SWTPC 6800

Dave Wade dave.g4ugm at gmail.com
Fri Aug 19 07:47:18 CDT 2016

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Mouse
> Sent: 19 August 2016 13:39
> To: cctalk at classiccmp.org
> Subject: Re: SWTPC 6800
> > 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
> 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
> is that the spec requires that any pin or combination of pins can be
> 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
> 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
> connectors wired by hand with that signal omitted, to isolate C from one
> the two terminals on that pin.  (I would actually go as far as to connect
> two pins, signal ground and C-to-T data, to one of the two terminals.)  It
> mean you can't type on both T1 and T2 (or, rather, typing will be ignored
> one of them); if you want that to work, you will need at least a few
> 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
> components like diodes and resistors.  You don't have to worry about
> like modulation schemes and lower-level protocols, the way you do with
> things like USB or Ethernet.

If you feel really lazy you can get serial switches in both 9 and 25-pin
varieties for almost less than the parts...



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