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running core without optocoupler


petrol vendor
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After inserting the optocoupler twisted 180° :cry: it stopped passing data.

Untill i get a new one, can i just place a wire link across the socket, conecting the midi in directly to the PIC? As i understand it´s there to isolate the midi in so in case of some fault the PIC won´t be damaged. I would be willing to take that risk for the moment if that´s the only drawback.

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Yes, you blowing up the PIC and having to wait for yet another part is the only drawback. Just like the only drawback of driving without a seatbelt is that you go flying out the window if you crash :-)

I'd simply wait for the new opto.

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Thank you for your help!

So in theory it would work?

I am willing to take your advice, please don´t get me wrong. I just try to understand what is going on with this little device (and also, well, waiting time is the hardest time...)

Why would connecting midi directly harm the pic under normal conditions? Is this common practice in digital systems whenever connecting to a device that does not share the same psu?

The interface i am using is usb powered, so no voltage above 5v should be expected. On analog circuits it is common to wire a small resistance in series with the input jack, and Diodes from the input to the power line(s) to protect the first active element in the circuit. All voltages above operating level will be current limited by the resistor and shunted into the supply rail via the diode. This can, for example easily happen when 48v phantom power is applied and the blocking caps let through a small burst onto the first gain stage.

Many opamps (and also digital chips?) already have these protection diodes built in. So small differences in signal level (two 5v psu will normally differ by a certain amount) can be tolerated.

Would you consider this circuitry an alternative to be on the safe side if i wish to go on just now..?

Or am i on the completely wrong track and we are not at all talking about overvoltage when looking at the purpose of the optocoupler?

Of course if we are talking about comercial units every possible protection should be applied, since the user might be doing something unpredicted as he is not the engeneer.

I once killed a friends novation bass station by feeding a 10v cv into the jack labled cv. The jack was not intended as a modulation input to connect to a modular synth. And it had no protection whatsoever...

Edited by petrol vendor
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It should work as long as you ensure that the grounds are connected together, and that the MIDI device doesn't output more than 5V at the Tx pin (which seems to be the case due to the USB powered approach)

Ground is normally not available at the MIDI IN socket to avoid ground loops (bad for audio devices).

But for this direct TTL connection you will need it.

Best Regards, Thorsten.

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