(1) Using single ended currents also works very well for this but I have never seen it used except when I did it.
David, it's really sad that you were not able to use a telephone.
Current loop existed long before MIDI was even an idea.
Original telephone system uses a current loop.
Off hook is signaled by closing the loop.
Rotary dial is mechanical timed opening/closing a switch.
The full duplex voice is carried on current.
Tteleprinters or Teletypes used current loop. Switches and relays to start with.
Current-mode logic is a differential digital current logic.
Video DVI & HDMI use this.
This logic is better for speed & distance when compared to voltage based logic.
Even for analog links, current is king.
The AD590 temperature sensor which is current based does not have the huge problems of the many voltage based temperature sensors.
Seph.b
If you think of led's you can have many in series.
Your control room could be a 1000' from other end.
You could have an LED in photo-coupler & a series connected LET showing link is good. You can connect many of these in series if you adjust the open circuit voltage source higher so that enough voltage is present to turn on all leds that form the series chain..
The transmitter is a normally closed switch. A transistor that is normally on.
You can have many transmitters connected with only one talking at a time. Again photo-coupler or opto-coupler makes this easy.
Need to remember ohn's law
Voltage = Current x Resistance.
You can use any one of the three to transmit information.
C