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Why is the midi out not opto-isolated?


dfro
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Years ago, I got one of the last E510 MIDI scanner kits that Maplin sold.  I also have Craig Anderton and friends' book, "Digital Projects for Musicians."  They both do some interesting things with the midi in/outs.  As with the midibox, the midi in of the MidiTools computer (DPFM book) is opto-isolated with a 6N138. In the MidiTools computer, a midi thru line is then split off of the midi in.  The signal goes through a 74HC04 hex inverter twice to buffer/invert/invert again the signal before reaching the midi thru jack.  Does this also help to clean up the signal - sharpen the rising/falling edges?  Does it help to isolate the signal? 

The E510 has no midi in.

The midi out of the MidiTools computer and the E510 also are run through a 74HC04/74LS04 hex inverter twice to buffer/invert/invert again the signal.  The E510 also splits off an extra midi out using the 74LS04.  The signal is inverted, then split, then the two lines are seperately inverted again and sent to the connectors/jacks.  Does anyone like this idea?  This seems like a simple and elegant way to give yourself several midi outs in case you want to, for example, drive a sound module from a midified organ and also run a signal into a computer sequencer to record the performance.  A very simple midi splitter module could be made to offer several identical buffered midi outs to drive different devices, either using the 74HC04 or and op amp voltage follower setup.  Any thoughts?

I hope I am getting the lingo correct.

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Don't know, but it's like that in everything, just with a few different  common opto types. I almost think I've seen that listed as part of the hardware spec. or something.

Maybe it's just required on that end of the link, so where it's on it's way out, the isolation is expected  to be taken care of in the next (receiving) device.

FWIW- In digging up various output circuits in the past, there was a wide assortment of techniques for cleaning up/boosting/inverting the line and what not. I think some cheap stuff, or adapter cables, may even omit  certain parts of the circuitry.

George 

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I was wondering why the midi out (tx pin) is not opto-isolated on the core module or the midi-merger with a 6N138, like the midi inputs are?

I think this is a fair question for people trying to learn new things here. Electrical isolation does not require an opto-isolator at both ends, only one. The original MIDI specification had the goal of providing a standard interface at the lowest cost possible to inspire manufactures to implement MIDI on their devices.

Isolation is achieved, in the case of MIDI, by having the input channel opto-isolated. The output channel does not need an opto-isolator because the input channel of the device it is connected to has the opto-isolator.

BTW: Another question is why are opto-isolators used at all? There are two primary reasons, both having to do with grounding. If opto-isolation was not used (e.g. there is a direct electrical connection between devices) there is an inherent danger that the ground levels between the two devices are different. This could lead to damage to one or both devices. Secondly, ground loops could cause problems with noise being introduced onto the MIDI signal causing erratic behavior.

I hope this helps :)

Regards,

Synapsys

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I think this is a fair question for people trying to learn new things here. Electrical isolation does not require an opto-isolator at both ends, only one.

Why?

The original MIDI specification had the goal of providing a standard interface at the lowest cost possible to inspire manufactures to implement MIDI on their devices.

But does this answer any quoestions? Are You the one we can say: "Synapsys" - the guy who specsed midi?

Isolation is achieved, in the case of MIDI, by having the input channel opto-isolated. The output channel does not need an opto-isolator because the input channel of the device it is connected to has the opto-isolator.

Please wiki me "isolation"?

BTW: Another question is why are opto-isolators used at all? There are two primary reasons, both having to do with grounding. If opto-isolation was not used (e.g. there is a direct electrical connection between devices) there is an inherent danger that the ground levels between the two devices are different. This could lead to damage to one or both devices. Secondly, ground loops could cause problems with noise being introduced onto the MIDI signal causing erratic behavior.

Yes - Why I smell so funny to customs dogs.. OR? Who gives a fuck?

I hope this helps :)

Moebius

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moebius,

Is this really how a High Guru of the Midibox Priesthood should be behaving?

No! Definitely not.

But as I said: Midi specification is as on seen: midi.org

It will give You the information You need. It's not up to me or anyone to "speculate" about this HERE...

So It's "opto-isolated current loop", where did that get You?! More questions? Lets see what other poople..whoops,. people can say.

Moebius

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I was wondering why the midi out (tx pin) is not opto-isolated on the core module or the midi-merger with a 6N138, like the midi inputs are?

Simple. Because the MIDI in on the next unit down the chain with have optoisolation, and so each piece of equipment will be electrically isolated.

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