[sdiy] Updated MIDI Spec for 3.3v

[Neil Johnson]

I remember discussing this very topic on the midi forum years ago....and it
took them until last year to finally publish something.

If I remember rightly it was on a thread about drawing power from MIDI.

Neil

On 11 Aug 2017 15:26, "MTG" <grant at musictechnologiesgroup.com> wrote:

> 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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