padpowell@wateng.UUCP (PAD Powell [Admin]) (11/07/83)
Having read the latest episodes on RS232, I have the following comments
about designers of RS232 interfaces.
1. I find it amusing that the people who have a $250K computer will cheat on
a $500 line interface. Sigh.
2. If you are really paranoid, having had your computer fried, you just might
want to do the following little thing.
1. Get hold of a designer, a bunch of RS232 interface chips,
a slog of dual optical isolators. You get 4 drivers/chip, 4 receivers
/chip., two isolators per chip. You will also need a 150ma power supply
WITH an isolated ground. This is standard.
2. Get your designer to build an isolator card:
RS232 -> optical -> your line card.
You may have to pull +5 V from the computer for pull ups,
or you may want to invest in another supply. Most supplies
have a +12V supply, and may have a separate ground for it,
so that is the easiest way to get power for the isolator
pull ups.
3. Now, consider the types of faults: floating ground, power Zaps.
The isolator board will usually catch floating grounds,
as long as you have run its ground to a GOOD earth
This is not as easy as you might suspect, as water pipes
are no longer "metal" everywhere. Sigh. Make DOUBLE
SURE that the ground (pin 1) signals runs to this earth
ground with heavy (#20) wire, and not silly wire wrap wire.
4. If you have some equipment that floats power and "common"
(pin 7), I would strongly recommend a 250 ohm resistor
in series with the connection from (pin 7) to the
power common. This will quickly help in finding those
guilty-of-incestuous-rape ground currents.}i
5. Now, you have to worry about Power Zaps. First, you can
forget about the line card, optical isolator, power supplies
should protect against almost all things except lightning.
I have found by a series of pragmatic tests that it
will even protect against static to a very large extent.
If you don't care about arcing the other equipment on
the card, read no further. If, however, you want to
make really sure, you have to provide individual isolation
supplies for each line. Costly? Yes and no. First,
buy a switching supply, and gut it, to provide a 10kHz
square output. Now go and get a slog of little pot cores,
look up your old magnetics, and start winding. A little
perusal of parts catalogs will locate some nice little
regulators.
6. Worry about location. DO NOT put the isolator stuff in the
same box/chassis as the Processor, unless there is no
alternative. If you have a buried floor, hide it there.
Put it under the breakout box for the cables.
Patrick ("We wired a Tire Plant, and this worked fantastically well")
Powell