eaaasy question..

why do we need a resistor (220Ohms) to the MIDI output?? ::slight_smile:


alright then, any place where I can read more info about the MIDI I/O circuits?

I know there is lot of info out there, but for some reason most of them mention the resistor (220Ohm) but without explaining why it should be used…


I think I remember that Midi works with some optocouplers? Might be there to limit the current, or as a pull-up, I don’t know. Any specific reason why you don’t want that resistor to be around?

And … errrr… what made you think that this is an easy question???


hi wayoda,

I’ve already put a 220 Ohms resistor at my MIDI output (following some examples). However, I would like to know exactly what I’m doing :slight_smile:

I thought it was easy because I can’t find any info…


Hi w,
here is a link for a page full of midi schematics http://home.concepts-ict.nl/~nctnico/midi.htm
On one of these pages it says that a midi-interface works by switching a current of ~5mA on and of.
(Switching currents as opposed to switching a Voltage-level is used in areas where there is a lot of (electronic) noise from the outside, like when a band is playing on stage.)

So the 220Ohms is there to limit the current to (roughly) 5 mA. As there is usually an Led on the other end of the Midi-Controler-circuit, I think a current from 3-20mA would be acceptable.

And here is the “official” midi-schematic
I guess it’s the midi-out section you are interested in. This has actually two resitors (plus another one on the midi-in receiver side).

But using only one resistor should be fine.


oh, i see…

thanks Eberhard :slight_smile: