14 May
1998
14 May
'98
5:26 a.m.
I used a TTY
ASR-33 as a printer for my PET years ago. My memory is
that if I left the receiver open circuit, i yammered away printing
nulls; if I shorted out the loop it didn't. I interfaced it using one
transistor - no power supply was necessary.
+----------------------
|
OUT b |/c
-------/\/\/\/---------| NPN TTY
|\e
PET |
-------------------------+----------------------
GND
For I've got a Kludge
And a good Kludge too ;-)
Seriously, that works. I've done it myself a few times. But it's not the
official way to drive a teletype - the external device should source the
loop current. And I prefer to teach people the right way and let them
find out the kludges for themselves. > Note: I never interfaced the TTY transmitter -
not even for reading
> paper tape (don't know why not)
IIRC, one of the problems I had was the high resistance of the transmit
loop - many kilohms, I think. It probably was mostly contact resistance
and highly non-linear i.e. apparent resistance would be less at higher
currents), but I wouldn't like to use only 5V. I'd suggest a reasonable
sized decoupling capacitor to get rid of contact noise, too - say, up to
1 microfarad.
Err, because the ASR33 reader is horrible and mangles
tapes? You have a
Trend, don't you?
This was at least 4 years before I ever _saw_ any other paper tape
stuff. Long before I got the Trend. Or even my little DSI box. (DSI =
Data Specialties Inc. They made a 30cps reader/punch unit that was sold
in the UK by a small company called Teleprinter Equipment Ltd. Never
met either of them anywhere else.)
Philip.