I am hoping to be able to take a serial protocol being sent over a 30V / 45mA current loop and translate the signal to send / receive from an Arduino's serial port.

My thoughts for reading the data into the Arduino are to use a TLP759 optocoupler and adapt the common MIDI IN circuit:



...since MIDI is also a current loop protocol. Obviously the values for the resistors would need to be different? I'm not certain the TLP759 is the right fit - I'm just going on the fact that it seems 30V / 45mA tolerant?

Haven't thought ahead to sending back from the Arduino - but perhaps a MIDI OUT circuit could be adapted as well? But the 30V / 5V conflict might cause problems??

Would be eternally grateful for any assistance!!

 

So, have had some progress on this, using an incredibly simple circuit consisting of just one 110 Ohm resistor to translate the ~2mA to 45mA current loop signal into a 0-5V voltage that can be directly read by an Arduino serial port! Like this:

The problem I have now is being able to transmit a signal back on the same loop... I have thought about triggering a digital potentiometer's resistance up and down to emulate the ~2mA to 45mA protocol, or using a transistor like this - but the problem is that by grounding the loop as shown in the above circuit I don't have a current anymore on the return loop to work with!

Am hoping for an equally simple / elegant solution for being able to talk back down the loop instead of just listening in?