20ma current loop to RS-232 conversion

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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.

TTY RS-232 Front.png TTY RS-232 Back.png

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.

RS-232 to ASR-33 Teletype Interface Schematic v1.1.jpg

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

My Altairclone TTY-RS232 Interface.png Teletype Model 33 ASR terminal block.png