MIDI Thru Functions as MIDI In???

[Mark Smart]

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?

Another thing I'm wondering is if it is harmful to do this for any length
of time. This strange "feature" is actually useful to me; I was
previously using a switch box to switch between controllers. Just having
them both hooked up all the time is very convenient. Much cheaper than
buying a MIDI merger.

I tried changing the programs all over the place on both the keyboard and
the processor at the same time, to see if it would screw up if MIDI data
came from both places simultaneously, but I couldn't get it to fail. This
would never happen in my live setup anyway.

I would appreciate anyone's opinions on this weird situation.

Thanks!


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*     Mark Smart                               *
*     Network Technician                       *
*     University Communications Inc. (UCI)     *
*     smart at medusa.nn.com                      *
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