Just found on net an idea to think about - 3.5mm jack to 5-DIN for MIDI connectors.
I plan to do this with my zynthian build. Problem is there are currently 2 standards on the pinout of the 3.5mm jack, one from korg and one from novation and you have to get the correct cable. You can read a bit more at https://www.midi.org/articles-old/updated-how-to-make-your-own-3-5mm-mini-stereo-trs-to-midi-5-pin-din-cables
EDIT: oh, that is discussed in the originally linked article, sorry I should read first. 
In effect they just reverse the connections on tip and ring…
A small DPDT switch could do that for you, but I have to say when you consider how standard MIDI has been for so long you’d have thought they would have done the courtesy of giving Dave Smith a call before they did this stuff…
I’d also be interested to know if they implement the opto isolator side of the interface?
IDK, but I expect they do. AFAICT its just a connector change since its used to interface with other MIDI gear
Switch is not required. One who can build Zynthian from available parts or order kit may solder wires on one way or another when needed. Just adding 3.5mm jack holes to official Zynthian case won’t hurt, as this jack is also common for connecting devices in MIDI world. For example Novation Launchpad Pro comes with two 3.5mm -> DIN5 cables in package.
True, but guess which cable you wont take to the gig 
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Switch then can be added to cable too… 
Or this and this for real road warrior, as those cables are universal anyway.
Zynthian box remains with one variant.
I’m thinking of a full wave bridge rectifier? I wonder if it would work?
That way the zynthian implementation wouldn’t care for midi in… And a schottky bridge rec would have a better voltage drop characteristic…
Interesting idea, but you can’t make output this way, so jack polarity remains anyway.
My last offer . .
A 3.5 mm jack to jack lead with a bridge rec in it and one plug labelled input and output with the bridge rec at the input end
And two separate but self polarity setting leads for 5pin din -> 3.5mm & 3.5mm -> 5 pin din.
You can see I enjoy this sort of thing, is it that time already nurse . . . ? 
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