Wiring terminals 300 ft. from the computer

[Jeff Liebermann]

rjw at atti07.ATT.COM (Ralph J. Winslow x7774) writes:

>-In <1990Sep19.083601.21364 at pilikia.pegasus.com> art at pilikia.pegasus.com
>	(Art Neilson) writes:
>->In <11774 at bsu-cs.bsu.edu> jennifer at bsu-cs.bsu.edu (Jennifer Freeman) writes:
>-> ... multi-user Unix Box running on a 386 or a 486.  One of the proposals is
>->to locate the computer about 300 feet away from where the bulk (10-14)
>->of the terminals will be.
>->What type of wiring would be needed for this?  Would shielded be ...

>That's true, but be careful to use twisted pairs if you use 4-wire.
>Actually, use twisted pair(s) whatever wire you use.

Wrong.  The purpose of twisted pair wire is to reduce coupling between
adjacent pairs and to reduce common mode pickup.  Since RS-232c is
a unipolar (unbalanced, common ground wire) scheme, neither of these
are applicable.  In fact, the twisted pair has a much higher capacitance
than unshielded twisted pair.  Adding a shield just increases
the capacitance (to ground).  The end effect is rise time deterioration.
The higher the frequency (baud rate), the worse the effect until no
signal makes it to the other end.

I have had no difficulties with 500ft runs of 4 conductor #26awg telephone
"station wire" at 19.2kb/s.  Yet I have several 30ft runs that barely
work.  The difference is that in the 30ft runs, the AC power grounds
were poor and about 10 volts (rms) of 60HZ crud appeared between the
computer and the terminal grounds.  Since the signal grounds between
the computer and terminals are all wired together (pin 7), the signal
appeared on top of the data.

The solution for this kind of grounding problem (found between buildings,
between floors, and creative electrical wiring) is to go balanced
(differential) drivers at one or both ends.  The idea is to eliminate
any ground loops.  There are "short haul modems" (which are sold
by Inmac, Black Box, Telebyte, and others for about $70/ea) which
convert unipolar RS-232 to balanced line and back.  Some are advertised
to go 6000 ft.  They require 4 wires per terminal.

Another solution is a cluster controller card.  I use Corollary products.
Their card will handle 4each of 8 port controllers to a max of 32 ports
per card.  Only 4 wires (balanced) go between the cpu and each cluster
controller.  I use these between floors and buildings.  One nice feature
is the ability to unplug a controller, watch the console complain, plug
it back, and continue without interruption.

-- 
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