[sdiy] Caution with DIY MIDI cabling

Ben Bradley ben.pi.bradley at gmail.com
Thu Dec 27 05:33:45 CET 2018

I recall the original MIDI cable spec (because I made my own cables
back then), it used ONLY the two signal wires, supposedly twisted in
the cable, with the idea that "it's a current loop, it doesn't need a
shield." It worked, but perhaps enough people had enough interference
to audio cables from their MIDI cables that the shield thing was added
to the spec. The original MIDI spec was fully isolated by the
optocoupler on the input. If the cable needs a shield it can be
telescoped (as in connected to ground, NOT one of the two MIDI
signals, at either end of the cable but NOT both).

On Wed, Dec 26, 2018 at 9:15 PM Michael E Caloroso
<mec.forumreader at gmail.com> wrote:
> I build my own I/O panels for my racks, and my own MIDI cables.  I
> recently uncovered a nasty bug that has perplexed me for years:
> If you use metal shell MIDI plugs on your DIY MIDI cables, DO NOT
> I was gigging my road system and was experiencing intermittent MIDI
> errors.  After finally being able to reproduce it at home, I figured
> out that the metal shell of the MIDI plug shorted to earth ground via
> the metal DIN jacks I was using on my DIY I/O panels.  Stray shield
> wires from the cable had contacted the cable clamp of the plug
> (despite my best attempts at trimming the stray wires), completing the
> short circuit of MIDI ground to earth ground.  MIDI gets very upset
> under this condition.
> I use an EBTech cable tester after I build all my DIY cables, and it
> does not have the capacity to detect a short from the shell to any of
> the pins - it's not supposed to.  Even with my diligent testing after
> completing a cable, this bug had eluded me for YEARS in my studio
> system.  Yes I was well aware that the MIDI specification grounding
> system is designed that way FOR A REASON.
> The only way to guarantee complete isolation of MIDI ground from earth
> on the metal shell of the plug was to disassemble the plug and add
> shrink tubing over the trimmed end of the cable.  This separates any
> stray shield wires from the metal clamp of the plug.  Once I fixed my
> cabling for the stage rig, it behaved MUCH better.
> Nasty bug, this.
> For years I had wondered why my SYSEX dumps were never 100% reliable,
> now I know why.
> That's just the STAGE rig.  Now I have to make the cable corrections
> to my STUDIO system, which has easily over 30 DIY cables that I built.
> Guess how I am spending my holiday break...
> Sent from my iGroundingCanBiteYou,
> MC
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