Akai MPD226 Service Manual page 14

Tests
Midi Loopback Test
This test requires a MIDI cable to be connected between the MIDI IN and MIDI OUT
connectors on the rear panel.
Test messages are sent to the MPD226 for transmission from the MIDI OUT connector on
the rear panel. The ADT1 Manufacturing Test app looks for these messages to be received
back on the MIDI IN connector.
There will be a short delay before the MIDI Loopback test runs, after which it will run
automatically with no involvement of the Tester and report its result.