[sdiy] MIDI input resistor

[Tom Wiltshire]

What's the purpose of the resistor from Pin 7 to ground seen in some 2N138/139 MIDI In optoisolator circuits?

Some schematics have it:

	http://www.ucapps.de/midimon/midimon.pdf

And others don't:

	http://www.sparkfun.com/datasheets/DevTools/Arduino/MIDI_Shield-v13.pdf

We've discussed the various values of the resistor between the output pin 6 and Vcc here before, so I won't go back over that. The summary IIRC was that different manufacturer's versions of the 2N138/139 seem to require different values, so pick something that works between 270R and about 1K8.

Thanks,
Tom