From Retrocomputing at 45 Baud

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Flickr Error ( ): PhotoID 8674190298 There are two complications with interfacing a teletype to a modern computer. The first is the slow 45.5 bit baud rate and the non-ASCII 5-bit Baudot code. The second is that it is necessary to step up the voltage to around 100V to overcome the (approximately) 4 Henries of the selector magnet, while supplying 60mA to the current loop.

The slow baud rate and format conversion is easy to do with a microcontroller like the Teensy, which can act as a normal USB serial device to allow operation with no special drivers on the host computer. It has a lookup table for the supported characters and maintains a state of which figure/letter rail is currently selected.

The roughly 5W power requirements of the coil are just within the voltage and current range allowed from a USB port as well. This design uses a boost converter to step up the 5V to approximately 100 V and is based on the Adafruit DC/DC boost calculator. The Teensy generates the PWM signal to drive the charge pump, but doesn't use any feedback to keep the circuit and software simple.

Flickr Error ( ): PhotoID 8846669076

The prototype in the top image uses a random inductor that is "big", but my second version uses a RLB9012-101KL 100mH high current coil. The IRF740 are higher current than necessary; IRF710 are suitable replacements.

The source code for the teletype interface board:

Flickr Error ( ): PhotoID 9767134104 Support for KSR models is now in functional with a simple 5V current loop. The 5-bit Baudot code and current input mode (Letters/Figures) are translated into ASCII. See it in action!