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

[Roman Sowa]

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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