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

[Michael]

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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