Peter Keller wrote:

> This is a control system with one pc an 4 workstations, DOS based, in
> 4 different rooms, one room for each person who is working on it.
> They cannot share the same room and they have different privileges.

  Well, if you prohibit simultaneity (wow!), you should have no
problem.  Just poll a port for four adjacent bits, mask it with a
"lockout" value and look for activity (lines go to "space").  Just set
mask for that line and use software UART on that line for the duration.

  When it either generates a "break" for too long (has been
disconnected) and as such is disabled in the "lockout" mask, or simply
becomes inactive for a time-out period, you go back to polling them all.

  Like a printer sharer.

  If all is idle for a longer time, check whether "lockouts" should
still be locked out.

  In fact, if you don't use regeneration (receive a character and re-
serialise it) but just straight feed-through, you could get by with a
12C508.  Four inputs, one output, pure "echo" logic.  Use a buffer on
the line in the other direction.

  May not work with PS/2 protocol as such, that could be more complex.
--
  Cheers,
        Paul B.