Bingo! MIDI cable connector is a German 5-pin DIN (Deutsches Institut für Normung)-connector. These used to be popular connectors on European HiFi-equipment (hey … 5 pins give you stereo in/out in one connector … simple and clever).
I have an old ADA MP-2, a midi controlled tube guitar pre-amp. The foot switch for it sends midi signals to the pre-amp and can use midi phantom power sent by the pre to the foot selector switch. The other two pins are for this midi phantom power. These are the only devices I’ve ever seen that use all five midi pins.
I suppose to sum up, if you want guarantee 100% MIDI compatibility with all devices, then all 5 pins should be wired. If not, and some day some piece of gear just isn’t working right, it will always be gnawing in the back of your mind that it might be the problem - even if it’s not. Even worse if it is the problem and you forgot that some of your cables are not fully wired.
I would disagree. Only 3 pins are standard. If device is using all 5 the 2 “non-standard” pins may have been wired to anything. If you connect 2 devices with different pin-outs together with 5-wire cable, you may have problems (or even smoke, if you’re unlucky). Anyway. I’ve always used 5-wire cables and never had any problems.
In that case the device is using custom midi cables so is itself not midi standard, and should specify this with a warning to the effect, for if one connects with a standard midi cable, chances are good its 5 conductor wired pin to pin. I too have only ever used 5 conductor cables I’ve tested all of mine and I purchased them as midi cables.
Perhaps I should have phrased my original statement to say ‘100% MIDI compatibility with all 100% MIDI compatibile devices’.