[sdiy] Updated MIDI Spec for 3.3v

[MTG]

Maybe I'm the last to know, but there appears to be an official word on 
this:

https://mitxela.com/other/ca33.pdf

I know people have been doing their own thing regarding the series 
resistors for the current loop.  I also noticed this caveat here:

https://www.midi.org/forum/492-certain-midi-equipment-for-3-3v:

"The MIDI specification specifies a 5 mA current loop, and shows three 
220 Ω resistors to limit the current when used with a 5 V transmitter.

When you have a 3.3 V transmitter, the two resistors in the transmitter 
must be smaller to get the same current, as shown in the Electrical 
Specification Update.

The receiver does not need to know what voltage the transmitter uses; it 
always uses the same 220 Ω resistor. All transmitters are designed to 
work with that same receiver circuit. This means that all transmitters 
and receivers are compatible.

(The receiver can use either 5 V or 3.3 V as supply for the 
optoisolator; this part of the circuit is isolated from the current 
loop, and does not affect compatibility of the MIDI interface. But note 
that only the PC900/H11L1 optoisolators can run at 3.3 V; other common 
models like the 6N137 or 6N138 require 5 V.)"

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