When digital technology became available, the idea of improving the 4-20 mA signal became the goal for every major instrumentation supplier. Using newly available cheap microprocessors, manufacturers developed various techniques to include additional information to be encoded digitally into the 4-20 mA current signal loop. As far as I can remember, the three major contenders that attempted to do this were Foxboro, with its FoxCom protocol, Yokogawa with its Brain protocol and Rosemount with the HART (Highway Addressable Remote Transducer) protocol.