This is an article culled from archive material, circa 1997.
A wee plastic box, a little bigger than a packet of cigarettes, with rows of
dedicated LED's for illustrating MIDI messages. It takes a 9v battery
or external PSU, and has a MIDI thru socket. It shows practically all
message types, but the device has a state machine/parsing bug which causes
it to indicate sporadic error conditions when receiving beat clock. It
is a useful device nonetheless, and definitely recommended for the
MIDI toolbox.