[sdiy] Updated MIDI Spec for 3.3v

Roman Sowa modular at go2.pl
Fri Aug 11 15:35:38 CEST 2017

You're not the last one to know, and I bet in 10 years from now there 
will be people, including gear makers, not aware of this.
BTW, my download date of that file is 1 year ago and I think it was 
there quite a long before.


W dniu 2017-08-11 o 15:21, MTG pisze:
> 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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