MIDI doesn't work [solved]

MIDI not working, arghhhh!

  • Maybe there’s a problem with the cables or the keyboard? Scoped the input of the optocoupler, ugly 50 Hz waveform with spikes at bits (this is normal for a current loop interface).
  • Scoped the output of the optocoupler, nothing.
  • Maybe the output of the opto is shorted? Use continuity tester: It is not.
  • Maybe there’s something wrong with the MCU pulling the line down? Removed the MCU, same problem.
  • Then maybe the opto is dead? Tried another opto, still no output.
  • Maybe there’s no power on the opto? Voltage on the +5V input of the opto is OK.
  • Maybe there’s one broken connections between the opto and the rest of the world? Check pins 1, 2, 3, 4, 5.

Dang, pin 5 was not grounded, it was probably a bad solder joint on the IC socket. Burnt a bit of the plastic at the bottom of the IC socket and added a big blob of solder on the top of the board.

MIDI is back.