[sdiy] Midi opto isolation

[Roy J. Tellason]

On Tuesday 01 January 2008 13:30, Grant Richter wrote:
> Ha, ha, ha...
>
> This is actually a throwback to before the 8 bit microprocessor world.
>
> Before the RS-232 suggestion, there was the 20 ma current loop.
>
> MIDI is based on that technology.
>
> These were used for teletype terminals, before the advent of the
> "glass teletype" you are using now.
>
> I guess the theory was that it would be harder to induce a noise
> current than a noise voltage. So they must have picked it for
> reliability, the interface parts cost is more expensive than RS-232.

Noise immunity is not the issue so much as ground isolation.  You start 
connecting together a bunch of different equipment without that and you'll 
end up with ground loops...

The current involved is a *lot* smaller than 20mA.

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