Quote Originally Posted by novino View Post
Yes I've been using an opto-isolator, but it was a 6n137. Does it make any difference with the 138 ?
It may.

The 137 has a higher input current requirement than the 138.
And I think MIDI uses a 5mA current loop for it's signal, which is right at the absolute minimum specified for the 137.

It may not be creating a good signal on the other side.
Can't say for sure though.
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