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.
Close-up of the interface board