20ma current loop to RS-232 conversion

20ma current loop <> RS-232 conversion

Vintage PCB

Dig this crazy circuit board man! Still in it's original packaging from Hobby World Electronics in Northridge California.

Additional Resources

• Helpful article, Teletype 101 from vintagecomputer.net
• Another circuit. From [Altairclone] Specifically designed to fit in a Teletype model 33 using only a 12vdc wall wart for power.
• Another circuit I found referenced on the greenkeys email list

My ASR hookup

I built the circuit described above from the altairclone site. I found the Teletype 101 article helpful for identifying the proper terminal strip locations. The interface works. I set baud to 110, with 7 bits and 'mark' parity in TeraTerm on a windows 8 notebook with a Gigaware USB<> serial convereter.

Schematic 1.1

From the original schematic at altairclone I added ASR terminal strip location numbers.

Wiring legend

                               screw        wire              schematic
Signal                        terminal      color             connection
(Teletype 101 nomenclature)   (on ASR)      (mine)          (altair clone nomenclature)  
---------------------------------------------------------------------
Transmitted data send (+)        7          orange          Data to Teletype +
Transmitted data return (-)      6          brown           Data to Teletype -

Received data send (+)           4          red             Data from Teletype +
Received data return (-)         3          black           Data from Teletype -

Image details