[sdiy] Caution with DIY MIDI cabling

[Ove Ridé]

I'm trying to figure out what would have gone wrong in your case. MIDI
ground (pin 2, or the middle of the 5 pins due to DIN5 pin numbering*)
is not part of the signal circuit. You should be able to connect it to
other grounds without much effect to the MIDI signal integrity.
(Although that could on the other hand introduce hum to audio
signals.) If you were saying that either the +5V pin or the signal pin
were connected to the otherwise floating mains earth then, yes, that
could certainly cause some troubles because those lines are actually
transmitting data.

(*) The DIN5 pin numbering is based on the DIN3 pin numbering where
pins 1-3 are at 90 degree intervals. Then pins 4 and 5 are added
between those, at the 45 degree positions.

On Thu, 27 Dec 2018 at 03:16, 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
> ALLOW MIDI GROUND TO COME IN CONTACT WITH THE SHELL.
>
> 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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-- 
/Ove

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