Wiring terminals 300 ft. from the computer

[Floyd Davidson]

In article <3927 at segue.segue.com> bruce at segue.segue.com (Bruce Adler) writes:
>In article <1990Sep25.052558.11002 at chinet.chi.il.us> floydd at chinet.chi.il.us (Floyd Davidson) writes:
>>Pray tell what does using twisted pair wire do on an unbalanced
>>circuit?  Nothing.  Twisted pair will do wonders for common mode
>>rejection on balance telephone lines, maybe on RS-422, but nothing
>>for RS-232.
>
>Not exactly nothing.  Try this experiment at home, hook up about 200-300 
>feet of twisted pair to the RD and TD pins of your favorite 9600 baud 
>terminal.  Leave the far end disconnected (this is the trick).  Now type 
>something on the keyboard.  The last time I tried this the unterminated 
>cable run magically echo-ed every character as if it was really attached 
>to a host port.  Can you say "inductive coupling"?  

Can you say "capacitive"?  If the line was terminated and some current
flowed or if it was so long (a bigger capacitor) that significant 
current flowed to charge it up, you might well get inductive
coupling.  But what you get with a couple hundred feet of unterminated
cable is the same thing you would get by just putting a capacitor
across the connector pins.  (Look up transmission line theory
for radio transmission and then realize that voice or even data
is the same thing at a lower frequency.)

Either way the last thing you want to use for RS-232 is twisted
pair.