SWTPC 6800

Mouse mouse at Rodents-Montreal.ORG
Fri Aug 19 07:39:04 CDT 2016

> If you have two serial devices on the same line and one is just
> listening while you work with the other, *can* that work, or would it
> just confuse things?

It depends on what you mean by "the same line".

For ease of language here, I'm going to assume that the devices are a
computer, C, and two terminals, T1 and T2.

If you connect all the pins, it will work fine for the signals that T1
and T2 are driving to the same state.  Signals driven to opposite
states may register as being in one of the two states or they may fall
into the undefined intermediate zone (between -3V and +3V, IIRC),
depending on the voltages T1 and T2 are trying to drive them to and the
exact impedances of the drivers.  (It shouldn't fry anything, though;
one really nice feature of RS-232C is that the spec requires that any
pin or combination of pins can be shorted together and/or to any
voltage source within the allowed range (-25V to +25V, IIRC)
indefinitely without damage.  I'm not sure this applies to ground pins,
though; it certainly doesn't in practice - I've seen ground loops.)

However, the terminal-driven data line (the one that T1 and T2 use to
send to C) is one of those signals.  I would suggest using a breakout
box, or two connectors wired by hand with that signal omitted, to
isolate C from one of the two terminals on that pin.  (I would actually
go as far as to connect only two pins, signal ground and C-to-T data,
to one of the two terminals.)  It will mean you can't type on both T1
and T2 (or, rather, typing will be ignored on one of them); if you want
that to work, you will need at least a few active components between
them - two diodes and a pullup resistor strikes me as the bare minimum,
and even then you may have to play with the resistor value to get the
voltages within the correct ranges.

Another nice feature of RS-232C is that it is electrically very simple.
You can throw together serial-line stuff with alligator clip leads and
discrete components like diodes and resistors.  You don't have to worry
about things like modulation schemes and lower-level protocols, the way
you do with things like USB or Ethernet.

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