So many modern microcontroller systems use serial communication both for programming, like the Arduino, and also for user input.In the past, serial connections were done with a current loop instead of voltage sensing.So if you want to control a current-loop device with modern tech, this ASR33 Teletype controller in combination with external relays will work great! Modern serial traces its legacy all the way back to the Murray-Baudot code and Depression-era teletypes.
Today it uses either TTL-level signalling or RS-232 signalling (5V logic and ±12 to 18V respectively).However, before RS-232, serial data was sent through a current loop.A constant 20mA (or, on even older systems, 60mA) flows through both ends of the link, and interrupting this current is how each bit is sent.
It was set up this way because of the electromechanical nature of teletypes.The current in the loop would directly drive coils to move selectors into position.CuriousMarc’s YouTube channel has an excellent series of videos showing both the full restoration of a WW2-era teletype as well as showing how current loops work to send data.
This Teletype controller has some neat features built in, including a copy of Colossal Cave Adventure, one of the very first text adventures ever created! It runs at the correct 110 baud for the ASR33 and can be connected to over WiFi for remote printing.You could use it to display messages sent to an MQTT topic, for example.For a video showing the ASR33 which this device was specifically built for, check out the embedded video below! Like this:Like Loading...
