Ohhh! I love this design …
You have to connect the LED to pins 1 & 2, and the MIDI current loop to 3 & 4. You can take a look to the v2 building tutorial, that include details about MIDI wiring:
(MIDI wiring has not changed from v2 to v3 kits, only connectors are different)
Regarding the electronic details, you have to realize that the activity LEDs use the current from the MIDI loop, being a “true” activity LED. So:
- When the MIDI loop is open (i mean the MIDI cable is not correctly connected on both sides/devices) , you wont see any light.
- When you see the LED blinking, it means there are some MIDI bits effectively flowing from sender to receiver, being both sides correctly connected according to MIDI specs (1).
In other words, the zynthian activity LED act as “MIDI loop” tester. If you have connected both sides and don’t see the LED blinking, this would mean that:
- The cable is broken
- Malfunction in the other device (connector or worst)
(1) As MIDI specs don’t say nothing about activity LEDs, I’ve adjusted the recommended R values for having a current loop very similar to the specs. For MIDI OUT & THRU, If you don’t want to use the LED, you MUST close the jumpers JP1 & JP2, and you will get 100% compliant MIDI circuits.
I tested the circuit with several devices and it worked perfectly with all of them … anyway, if you have problems with some devices, you could try to close JP1 & JP2.