[sdiy] Midi opto isolation

[harrybissell at wowway.com]

Yes, I agree. The problem is with devices that did not follow the
spec and round pin 2.  My Midi-Bass II did exactly that, and even
shorted the +5V ground directly to the chassis as well. Must have
worked real well... in the lab :^P

H^) harry



On Wed, 02 Jan 2008 18:56:10 +0000, Seb Francis wrote
> 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


Harry Bissell & Nora Abdullah 4eva