On Windows (and, eventually, Linux), the MidiOutputActor sends midi commands to the default midi device on the sound card.
To use the MidiOutputActor implementation, load midi.so and create an actor of type MidiOutputActor. Then use BeginSound to create a handler, to which you can send SetInput messages to attach the midi outputs of handlers of other midi actors to the computer's serial port output. For example:
aIn = Create MidiInputActor;
sIn = BeginSound aIn;
aOut = Create MidiOutputActor;
sOut = BeginSound aOut SetInput sIn;
implements a simple midi "thru box".
If the midi synthesizer you use is SGI's own "midisynth" software, we recommend that you set the output sampling rate of both VSS and apanel to 44100 Hz. Otherwise, timings and pitches may be incorrect. You may also need SGI's "synthpanel" (or "killall midisynth", brute-force) to control the midisynth synthesizer.
We recommend a Macintosh-style serial-to-midi conversion box which uses its own power supply instead of being parasitically powered from the SGI's serial line.