Hallo Anthony, I try to answer your enquiry of two days ago:
> My question regards the wiring on the input jack. It has only two
> wires soldered to it and one is ground (center pin 2). Can a MIDI
> system function with just the ground and the 'signal' input on pin 4
> but without a 'sink' or 'current source' connection on pin 5?
I do not know whether it might work, but this is certainly not
correct. MIDI-In (pin 4(+) and pin 5(-)) are directly routed to the
optical isolator of the receiving circuit.
A good description and sketch can be found here:
The Ground potential of the transmitter end may be used for shielding
purposes but should _never_ be connected at the receiver end.
Otherwise the basic idea of optical isolation would be violated.
This does not mean that there are existing lots of commercially
available boxes which show this mistake. I remember even a Yamaha
device where I had to cut printed circuit routes in order to have it
follow the rules.
Best regards from rainy Germany,