T3 MIDI Out resistor calculations

yeahtuna

Well-known member
As per the following page, T3 devices should use 47 ohm resistors on MIDI Out. How are these values calculated? What amount of loop current will those resistors provide?
https://www.pjrc.com/teensy/td_libs_MIDI.html

I have a customer that is suggesting that my MIDI out circuit is causing his CME WIDI Master dongle to fail, and I'm trying to investigate if there's any way my MIDI Out circuit could be the culprit.

Also, in the unlikely event that pin 4 gets shorted to ground, would that sink enough current to kill the TX pin? Are we advised to to use a buffer?

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Dug into this quite a while ago after finding that some of my Teensy Midi (TX) projects did not work with all available Midi gear.

Thinking that the Midi TX - Rx pathway is a current loop and ever suspicious of "standards", breadboard replicated various manufacturers implementations and discovered that measured static loop current might be anywhere between 5 and 11 Ma.

Not sure of the purpose of the dual diode though.

They are TVS diodes for ESD protection.

Is the upper resistor (pin 4) tied to 3.3v? It seems connected to ground in your schematic

Ya, that's 3.3v. Not the best choice of symbols.

I suppose I can wire the circuit on a breadboard and instead of connecting pin 5 to the MIDI TX pin, I can attach it to ground and then I should be able to get a good current reading from my multimeter.

So I wired up the MIDI out with pin 5 going through the 47 ohm and then ground and then attached it to a MIDI In circuit with an PCV900 and 220 ohm resistor and measured the current. The MIDI Out circuit produces a loop current of 5.9 mA with the LED attached, 6.7 mA with the LED removed. I thought that because the LED was in parallel, it wouldn't affect the loop current that the MIDI in sees.

The measured forward voltage of the PCV900 is 1.1v. So doing the math, the loop current should be (3.3 - 1.1) / (47 + 47 + 220), which is about 7mA. So it seems that everything is in order, although I still don't really understand why the LED would affect the current running through the PCV900.

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Just opened up the box, I'd used 22R.