Preventing serial cable interference

[Terry Ingoldsby]

In article <33 at octel.UUCP>, mike at octel.UUCP (Michael D. Crawford) writes:
> We have a great deal of trouble here with our Sun serial ports either hanging up
> (meaning rebooting fixes them) or being blown (meaning we need to replace the
> board).
> 
> Sun Service says it is from our long serial cables being left unplugged at the
> far end, or being plugged into powered-off equipment, which causes the 
...
> What I would like to know is if there is some kind of isolator I could make
> or buy that would overcome this problem, at least some kind of terminator
> I could plug in the other end of the cables when they are not being used.

Make up a connector with terminating resistors that could be plugged into
the end of the cable that is dangling free.  The connector should consist of
all active lines connected via separate resistors to pin 7 (signal ground).
I would try 1K resistors for starters.  This should provide a low enough
impedance to keep any inputs from picking up noise, and still be high
enough resistance so that any outputs won't drive excessive current.  I
think most RS232 outputs are protected anyway, but better safe than sorry.




-- 
  Terry Ingoldsby                       ctycal!ingoldsb at calgary.UUCP
  Land Information Systems                           or
  The City of Calgary         ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb