MIDI-Thru problem ?

[Michael Buchstaller]

OK group, Hi Paul,

thank you all for your helpful suggestions.

*finally* i found out why it did not work.

>My circuit ist here: http://www.takeonetech.de/midi.gif

The value of the pullup resistor at pin 6 of the Optocoupler was too
high. The 2.7 K was w value that i have seen in another application, and it
looked to be in the right ballpark, so i used it.

Close examination of the incoming MIDI signal and the input at the processor
showed up that the rising edge of the signal was OK, but the falling edge did
have w delay of approx. 10 us. Not a big deal at 31 KHz, but when that is
repeated 2 times, the signal becomes unreadable and smeared (but when
examined with a scope, still looks fine - only comparson with the input of the
very first MIDI device in the chain shows that some "peaks" get lost)

I decreased the value to 220 Ohms, and the delay dropped to 2 us, and
nov everything works correctly.
Now i will have to look un the 6N138 datasheets for the max. collector current
of the output transistor, maybe i could safely reduce the resistor to say 100 ohms
to improve that behaviour even more.


-Michael Buchstaller