logic circuitry is powered by the computer and/or connected MIDI devices (no external power supply required)
if the Amiga is switched off or the serial port is active, all OUT ports are switched to THRU mode, i.e. sending out the MIDI commands from IN port to all OUT ports
connects to the Amiga via the serial port, provides serial interface pass-through
Notes
some MIDI devices will switch off their MIDI port if no receiver is present, so the THRU mode cannot work - revision 2 of this expansion features a power supply connector to stay powered in such cases
revision 2 features a switch at the bottom to change one port from THRU to OUT mode only