[GreenKeys] ASR-33 20 mA output converted to RS-232

[Federico BERGENTI]

Hi Mike,


my short answer to your question is: most Model 33's have passive transmit and receive loops (note: most, not all).


I have a Model 33 with both passive loops that looks very similar to the one shown in the page that links to the schematics for the converter you downloaded (http://altairclone.com/teletype.htm).


However, I also have a second Model 33 which has both active loops. The teletype provides 30V to drive the loops, and the call control unit (the part at the right side of the teletype which includes the power supply) is completely different.

So, I would first measure the voltages at open loops (rather than currents). In the unlikely case that you find 24/60V, than your teletype has active loops. On the contrary, I would conclude that it has passive loops.

I successfully obtained RS-232 signals from both my teletypes using a  customizable converter very similar to this one:

https://commons.wikimedia.org/wiki/File:Rscl.jpg#/media/File:Rscl.jpg

You can probably find it on eBay for $10-20.

Hope it helps.

Cheers
Federico

________________________________
From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net> on behalf of Michael via GreenKeys <greenkeys at mailman.qth.net>
Sent: Saturday, August 25, 2018 10:30 PM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] ASR-33 20 mA output converted to RS-232

I have an ASR-33 Teletype. I want to convert the 20 mA Transmit and Receive loops to RS-232 (so I can interface to the iTelex extra serial port on the TW39 board.) I do not have a copy of the circuitry for the model 33 and I have some questions about whether both the loops are self powered from the Model 33. When I close the Receive Data loop with an Ammeter, the 33 stops cycling and I read 11 mA n on the meter, so I think that loop is self powered, although it should be 20 mA. The Transmit Data loop - I cannot tell if it is self powered or not. You can buy a B&B converter for $80, but I also found a simple circuit on the internet posted by someone that built this interface for their model 33 to an Altair computer a long time ago. It is very simple, only a few transistors, resistors and small 12 VDC power supply that goes right inside the model 33 case. I have attached the schematic. It looks like it would work - and cost about $10 and also be fun to build!!!!  Any advice or suggestions would be greatly appreciated.

Mike from NJ

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