That's it Tony, and yes... I'm serious. They connected the ASR-33 across an
inverter and shorted it out when the relay transmits. The R107 is RTL (with
a real transistor) so it can't damage it.
I made the change and...... the PDP 8/s is now running DECUS tic tac
toe!!!!!!!!!!!!!!!!!!!!!!
I got the docs finally on the PT08 and found a -3V reference supply was in
the wrong slot... That would account for the problem I had initially.
Otherwise the entire system is now happy.
BTW: I also picked up the ASR 33 schematics and the "Teletype Service
Manual - ASR 33" as well.
Thanks for the help.
john
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Saturday, October 16, 1999 2:33 PM
Subject: Re: Urgent Current Loop Question - ASR 33 / PT08
>>
>> Before I attempt to figure this one out I would like to know if anyone
else
>> can help me.
>
>I know the ASR33 pretty well, but I've never seen an 8/s. But maybe I can
>help. I'm going to assume you have the sort of ASR33 that's normally used
>with computers -- that's to say one with a current loop interface.
>There's s 3 position knob on the front of the machine (line/off/local)
>and no other controls on the 'call control unit' (electronics module).
>
>>
>> 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..
>
>Are you sure this is right. It sounds crazy to me (and now I'll get a
>pile of replies pointing out how DEC were being clever here)...
>
>The ASR33 transmit loop is electrically a switch. Actually it's a complex
>array of switches. but in the end, it either shorts the 2 Tx terminals or
>opens them. There is no voltage on either terminal (in a 'stock ASR33')
>from the ASR33 electronics. There are no other components involved.
>
>Connecting a switch between the input and output of an inverter is not
>normally a good way to get a clean logic signal ;-)
>
>The normal circuit (used in all sorts of machines) is to connect one of
>the tty Tx wires to one supply rail, possibly through a small resistor
>to limit the current in the event of a short somewhere). The other tty Tx
>wire to the other supply rail through a (larger) resistor. And then to
>monitor the voltage at the 'top' of that resistor, say by connecting it
>to the input of a logic gate.
>
>>
>> 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???
>
>Well, I could understand connecting the tty Tx loop between the input of
>one inverter and the output of another one, especially if the input of
>the latter was tied to a known logic level (so the output is just a
>constant voltage source, etc). This makes an assumption about how
>floating inputs behave, which may be OK on transistorised logic like this.
>
>You are _sure_ that you've connected to the right pins on the R107
>module, and that in fact that's the right module to be using?
>
>> #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????
>
>Sure. Just make sure the TTY contacts don't have to carry high currents
>(like don't connect the loop across the PSU) unless you like repairing
>burnt/melted switches...
>
>>
>> I just want to make sure the ASR 33 does not have any active part that
will
>> eat my PT08 for breakfast.
>
>Not that I know of...
>
>-tony
>
Received on Sun Oct 17 1999 - 08:26:50 BST