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

René Schmitz synth at schmitzbits.de
Fri Jun 28 14:11:18 CEST 2024


AC optocouplers are an option too. I havent found one that is fast 
enough out of the box though.

There are ways you can make them faster, but I'm not sure I would want 
to rely on that.

These seem intended for sensing 50Hz power grid frequency.


Am 28.06.2024 um 13:35 schrieb Roman Sowa via Synth-diy:
> There is such thing as optocoupler with 2 LEDs back to back, so 
> polarity does not matter for them. I used them long time ago in one of 
> my MIDI2CV converters. Unfortunately it's not very fast, so it needs a 
> little trick to make it work at MIDI speed. I think it was ACPL-214.
> Fun fact - connecting MIDI jack in reverse is number 1 reason for user 
> claims that the product doesn't work.

All it takes is using the wrong flavour 5pin to TRS. Or that wonky 5pin 
cable that is wired incorrectly. If you combine the two it's working fine.


Cheers,

  René


>
> Roman
>
> W dniu 2024-06-28 o 00:06, Mattias Rickardsson pisze:
>> 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
>>     <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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