RS-232 pin 1, line noise

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Colin Kelley

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Nov 3, 1986, 10:39:03 PM11/3/86
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I recently set up an Epson printer with a serial interface to an ABCD switch
(i.e. a 1-of-4 switch) in order to share the printer between 4 PClones. The
Epson serial interface manual was very good, and I had no trouble getting
it to work with hardware handshaking, using only SG, RD, and DTR on the
printer end. However, I didn't (and still don't!) know what to do with pin 1
(Frame Ground, or Protective Ground).

I'm now finding that the longest cable (~50 feet) is having trouble with
garbage occasionally introduced by noise. I tried dropping the baud rate
from 9600 to 2400 (still faster than the printer) but that didn't help.
How can I remedy this? Should FGs of each device been tied together? Could
not doing this have lead to line noise?

Also, the cable is shielded, but I didn't do anything with the shielding.
Should it be grounded to the connectors, or one of the pins? On one or both
of the machines?

I do have maybe 10 feet of slack which I could cut of if shortening the cable
20% might help...(The other three cables are closer to 40 feet, and they all
work fine. However, the cable in question goes through 2 walls, and may be
passing some noise source that I'm not aware of.)

Thanks for any help!

-Colin Kelley ..{cbmvax,pyrnj,psuvax1}!vu-vsli!colin

John Plocher

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Nov 4, 1986, 5:33:14 PM11/4/86
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Colin Kelley cries loudly for help with RS-232 interfacing:

> However, I didn't (and still don't!) know what to do with pin 1
>(Frame Ground, or Protective Ground).

Frame Ground means just that - GROUND. Tie this to a real Earth Ground (a
cold water pipe, the ground pin of the nearest AC outlet...)

>I'm now finding that the longest cable (~50 feet) is having trouble with
>garbage occasionally introduced by noise.

I'm not suprised - fluoresent lights are REAL BAD in the noise dept...
do *you* have any of them near the cable?

>How can I remedy this? Should FGs of each device been tied together? Could

YES

>not doing this have lead to line noise?

YES

(I was there once, also. RS-232's a die, then you bitch...)
--
"Never trust an idea you get sitting down" - Nietzche
------------ {harvard,seismo}!uwvax!uwmacc!uwhsms!plocher (work)
John Plocher {harvard,seismo}!uwvax!puff!plocher (school)
------------ decvax!encore!vaxine!spark!121!0!John_Plocher (FidoNet)

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