[sdiy] MIDI processor - power from midi in?

[Colin f]

> Standard MIDI has only two pins used, they are a
> current loop that supplies an LED in an optocoupler at
> the receive end.

The standard midi output is a 220 ohm resistor to +5v which connects to the
input anode, and a 220 ohm resistor to an open collector output (the data
signal) for the input cathode.
There should also be a 220 ohm series resistor at the receiver end, giving a
total of 660 ohms. At 5v less the diode drop for the LED in a typical
opto-isolator, that gives an 'on' current of roughly 5mA.

You are not supposed to connect the ground line at a receiver, but if you
do, you can take a +5v supply between the anode line (pin 4) and ground (pin
2).
The supply voltage will drop as you would expect given a 220 ohm resistor in
the supply line, but you can draw a fair few milliamps and get away with it.
If you get ground loops though, it's your own fault.

Cheers,
Colin f