[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

Fri Sep 11 11:21:18 CEST 2015

I'm jumping in a bit late, and it is so interesting subject.

It would be awesome to have well defined new physical interface, but 
since it stays like that over 30 years now, noone would dare to make 
something incompatible with older units. I always dreamt about MIDI over 
light, using the same stuff as SPDIF. The plastic lightpipes are dirt 
cheap, TX/RX modules are no fortune either. And if some manufacturer 
would design DIN socket wih center hole for optical plug, imagine 
possibilities! Something like XLR/TRS combos.

Anyway back to reality.

You can draw power from MIDI line because:
1. it's been done since the next day MIDI was invented
2. small amount of power MIDI line provides is more than enough for 
today's micros
3. no need for battery or walwart gives so much more flexibility that 
customers can accept a few limitations, like not 100% compatibility
4. most of MIDI circuits use the same topology with a resistor connected 
to +5V

OTOH you cannot draw power from MIDI because:
1. not every gear has pin 2 connected to GND
2. some MIDI OUT interfaces turn off a few ms after sending a byte, so 
there's no voltage between any pins during idle time
3. such devices can be chained, but if you chain several of those, 
obviously at some point there will be not enough power for all. There's 
no way to prevent users from chaining them without limits and then complain.

Roman

W dniu 2015-09-11 o 00:42, eidorian at aladan.net pisze:
> Thanks Neil, this is an interesting (if somewhat disappointing) read.
> It's a shame they haven't come out with a more forceful position on this
> given how long they're had to do so.
>
> That said, it's pretty clear that to add any sort of genuinely _useful_
> power specification to the MIDI standard risks introducing
> incompatibilities that are just not safely backwards compatible with
> existing equipment.  I think the best approach would be a new physical
> interface, with simple (i.e. unpowered, ideally) but well-defined
> converters to allow for integration between old and new.  I won't hold
> my breath though :)
>
> And yeah, I personally won't be powering anything from MIDI in the near
> future, and I have no plans to use any MIDI Solutions gear!
>
> Cheers,
> A.
>
>
> On 2015-09-10 18:30, Neil Johnson wrote:
>> Found the discussion I had with the MMA on this issue:
>>
>>
>> http://www.midi.org/cgi-bin/forum/forum.pl/forum.pl?m-1171180161/s-all/
>>
>> Neil
>>
>> On 10 September 2015 at 09:58, Neil Johnson <neil.johnson71 at gmail.com>
>> wrote:
>>> What I find particularly funny/frustrsating/disappointing is how many
>>> people don't understand the MIDI spec.
>>> All this talk of "5V" and "220R resistors" completely misses the point.
>>>
>>> *sigh*
>>>
>>> Here's a clue: the text of the standard makes no mention of voltage.
>>> Chapter 2, page 1, "HARDWARE".  The circuit given on page 2 is an
>>> illustration of one implementation.  I've also discussed the wording
>>> in the standard with the MMA and their response to the vagueness in
>>> the standard is "well no-one's pointed this out before, and we don't
>>> quite understand why the spec was written in this way, so, umm, la la
>>> la la..."
>>>
>>>
>>> Neil
>>>
>>>
>>> On 10 September 2015 at 09:29, Ove Ridé <nitro2k01 at gmail.com> wrote:
>>>> On 10 September 2015 at 09:40,  <eidorian at aladan.net> wrote:
>>>>> I just re-read the original post, and it might be that JP isn't
>>>>> asking how
>>>>> he can get power from a MIDI cable, he might actually be asking a
>>>>> subtly
>>>>> different question: how can he *send* more power over the two MIDI
>>>>> pins
>>>>> (presumably without affecting the ability of those pins to transmit
>>>>> MIDI
>>>>> to
>>>>> older devices at the same time).
>>>>
>>>> The MIDI-DIN states that pin 2 should be connected, if nothing else,
>>>> through
>>>> the shield. Are there really cables around that have pin 2 completely
>>>> unconnected?
>>>>
>>>>> But yeah, there's no fun to be had doing this.  Any device that
>>>>> requires
>>>>> your "high powered" MIDI-out ports for power is going to damage your
>>>>> "normal" gear if you ever plug it in accidentally :-(
>>>>
>>>> I don't see how. A compliant, regular output device should have 220 ohm
>>>> resistors in series on both pin 4 and 5 to limit the current and should
>>>> handle even being indefinitely shorted. Likewise, a regular input
>>>> device
>>>> will have 220 ohm in series with the optocoupler. 5/220=23 mA and
>>>> that's
>>>> being really pessimistic and ignoring the LED forward voltage drop.
>>>>
>>>> --
>>>> /Ove
>>>>
>>>> Blog: <http://blog.gg8.se/>
>>>>
>>>> "Here is Evergreen City. Evergreen is the color of green forever."
>>>>
>>>>
>>>> --
>>>> /Ove
>>>>
>>>> Blog: <http://blog.gg8.se/>
>>>>
>>>> "Here is Evergreen City. Evergreen is the color of green forever."
>>>>
>>>>
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>>>>
>>>
>>>
>>>
>>> --
>>> --
>>> http://www.njohnson.co.uk
>
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