Opto-Isolated RS232 Interface

This design is for a simple half-duplex optically isolated interface that converts a 20 mA current loop (connected to J2) into an RS232 signal (on J1) which can be monitored by, say, a laptop PC. In the author’s case, the system operates at 1200 baud. The signal to be monitored should be a fully digital on-off communications signal, rather than the usual 4/20 mA (industrial) analogue transmission standard.