[sdiy] MIDI with +3.3V supply

[Seb Francis]

I'm just designing the MIDI I/O and am having to slightly re-think the 
way I normally do things as I'm using 3.3V power.

MIDI is a 5mA current loop.
Approx 1.3V is lost in the opto-isolator LED.
With 5V power, that leaves 3.7V which means a resistance of 740R is 
needed in the loop.
So allowing for a bit of loss in the MIDI cables, etc. the normal 
practice seems to be to use 3x 220R resistors, 1 on the input and 1 
either side of the output.

But I'm not sure if this is an actual standard that there should be 220R 
on the input side?  All the MIDI spec information I've seen just states 
that it's a 5mA current loop, but not how the resistance should be 
divided between input and output.

Does anyone know the truth of this?

Assuming that all MIDI inputs do indeed have 220R and ~1.3V 
opto-isolators, then when driving an output with only 3.3V one should 
presumably use something like 2x 91R resistors (or a bit less) to get a 
5mA current loop.  This would seem to offer less protection if someone 
plugged an output into an output.  (Actually I've just noticed that the 
new dsPIC33Fs have an absmax of 4mA per I/O pin - was 25mA in older PICs 
and dsPICs - so probably necessary to use a buffer on the UART output now.)

Anyone have any experience doing MIDI with 3.3V?

I'm assuming the output side of a 6N138 is going to work fine with 3.3V 
- I couldn't see anything in the datasheet which suggested otherwise, 
although they do talk about 5V in the examples.

By the way, what do you do with the Vb pin?  I've always connected it to 
ground via a 1K5 resistor, but the datasheet shows it as unconnected.

Seb