[sdiy] MIDI cable impedance and MIDI In ESD protection?

René Schmitz synth at schmitzbits.de
Fri Jun 28 11:50:57 CEST 2024


If you have the space and budget, use two optocouplers: one for each 
direction, that allows you even to sense which direction it is.

(So you can change the output too...)


Where do I collect my prize? :)


Best,

  René


Am 28.06.2024 um 00:06 schrieb Mattias Rickardsson:
> On Thu, 27 Jun 2024 at 20:55, Andrei Kudryavtsev 
> <andrei.kudryavtsev at gmail.com> wrote:
>
>     Speaking of TRS mess, found this one in the recent application
>     list for MIDI innovations:
>     https://midi.org/innovation-award/psimidi
>
>
> Oops, that has been solved in commercial products for over 5 years.
> But without those extra diode voltage drops!
>
> (Assuming 1.9 V drop for the optocoupler and 2 x 0.3 V drop for the 
> Schottkys, and the new 3.3 V (+/- 5 % on the lower side) logic level, 
> there's 0.635 V left for the 220 ohm resistor. The current is then 
> only 2.9 mA, far below the required 5 mA. Oops2!)
>
> (It would need to have 127 ohms instead of 220. But with 5 V (+/- 10 % 
> on the higher side) logic level, and 1.9 + 0.3 + 0.3 V voltage drops, 
> you get 3 V over the resistor. 23.6 mA is above the specified Abs Max 
> input current of the 6N138 optocoupler suggested in the MIDI spec, 
> even if the 6N137 used here will survive. Oops3!)
>
> I'm sure similar gender-agnostic TRS MIDI In jack solutions must have 
> been implemented by several manufacturers by now? It was a bit weird 
> to see different brands using different polarities when TRS MIDI 
> emerged, and even more strange to see MMA omit a solution for it in 
> the new standard. :-)
>
> Thanks for the reading tip, Andrei!
>
> /mr
>
> PS: I wrote "TS" in my first message, mistake - should have been 
> "TRS". Sorry.
>
>
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-- 
--
synth at schmitzbits.de
http://schmitzbits.de
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