This was defined by Opto-22, an American company who specialised in producing opto-isolated industrial I/O.

Between each signal wire is a ground wire to prevent cross-talk.

My only grumble is that the channel numbering starts from pin 47 and works backware to pin 1. If they had made pin 1 as 5V  then the channels would have incremented nicely into wider cables. But it is too late to change now... 

It was such a common standard that Arcom created an adapter board. This routed their four channels to two Opto-22 connectors.