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<div class="moz-text-flowed" style="font-family: -moz-fixed;
font-size: 14px;" lang="x-unicode">Maybe I'm the last to know, but
there appears to be an official word on this:
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<br>
<a class="moz-txt-link-freetext"
href="https://mitxela.com/other/ca33.pdf">https://mitxela.com/other/ca33.pdf</a>
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I know people have been doing their own thing regarding the series
resistors for the current loop. I also noticed this caveat here:
<br>
<br>
<a class="moz-txt-link-freetext"
href="https://www.midi.org/forum/492-certain-midi-equipment-for-3-3v">https://www.midi.org/forum/492-certain-midi-equipment-for-3-3v</a>:
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"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.
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<br>
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.
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<br>
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.
<br>
<br>
(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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