I built a current-loop to USB interface from scratch, powered by a Teensy microcontroller. It's a hobbyist build, not intended to be commercially replicable. A very cool way to learn about current-loop interfaces nevertheless.
Project notes and pictures: PDF and PowerPoint.
The key features are:
- 20mA current loop @ 110 baud, interface to USB Serial. The USB stuff is done with a Teensy microcontroller.
- Duplex operation, with separate transmit and receive loops.
- The loops are active (integrated current source), since the ASR33 is passive.
- Monitor loop activity with LEDs.
- Some character translation and processing is done in Teensy firmware.
Additional information:
- SPICE diagrams for the current-loop interface.
- Older pictures from a discarded first build.