From bruce@ashbysolutions.com Tue Oct 09 12:29:49 2001
Subject:Re: Merge box problem

James,

At 06:49 P 10/9/2001 +0100, you wrote:
>Bruce wrote
>
><< Line-powering in MIDI is a tricky concept. The MIDI specification
>(v1.0) doesn't even address it, so every unit that uses the "feature" is
>working in a gray area. I've never used the Philip Rees box, but I had
>problems
>with my MIDI Solutions router in certain configurations. >>
>
>The word according to Roland was that the MIDI out port should send an
>"I'm alive" pulse every so many ms. Some keyboards don't adhere to this
>spec so they can't power these line powered units.

Ah, tech support teams -- gotta love 'em. Unfortunately, that explanation
has little to do with the problem...

MIDI communication hardware is designed so that a logic '0' (i.e., no data)
is created by turning the current on in the circuit, while logic '1' is
current off. So therefore, sending no data at all produces a continuous
current to the output devices, allowing them to charge up. The problem
time is when lots of devices are talking on the cable, where a long stream
of '1' bits can starve the line-powered device.

What Roland is talking about is called "Active Sensing," and is used as a
software handshake between units. Unfortunately, this completely optional
feature messes up many MIDI devices, particularly when using several MIDI
channels at once, because it requires a device on the same channel to
respond quickly with an "I'm alive, too" message. For example, the V3 has
problems with these messages: I had lots of stuck notes using my Roland
D-50 as the controller for my V3, until I used a MIDI Solutions Router to
filter out the Active Sensing; after that, the V3 settled down. As another
example, MIDIman USB interfaces appear to filter out the messages, even
when used in "thru" mode: I can see the pulses on the input LEDs, but no
corresponding pulses on the outputs. This is probably done as a safety
measure to prevent lockups in a complex mapping.

>All my Roland keyboards work fine with my MIDI solutions merger, but I
>have some master keyboards made by less 'well known' manufacturers that
>mean I always have to make sure I am merging a Roland MIDI out with one of
>these for the merger to work properly. The cheapo controller I use for my
>pedalboard is guilty in this respect. Seems that not everyone has
>implemented the original spec, because if they did, these line powered
>units would work just fine.

It more than likely has to do with other issues, besides Active
Sensing. Some manufacturers don't provide a ground connection on the MIDI
Out jacks, which can inhibit self-powering. Others don't use 5V as the
power source, and even others turn off the power when no data is present,
to save batteries. (This last case is the only place where Roland's
explanation might make any sense -- but even there, the Roland synth is
sending Active Sense pulses every 300mS, so the next device in the chain
should never go to sleep.)

And as I mentioned earlier, I've had MIDI merge issues on Roland gear, too.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™
www.ashbysolutions.com
CloneWheel Support Group moderator
978.386.7389 voice
978.964.0547 fax
bruce@ashbysolutions.com