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I'm developing a project on a board that uses 3.3V. And there has to be just one MIDI out. I tried connecting ground, vcc and data pins to a midi jack and it works fine with Kemper profiling amp. I tried it without any resistors as well as 220 ohm and 100 ohm on the 3.3V power pin (vcc) and there was no difference.
Is it better to use 5V than 3.3V?
What resistors are needed for each case and why?

Thank you!
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The Electrical Specification Update (PDF) shows what resistors to use if the output is powered with 3.3 V.

MIDI requires a current of at least (and not much larger than) 5 mA. Resistors of 10 Ω and 33 Ω are recommended to achieve this with 3.3 V; your device might work with other values, but there is no guarantee that this works with other devices.
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  1. more than a month ago
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Clemens Ladisch wrote:

The Electrical Specification Update (PDF) shows what resistors to use if the output is powered with 3.3 V.

MIDI requires a current of at least (and not much larger than) 5 mA. Resistors of 10 Ω and 33 Ω are recommended to achieve this with 3.3 V; your device might work with other values, but there is no guarantee that this works with other devices.


Thanks! If the current should be around 5mA, why is it not 660 ohms (3.3/0.005 according to ohms law)? 3.3/10 gives 330 mA and 3.3/33 gives 100 mA.
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Clemens Ladisch wrote:

The Electrical Specification Update (PDF) shows what resistors to use if the output is powered with 3.3 V.

MIDI requires a current of at least (and not much larger than) 5 mA. Resistors of 10 Ω and 33 Ω are recommended to achieve this with 3.3 V; your device might work with other values, but there is no guarantee that this works with other devices.


Thanks! If the current should be around 5mA, why is it not 660 ohms (3.3/0.005 according to ohms law)? 3.3/10 gives 330 mA and 3.3/33 gives 100 mA.
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This is not the correct way to compute the current; you have to add up the voltage drops of all three resistors (the output's two, and the receiver's 220 Ω one) and the optocoupler's LED. (See the specification.)
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Clemens Ladisch wrote:

This is not the correct way to compute the current; you have to add up the voltage drops of all three resistors (the output's two, and the receiver's 220 Ω one) and the optocoupler's LED. (See the specification.)

Thank you, found it in the specification :)
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What happens if a 5V midi device is connected to a 3.3 volt midi device?
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I am expecting to measure 5mA of current drawn between two devices, however it is much less than that, about 0.1 mA. But things seem to be working ok. I have the current meter in series with the 5 volt line, and later in series with the MIDI data, but there is much less than 5mA. Is that correct?
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A MIDI line is idle most of the time, so the average current is not 2.5 mA, but approximately zero.

To measure the current during a zero bit, you need a faster device, such as an oscilloscope.
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