ASR-33 on RS232

As everybody knows, the ASR-33 Teletype  has a serial 20 mA current loop interface.

ASR 33 wiki Bearbeitet

That means:

  • yes, he transmits characters serially in the same style as RS232 . Baudrate, bitwidth, parity and stopbits are given by "110 7E2" .
  • yes, characters are encoded in ASCII. But you can only transmit or receive UPPERCASE chars.
  • no, its interface wires are not compatible with standard RS232 ports.
    And you have a pair of wires for each signal: TxD+, TxD-, RxD+, RxD-.


The ASR-33 also has a papertape puncher and a paper tape reader. Punching is controlled only manually: you press the transparent "ON" button, and everything printed is also punched.

Control of the Papertape Reader a bit different, three conditions are involved:

  1. it runs only when tape is inserted.
  2. in "LOCAL" mode it starts reading the tape when you set its mode Switch to "START".
  3. in "LINE" Mode still nothing happens, until the remote computer activates another signal: "READER ENABLE".
    This way the Reader is under remote program control.

"READER ENABLE" is also a 20mA-like signal, but not part of the serial interface. We have two more wires: "RDR ENB+" and "RDR ENB-".


now is a multifunction board, which connects ASR-33 and similar teletypes to the typical Retro-Nerd environment (neither the first nor the last project of this kind).

tty2pi case closed

  • First, "tty2pi" contains a 20mA - RS232 converter ... I copied the  "deramp" circuit.
  • RS232 signals are routed out on a DSUB-9 male connector.
  • Onboard is also a Raspberry Pi Zero W, with another RS232 interface.
  • The RPi can be used to forward serial data stream via USB or WLAN to other devices.
  • RPi can also host simulators like SimH, which run connected to ASR-33 then.
  • RPi Zero gets 4 regular USB ports with the USB HUB HAT. You connect desktop mouse/keyboard and a 3rd RS232 ports here.
  • "READER ENABLE" can be driven via DTR on the DSUB-9 male, and by an RPi GPIO.


tty2pi front


tty2pi back


Resources for this project are found at .