Preventing serial cable interference

[Charles Hedrick]

The referenced article recommends a terminator that you can plug in
when a line is open.  That is a clean solution, but it's inconvenient,
because users have to remember to plug in the terminator.  Some kinds
of terminals because unterminated when they are turned off.  You may
be able to get around this by installing terminating resistors
permanently, probably in the machine room.

We saw similar problems with open lines on a DEC-20 about 10 years
ago.  DEC field service whipped up an ECO for us using parts from
Radio Shack.  (It was thereafter known as "the Radio Shack ECO", and
was dutifully installed by field service in all of our newer DEC-20's
as they arrived.)  I didn't look at the details, but I believe they
did pretty much what was described here, except they used larger value
resistors.  They found a value that provided enough termination to
prevent open lines from ringing, but didn't interfere with normal
communications when something was plugged in.  I don't know what value
they used.  Presumably somewhere between 10K and 100K.  You might
experiment.