I've got a little MIDI adapter for an old macintosh, it simply plugs into one of the serial port and provides one input and a couple of outputs.
It works with "MIDI Manager", which is system software from Apple and provides an application called "PatchBay" which lets you plug and play with physical and logical midi connections.
The idea is that "MIDI Manager" aware applications publish some inputs and outputs, then the patch bay can be used to wire them up, just like the mess of wires between keyboards, synths, drumboxes and expanders.
I used to use it with a Yamaha QY10.
Make me an offer, I'm in New Zealand.