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

[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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