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ASR33 - Starting the tape reade in line mode

spiceminer

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Hi everybody!

I would like to use the tape reader of my Teleprint 390 (=ASR 33) Teletype.

It works fine in local mode. But when I am in line mode, the reader will not start by shifting the "start" lever forward.

Anybody familiar with that behaviour?

Regards
Stephan
 

durgadas311

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As I recall, the tape reader switch had 3 positions - stop, auto, start. I think Stop and Auto were static positions, while start was momentary (spring loaded return to "auto"). In auto mode, the teletype would respond to Ctrl-S and Ctrl-Q in order to stop and start (respectively) the tape. (this may have been the origins of the XON/XOFF protocol used to do software flow control on serial ports - not sure which came first).

If Ctrl-Q (XON) does not start the tape with the lever positioned on "auto", and manually pushing the lever to "start" and releasing does not start the tape, it sounds like there is some basic failure of the tape read mechanism. But again, I'm getting out of my depth. Is there any sort of mechanical activity when you push to "start"? Or is it as if the mechanism is disconnected? You say that it works in local mode, which is odd because the mechanics are the same. I wondered if there was some hardware handshake going on with your unit, whereby the computer needs to raise CTS or some such thing in order to allow the tape to run. I'm not sure if anyone ever rigged up a TTY to do that, I never saw it but really did not work with many situations. Are you able to communicate from the keyboard to your computer, and computer to print/punch unit?
 

spiceminer

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As I recall, the tape reader switch had 3 positions - stop, auto, start.

Thanks very much for your comment. I tried it out again with no success....

My machine has only "START", "STOP" and "FREE" positions - no idea what "free" means.
My switch will rest in all three positions.

In none of the three switch settings, the reader will start on CTRL-S or -Q issued from the terminal which I have connected to the machine (in line mode).

I have hooked the machine up to an MS DOS PC using a 20mA current loop interface. I can print and punch, and read the tapes back in local mode. I can see the characters on the PC-Terminal (KERMIT) when I press keys on the ASR 33 keyboard.

There are two pin at the data cable from the machine where I do not know iths purpose. They lead to a solenoid of a reed contact relais... I have trouble to understand its function. I do not have a schematic of the "Teleprint" electronics boards unfortunately which are not the same as in the original Teletype 33.

I am sure it works as the previous owner told me the reader was used to read programs to a SIEMENS 300 computer (back in the 70s)

Regards,
Stephan
 

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durgadas311

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"free" means that the tape feed sprocket is "free", and the tape can be manually pulled back/forward to position it.

The looks like a standard KSR/ASR-33 frame, re-packaged for another purpose. Can't tell for sure whether it is KSR (no XON/XOFF) or ASR.

Let me ask a basic question. When in "line" mode and you push the tape reader lever to "start", is there a position slightly past "start" that is spring-loaded? (picture seems to indicate no space for that) Typically, the "start" function is a momentary switch, but there were many models and I don't remember all the ones I worked with.

Also, when you say the reader doesn't work, is it that the characters on the tape do not print? or that the tape doesn't advance at all? When in "line" mode, characters you type/read will only appear on the paper if the remote machine (computer) "echoes" them back. In "local" mode, the tty output is looped back to input, so everything you type/read is implicitly echoed.
 

spiceminer

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"free" means that the tape feed sprocket is "free", and the tape can be manually pulled back/forward to position it.

Yes you are right about the "free" position, great, thanks for the information :) !

I have no idea if the machine is ASR or KSR... I have no documentation, but it looks like this machine here http://www.computinghistory.org.uk/news/16769/Military-Computers-Preserved/

There is no position past the start switch.

In "LINE MODE", the reader does not advance the paper strip. When the reader is in start position with a paper tape loaded, it starts immediately reading and printing when I turn the mode switch from "LINE" to "LOCAL".
 

durgadas311

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That is starting to sound like some sort of remote control to prevent the tape reader from going off before the other end was ready. Maybe that relay you saw is connected to enabling/starting the tape reader. You'd either have to experiment with it or remove all the covers and examine the reader mechanism. However, I'm not sure where the electrical-to-mechanical link might be - it could be nowhere near the reader itself (as I recall, there are linkage arms all over the place there, extending across the keyboard and even back into the print unit).
 

antiquekid3

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Sounds like it might have a reader-run circuit installed. This was somewhat common for DEC machines. You'll probably find a card with a relay on it if so.
 

NeXT

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My teletype machine has the Automatic Reader and thus four switch positions in this order:

-Manual start
-Auto
-Manual Stop
-Free

You already know what "Free" does and "Manual Start" and "Manual Stop" are momentary and return to "Auto" when released.

The Automatic Reader Run mechanics are two momentary switches inside the teletype.
 

chris-uk

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I believe a KSR33 is like an ASR33, but `keyboard only`, IE: No tape reader/punch.
and a Data Dynamics 390 with tape reader/punch is based on an ASR33

I have a 390 printer manual (DDTM 025 Issue 3, 1971), which shows that start=auto and so for online reading the lever should be set to `start`
and Ctrl+Q (DC1) should be sent from the host to start the reader.

It is stopped by sending Ctrl+S (DC3) or moving the lever away from `start`

Not sure if this is sensible, but it may be worth checking the parity - it should be set up for `even`, but I think it can be set up for `odd`

If not something like that, then perhaps some physical fault.
I am part way through scanning the manual, which is over one inch thick, but won't have time to finish for a few weeks.
 

durgadas311

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I had completely forgotten about parity. Yes, that could prevent XON from working (if the teletype has that feature enabled). However, I'm not sure if even parity is the answer. Ctrl-Q/XON/DC1 with even parity is the same as "no parity" (8n1). But, if your TTY operates with odd parity, that would matter. I just don't recall received parity mattering, though, at least on the devices I used to use.

If the odd relay is a "reader run" circuit, then that may be how you have to start the reader. You said there were two wires on the interface that connected to it? What you'd have to determine is whether you short the two wires, apply voltage, or something else. Perhaps some circuit analysis first would be wise.
 

m_thompson

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DEC commonly added a small circuit board holding a relay to the ASR33 so they could remotely control the reader. There was an additional 20mA circuit from the computer's serial port that would turn off the reader until the character had been read from the buffer.
 
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