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

[Neil Johnson]

There are many synth schematics on the web from Kurzweil, Roland, etc.
In all the ones I've seen there are filter caps on the MIDI
in/out/thru pins.
I'm looking at the K2500 schematic right now (google it) and it has
lots of 1n0 capacitors.  Good for RF as Roman says.  For ESD - use TVS
diodes IF YOU NEED TO.

Neil

On Wed, 26 Jun 2024 at 14:57, Roman Sowa via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> Up to 4n7 the caps will not affect the shape of the MIDI pulses dramaticaly.
> And as long both are the same on pins 4 and 5, there should not appear
> any false spikes caused by interferences from the cable. With
> recommended 100nF from pin-2 to GND that's not even a concern anymore.
> I'd say 1nF will be good choice. But will that supress everything? RF
> for sure, but ESD? Maybe better to use TVS across the LED and another
> one, say 300V or more, from there to GND, as it was already suggested by
> Ben. Their capacitance in nowhere near our concern here.
>
> Yes, it is alowed, since it's recommended to have 100nF cap to GND at
> the shield, which creates bigger path for AC than those tiny caps on
> data lines. Those caps still do not create any ground loops, which was
> the only reason to introduce the opto in MIDI, AFAIK.
>
> Roman
>
> W dniu 2024-06-26 o 15:14, Mattias Rickardsson pisze:
> > This was one of my main concerns: Is it "allowed" to have the
> > optocoupler LED pins connected via capacitors to the ground of the MIDI
> > receiver, from which it is supposed to be galvanically isolated?
> >
> > (The MIDI standard mentions allowing two small caps to receiver ground
> > from MIDI In pin 2 (transmitter ground) for RF shielding and from MIDI
> > In shield for EMI/EMC performance, but nothing about the two MIDI In
> > signal pins 4 & 5.)
> > /mr
> >
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