[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

[Gordonjcp]

On Thu, Sep 10, 2015 at 11:31:51PM +0100, Richie Burnett wrote:
> >Interestingly, the standard shows a single 220R resistor in the input
> >circuit, although I have seen several manufacturers (e.g., Roland)
> >split that into a 100R and a 120R resistor, one each side of the
> >opto-isolator.
> 
> Very strange, there must be some reason for that.  There's no way
> they'd spend an extra half cent on every unit manufactured without
> some tangible benefit.  (Maybe the product already used loads of
> 100R and 120R resistors, but didn't use a 220R anywhere else!?)
> 

I always assumed it was a combination of terminating the lines with a non-zero impedance, and limiting current in case of a short-to-rail fault in something plugged into it.

You can see how if pin 4 became shorted to pin 2 in the cable and there wasn't a resistor, it would deck the 5V rail and possibly do a bit of damage.  Just look at the power supplies in some old gear, they're like MIG welders!

-- 
Gordonjcp MM0YEQ