Started working on a dongle to connect the Sound Blaster MIDI cable to the Altair, or perhaps a Raspberry Pi, or else an Arduino - or perhaps it can be made to work with "all of the above."  Here we are receiving some MIDI data from a Yamaha keyboard which is playing some material in demo mode.  I scavenged a DB-15 female connector to IDC connector via ribbon cable from an old PC, that probably went to the recycler years ago, and that is how the Sound Blaster MIDI cable normally connects to the game port of a PC, that is if it still has a traditional game port.  Added some IDC header pins and other goodies to some perf board, and presto, "instant" MIDI to "whatever" dongle, well, hopefully soon enough.  Now as far as the Sound Blaster-compatible MIDI cables go, that part is pretty simple to wire up.  Apparently, pins 4 and 5 are connected to the ground, then +5 volts should connect to pin 8, and that provides the power to some internal circuitry that is built into the cable assembly.  Finally, MIDI data from a remote device will appear on pin 15, whereas pin 12 can be used to send MIDI data, which will need to be clocked at 31250 bits/second.