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Path: utzoo!mnetor!seismo!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: net.dcom
Subject: Re: unasked-for autobauding -- answers
Message-ID: <1352@kitty.UUCP>
Date: Fri, 17-Oct-86 10:31:03 EDT
Article-I.D.: kitty.1352
Posted: Fri Oct 17 10:31:03 1986
Date-Received: Sat, 18-Oct-86 03:10:57 EDT
References: <7208@utzoo.UUCP>, <152@dlb.UUCP> <7229@utzoo.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 31
Keywords: autobaud
Summary: Crosstalk on long data cables

In article <7229@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes:
> ...
> Martin Minow suggested that badly-balanced lines can echo to the host to
> some degree; apparently DEC has patented the use of this phenomenon for
> testing some types of equipment!
> ...

	Almost everybody - including myself - exceeds the RS-232C distance
specification of 50 feet (actually, the spec says 2500 pF of capacitance,
which usually equates to 50 feet of cable).  Exceeding this specification
can be an invitation to spurious character echo, especially if "homemade"
cables are incorrectly wired.  The most common wiring error that I have
seen involves the use of multi-conductor cable having wire in pairs; some
people think that TD (BA) and RD (BB) should be in the same pair - this is
the WORST thing that anyone can do, since these leads are hardly a balanced
pair!  If you're going to use twisted pair cable, pair TD and RD each with
ground if there are enough conductors; if not, at least pair them with some
lead that is "static" in state - like DSR (CC) or DTR (CD).  If you're really
determined to "push" a RS-232C line to a long length, the best way is to use
multi-conductor cable having individually shielded (i.e., coaxial) wires.
	An even worse echo problem can result through the use of RS-422 as
a limited-distance modem.  RS-422 lines should have a termination resistor.
Not all RS-422 interfaces have this resistor, because they assume that the
_other_ end will have the termination resistor.  So, if you connect two
RS-422 devices together, and neither one provides the termination resistor -
chances are there will be trouble over any significant cable length.

==>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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