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

[Mike Bryant]

If you look for them, there are quite a few 4 pin optoisolators which work fine, and two of them take less space and cost less than a single 137.

________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of René Schmitz <synth at schmitzbits.de>
Sent: 28 June 2024 10:50
To: synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] MIDI cable impedance and MIDI In ESD protection?



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<mailto: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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