Urgent Current Loop Question - ASR 33 / PT08

From: daniel <daniel_at_internet.look.ca>
Date: Fri Oct 15 23:36:42 1999

Before I attempt to figure this one out I would like to know if anyone else
can help me.

I hooked up an ASR-33 (non DEC) to my PDP-8/s. I ran a maindec program and
the PDP could talk to the teletype no problem, never an error.

After spending 4 1/2 hours cleaning up bad solder joints on the receiver I
got it up and running.... *kind of*

The problem is this: The ASR-33 is interfaced to the PDP currently by tying
one receive line to the input of a R107 (transistor inverter) and the other
receiver line to the output of the same inverter. Then, the signal is
*conditioned* (inverted) a few times before it is passed onto shift
registers,,, etc..

If I hook up the teletype the WAY the circuit exists now I get a 0-1V data
signal (notice positive) out of the first inverter which of course is
totally incompatible with the others and the signal never passes through
beyond the first inverter.

If I disconnect the FIRST inverter from the second one and LEAVE the one
receive line in the input of the first inverter ("floating") and hook up the
SECOND receive line to the input of the now *disconnected* second inverter I
get data. Most of the time good, but some bad characters get through.

This is obviously not a working current loop solution.

If the teletype is passive should I just connect the receive line on the
input of the first inverter to ground or -3V???

Picture below:
----------------------------------------------------------------------------
-------------------------


The way it was:

Receive #1 -----------|>o----------(Receive #2)----------|>o---------|>o---
etc..

The SIGNAL at receive #2 was 0 -> +1 Volt (data)


The way it is now:

Receive #1-------|>o--- [broken] Receive
#2 -------|>o-------|>o------|>o----------

Clean looking data but flaky (prob due to the other *floating line*.


#1) As I don't have docs nearby on the ASR-33 and have never had a VERY
simple interface like this CAN I short the one receive line to ground or -3
volts????

I just want to make sure the ASR 33 does not have any active part that will
eat my PT08 for breakfast.

john
Received on Fri Oct 15 1999 - 23:36:42 BST