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

[rsdio at audiobanshee.com]

On Sep 11, 2015, at 12:11 AM, Gordonjcp <gordonjcp at gjcp.net> wrote:
>> 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,

Terminating a line involves adding a path to ground and/or a path to the power rail - i.e. a shunt to DC. You can't terminate a line with a series resistor. Actually, you can terminate a line with a series resistor if the final load is referenced to ground, but in the case of an opto-isolator there is no reference. Termination has to allow the current to be diverted so that it does not reflect back to the source. But termination is about voltage signals. With current loop signals, I don't think that termination even comes into play. If it did, then you'd need something other than series termination.


> and limiting current in case of a short-to-rail fault in something plugged into it.

I think you're right on the money here.

Brian