface.
It is easy to go from RS-232 to current loop if you can wire up an
optoisolator, a transistor or two, and the connectors and power
supply.

	Rob