I'm thinking about doing a MIDI project for fun and I was looking at example circuits.
I see that many people ground PIN 2 on MIDI OUT and I'm wondering why -- pins 4 & 5 on MIDI out drive the opto-isolator on the MIDI INput side (pins 5 & 4 respectively).
From my recollection of the MIDI spec is ONLY ONE end should ground the shield of the cable. Thus keeping isolation if optos are used but maintaining some shielding without ground loops.
Then again it is several years since I looked at MIDI circuits.
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Paul Carpenter | paul@pcserviceselectronics.co.uk
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The standard is that the cable should have the shield connected to pin 2 at both ends. In all equipment pin 2 is grounded at the midi out sockets, including midi through, but pin 2 at the in socket is not connected. This puts a grounded shield over the optoisolated current loop in the cable but does not connect equipment electrically.
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Ian Stirling, G4ICV, AB2GR, Long Valley, New Jersey, USA.
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You've received some good responses to your question, so I'll just turn you on to a couple of MIDI resources that you may find useful.
"The MIDI Technical Fanatic's Brainwashing Center" at
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has just about everything one might need to know about MIDI.
"The Urge To Merge" at
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This site provides schematic, source code, and more concerning one of my MIDI projects that implements an 8051-based merger with footswitch inputs and relay outputs.
Have fun! MIDI is definately a pleasant medium with which one can bring one's music and embedded processing skills together!
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Michael Kesti | "And like, one and one don't make
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