MIDI Expander Box
 

For those who use MIDI-capable musical instruments: I've got a nearly
25-year old MD 80 MIDI expander (thru) box that works fine but I've
noticed that connecting the MIDI out from some devices to an input on
the MD 80 doesn't always result in a signal at any of the MD 80's 4
output (thru) ports as there should be. Yet connecting a different MIDI
device the same way works. I'm a bit puzzled as to this incompatibility
given the relative simplicity of a MIDI connection. Your comment and
time is greatly appreciated. Sincerely,
--
J. B. Wood e-mail:

MIDI Expander Box
 

"J.B. Wood" wrote in message ...

For those who use MIDI-capable musical instruments: I've got a nearly
25-year old MD 80 MIDI expander (thru) box that works fine but I've
noticed that connecting the MIDI out from some devices to an input on
the MD 80 doesn't always result in a signal at any of the MD 80's 4
output (thru) ports as there should be. Yet connecting a different MIDI
device the same way works. I'm a bit puzzled as to this incompatibility
given the relative simplicity of a MIDI connection. Your comment and
time is greatly appreciated. Sincerely,
--
J. B. Wood e-mail:






Perhaps the MD80 doesn't like MIDI active sensing?

A device with active sensing will send sense data every 250 mSec I think.
Anyway you can find out which devices do and don't do this by inserting an
LED into the two pins either side of centre of the MIDI out socket, and see
if it flashes regularly.
(The LED has to be connected the right way round)




Gareth.

MIDI Expander Box
 

On 06/02/2014 12:44 PM, Gareth Magennis wrote:

Perhaps the MD80 doesn't like MIDI active sensing?

A device with active sensing will send sense data every 250 mSec I think.
Anyway you can find out which devices do and don't do this by inserting an LED into the two pins either side of centre of the MIDI out socket, and see if it flashes regularly.
(The LED has to be connected the right way round)

Thanks for taking the time and providing the tip, Gareth. Sincerely,
--
J. B. Wood e-mail:

MIDI Expander Box
 

J.B. Wood wrote:

For those who use MIDI-capable musical instruments: I've got a nearly
25-year old MD 80 MIDI expander (thru) box that works fine but I've
noticed that connecting the MIDI out from some devices to an input on
the MD 80 doesn't always result in a signal at any of the MD 80's 4
output (thru) ports as there should be. Yet connecting a different MIDI
device the same way works. I'm a bit puzzled as to this incompatibility
given the relative simplicity of a MIDI connection. Your comment and
time is greatly appreciated. Sincerely,

That device looks suspiciously similar to the old Yamaha YME8 thru
box I bought in the late eighties and used for decades. A thru box is
usually a very simple circuit: one optoisolator at each input followed
by a buffer, say one or more 74ls04 or the like with their outputs
wired as a current loop. There is not much that could fail, other than
maybe the power supply filtering capacitors both on the device and the
wall wart. I would check those first.

MIDI Expander Box
 

When I built my MIDI pipe organ in '89 the boards I got for 8031 processor included MC1488 RS-232 line drivers. I ended up using the 1488s to drive MIDI which work better than the TTL chips. Add a diode to the output to prevent the MIDI from going seriously negative. The 1488s have current limiting and do not over drive the optocoupler input. Most were using Sharp PC-900s but I used 6N137s because they were easier to get at the time and work fine also.

The box you describe would have zero latency and pass any MIDI data straight through. While it would square up the transitions it would not correct any bit timing issues. A processor would clean up the possible timing issues but would have at least one byte of timing delay and more if you attempted to actually process the MIDI data. It's not difficult to do even with those old processors.

G²