[sdiy] Updated MIDI Spec for 3.3v
grant at musictechnologiesgroup.com
Fri Aug 11 15:21:46 CEST 2017
Maybe I'm the last to know, but there appears to be an official word on
I know people have been doing their own thing regarding the series
resistors for the current loop. I also noticed this caveat here:
"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
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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