19 Aug
2016
19 Aug
'16
5:47 a.m.
-----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
T2 are
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 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.
If you feel really lazy you can get serial switches in both 9 and 25-pin
varieties for almost less than the parts...
http://www.ebay.com/itm/131887801860
Dave
G4UGM
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