[sdiy] MIDI with +3.3V supply

Simon Brouwer simon.oo.o at xs4all.nl
Sun Nov 16 01:03:07 CET 2008

Seb Francis schreef:
> I'm just designing the MIDI I/O and am having to slightly re-think the 
> way I normally do things as I'm using 3.3V power.
> MIDI is a 5mA current loop.
> Approx 1.3V is lost in the opto-isolator LED.
> With 5V power, that leaves 3.7V which means a resistance of 740R is 
> needed in the loop.
> So allowing for a bit of loss in the MIDI cables, etc. the normal 
> practice seems to be to use 3x 220R resistors, 1 on the input and 1 
> either side of the output.
> But I'm not sure if this is an actual standard that there should be 
> 220R on the input side?  All the MIDI spec information I've seen just 
> states that it's a 5mA current loop, but not how the resistance should 
> be divided between input and output.
> Does anyone know the truth of this?
The three 220 ohm resistors are what is shown in the reference circuit 
in MIDI specification 1.0.
The note about what drivers can be used is missing from my photocopy but 
the guy I got it from wrote on it "can be transistors or logic gates". 
Probably a minimum low level of a few hundred millivolts is assumed in 
the calculation of the resistors. A MIDI cable would not have more than 
a few ohms resistance.
I would expect the 220 ohms and a LED that is comparable to Sharp PC900 
or 6N139 (the two types that "have been found acceptable")  in any 
standard MIDI input. Forward voltage of a PC900 @4mA is 1.4V max.

Vriendelijke groet, 
Simon Brouwer. 

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