RS-232 Interface for the Teletype ASR-33

The Teletype ASR-33 uses 20 ma current-loop for serial communication instead of RS-232. In order to use the Teletype with the Altair Clone, or any other computer not configured for 20 ma current-loop, an RS-232 to current-loop converter is required.

The easiest way to interface a Teletype to an RS-232 serial port is with this Plug and Play Teletype to RS-232 Adapter.

If you'd rather build something yourself, following is an easy to build RS-232 to current-loop interface for the Teletype. This circuit is particular to the Teletype ASR-33 current-loop interface and is not recommended as a general purpose RS-232 to current-loop converter. The circuit for this converter is very similar to the current-loop interface used by MITS on their serial I/O boards.

Here is a link to the schematic for the converter.

While the converter can be used externally to the Teletype, this converter can easily be integrated directly into the Teletype. The picture on the left shows the converter circuit board mounted in the front half of the UCC. Power for the converter is supplied by a 12 volt A/C adapter plugged into the convenience outlet built into the UCC. The Teletype cover has sufficient clearance to fit over the A/C adapter.

The picture on the right shows the two cables that connect to the interface board. One cable houses the four current-loop wires that connect to screw terminals at the back of the UCC. The second cable is a 10 foot long RS-232 cable that goes through a strain relief at the back of the Teletype and then terminates into a male DB-25 connector for easy connection to a computer.

RS232-1 RS232-3

Close-up of the interface board