The PMA-5 has a computer switch on its side.
When the computer switch is set to MIDI:
The MIDI IN and OUT connectors work the same as they would on any MIDI instrument. To use it with a computer, a MIDI interface is required. The computer connection is inactive in this mode.
When the computer switch is set to PC-2 or Mac:
The PMA-5 acts as it's own interface and can be connected directly to a serial port on a computer. The MIDI OUT connector is disabled but the MIDI IN connector is still active. A keyboard controller (or other MIDI device) can be connected to the MIDI IN and its signal will go through the device (without triggering any sounds) and will be passed over the serial line to the computer. If the sequencing software has MIDI echo (sometimes referred to as Soft Thru or MIDI Thru) enabled, the MIDI data from the keyboard controller will then be sent back to the PMA-5 and will trigger a sound. Since the MIDI OUT is disabled, it is not possible to trigger sounds in external MIDI devices.
When the computer switch is set to MIDI:
The MIDI IN and OUT connectors work the same as they would on any MIDI instrument. To use it with a computer, a MIDI interface is required. The computer connection is inactive in this mode.
When the computer switch is set to PC-2 or Mac:
The PMA-5 acts as it's own interface and can be connected directly to a serial port on a computer. The MIDI OUT connector is disabled but the MIDI IN connector is still active. A keyboard controller (or other MIDI device) can be connected to the MIDI IN and its signal will go through the device (without triggering any sounds) and will be passed over the serial line to the computer. If the sequencing software has MIDI echo (sometimes referred to as Soft Thru or MIDI Thru) enabled, the MIDI data from the keyboard controller will then be sent back to the PMA-5 and will trigger a sound. Since the MIDI OUT is disabled, it is not possible to trigger sounds in external MIDI devices.