From bruce@ashbysolutions.com Thu Jan 03 06:23:44 2002
Subject:Re: midi

Larry,

>So I finally painted myself into a corner, midi-wise. I need to
>be able to switch the midi stream on/off. Used to do this on the
>audio side, but with FrankenClone RevB, need it on the midi side.
>
>Guessing this will work: midi spec shows pins 2,4,and 5 in use. 2 is
>ground, 4 is +5VDC, and 5 is data stream.
>
>Will a simple SPST switch on pin 5 work to kill the data stream and
>be able to re-extablish communication?? Assuming, of course, using
>the switch when not holding down notes or other simultaneous functions.

This will work, but be extra careful if you're planning on kicking it in midstream. The software side of the MIDI spec makes the assumption that the connection is always in place, and things like running status and MIDI clocks can get messed up if only part of the data is received.

>Not the most elegant way to work it but it'll sure get me going
>if I can use such a system. BTW, thought the midi spec shows pin2 as
>ground/shield, I've yet to cut open a "midi cable" that did this.

The MIDI spec shows Pin 2 as ground on the OUT and THRU jacks only; on the IN jack it is "NC" (no connect). Pin 2 is really only used as a method to get ground to the shield, not as a full connection, so cable manufacturers that use DIN jacks that provide a direct shield path will often not bother with trying to tie the shield to Pin 2.

Also, just for completeness: Pin 4 is not raw +5VDC; it's +5VDC though 220 ohms, similar to what computer manufacturers do on their ports.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ http://consult.ashbysolutions.com
CloneWheel Support Group and HiNote moderator
978.386.7389 voice/fax
bruce@ashbysolutions.com