MIDI Thru Functions as MIDI In???

[Matthew S. Padden]

Hi..

>
>I made a really weird discovery last night. I have a Roland GR-50 guitar
>synth which I drive sometimes with a keyboard and sometimes with a guitar
>processor (to change programs). Last night I accidentally hooked things up
>like this:
>
>
>guitar processor------------>Gr-50 MIDI In
>MIDI keyboard controller---->GR-50 MIDI Thru
>
>This was, of course, a totally stupid mistake. When I did it, however, I
>was astonished to find that the GR-50 reponded to MIDI data from EITHER the
>guitar processor OR the keyboard!! So MIDI data going INTO the Thru port
>is being treated as input data!
>
>I assume that this means that the Thru port is just wired in parallel
>with the MIDI In port, but I was under the impression that all MIDI ports
>had opto-isolators to prevent this kind of thing from happening. Can
>someone explain this to me?
>

I think that only the MIDI IN ports are opto-isolated, not the THRU or OUT 
ports. I assume it's to help with ground isolation or something like that 
(haven't checked my notes for a while..)

So maybe it is wired parallel; the THRU is sourced from the IN after the 
opto-isolator. This would explain why sending things to the THRU port would 
seem to work. But, with the above scenario, you should get all kinds of 
nightmares when both IN and THRU are receiving things, like you tried. Very 
strange...

--
Matthew S. Padden
Computer Music Research Group
Music Dept.
Huddersfield University
Queensgate
Huddersfield
England HD1 3DH
p: +44 1484 422288 x2402
f: +44 1484 472656
e: m.s.padden at hud.ac.uk