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

[Neil Johnson]

Richie,

Any manufacturer who follows the circuit in the standard would most
likely be OK, since they present an opto-isolator at the input.
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.

For EMC/RFI/noise reasons I would recommend have equal source
impedances on the two lines to maintain a semblence of balanced
impedances.

But, yes, in principle there's nothing stopping you running a MIDI
interface from a weird voltage rails, e.g. 0V and -12V as found in
organs, etc.

Neil




On 10 September 2015 at 23:04, Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
> Hi Tom,
>
> Thanks for clarifying that you can just change the resistor values to make
> the current right to make MIDI work with 3.3V logic.  I did wonder whether
> you still needed a +5V line just to tie up pin 4 of the MIDI output jack.
>
> Since MIDI is a current loop standard, I should in theory be able to
> implement a MIDI output port by grounding pin 5 of the DIN socket, and
> switching a current-source in the high-side connected to pin 4 of the DIN
> socket then?  That would still meet the standard in terms of an on/off
> modulated 5mA current loop and drive the floating opto-coupler at a MIDI
> input port just fine.  (I wonder if anyone does this?)  I'd imagine that it
> would mess up any product that anticipated the "Electrical Specification
> Diagram" example shown in the MIDI spec and planned to draw power from pin 4
> though?
>
> -Richie,



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