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.

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.

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.

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.)

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 🙂

What happens if a 5V midi device is connected to a 3.3 volt midi device?

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?

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.

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 🙂

Hello
I have same problem...connecting midi device to 3.3v (Arduino MKR 1010).
What is the correct schematic for this connection ?
All the schematic I've found says only 220ohm resistors...but output signal is unable to talk to some devices..
Please help
Roberto

Are you using a MIDI output, or a MIDI input, or both?

A 3.3 V MIDI output must use different resistors (see the Electrical Specification linked above). A 3.3 V MIDI input does not need to change anything.