ASR-33 on RS232
As everybody knows, the ASR-33 Teletype has a serial 20 mA current loop interface.
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-.
Papertape READER ENABLE
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:
- it runs only when tape is inserted.
- in "LOCAL" mode it starts reading the tape when you set its mode Switch to "START".
- 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-".
TTY2PI
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).
- 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.
Downloads
Resources for this project are found at files.retrocmp.com/tty2pi .