While doing some research to build a simple MIDI tester, I started to study the “real world” MIDI electrical specifications out there. I also experimented with various MIDI equipment and measured some voltages.
A simple MIDI tester
MIDI specifications imply that a MIDI OUT circuit always provides +5 Volts, on pin 4, in reference to circuit ground, present on pin 2. Pin 5 carries the actual MIDI signal and switches from +5 volts to zero Volts, referenced to the ground present on pin 2.
To test if a MIDI OUT circuit adheres to the specs, I use this simple circuit:
The red LED will indicate that pin 4 supplies 5 Volts and that pin 2 is connected to ground. Use a small LED with low current draw. You might want to replace the 220 Ohm resistor with 1K (that’s what my final design uses) or even 10K so that the LED doesn’t steal all the current from pin 4. With a 1K resistor, this circuit draws 1.15 mA (0.15 with a 10K resistor).
The green LED will flash when receiving MIDI messages.
I built this circuit using a leftover midi cable, the LEDs and some hot glue
Many MIDI circuits will fail to light the red LED. It means that pin 2 on the MIDI OUT is not connected to ground. Some designers connect it to chassis ground. Others leave it unconnected. That’s OK as long as the green LED flashes. The red LED will indicate that a small electronic circuit could use the power provided between pin 2 and pin 4. For example, on my AKAI MPK261 keyboard, the MIDI OUT port provides just enough power for an Arduino (about 9.5 mA).
The circuit can be modified to better conform to the midi.org specifications by adding a 1N4148 diode with a 220 Ohm resistor. That would protect the LED and reduce current draw. It might not be worth the cost of the parts… I tested it but kept the simple design.