ASR33

From: Philip.Belben_at_powertech.co.uk <(Philip.Belben_at_powertech.co.uk)>
Date: Thu May 14 12:26:13 1998

>> 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.

Oh, yes. Absolutely. But I was disagreeing with your assertion that
while 5V might do for the transmit loop, you'd "need" 12V for the
receive loop.
 
>> 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.
Received on Thu May 14 1998 - 12:26:13 BST