[sdiy] Midi opto isolation

[Seb Francis]

harrybissell at wowway.com wrote:
> Many problems can be cuased by incorrect design in the MIDI
> isolation / cables.  The cable "ground" should only be connected at
> one end (the transmitter).  The receiver should only have the two
> LED pins connected.  A sure sign of this is if one order of modules
> a-b-c works but a-c-b does not.
>
> Look for genuine MIDI cables that have onlt two conductors. This will
> ~not~ work for many drum machines that use the extra pins.
>   

I don't think this is quite right: MIDI cables are twisted pair with 
shield, connected at both ends (i.e. 3 conductors total)
Ground loops are prevented by the fact that the shield connection (pin 
2) is connected only in OUT or THRU sockets and not IN sockets.

Using 'audio' cables rather than proper MIDI cables causes problems 
because often these cables have the shield connected to the DIN plug 
metal shell and so the chassis of the pieces of equipment become linked 
and hence ground loops.  Probably also the fact that such cables aren't 
twisted pair may have some effect for longer cables.

Seb