[sdiy] Midi powered?

[Jim Robertson]

Not sure about the 3 connection MIDI hookup.  Never
seen that before.  But without digging into the MIDI
spec.....

Standard MIDI connections should be operating as a
two-wire "current loop".  When the connection is idle
(no data), the receiving device would see a steady +5V
level.  During data transmission, the start bit, stop
bit, and data bits with a value of "1" are represented
by a 0V level.  

I believe that devices powered from the MIDI line are
simply filtering the "data" input itself (via a
capacitor) to provide a steady +5V to power a circuit
with very modest consumption.  

Since the exact MIDI output circuitry used by any
given manufacturer may vary somewhat in its design, it
would follow that each device will have a different
tolerance for powering external devices.  Most MIDI
inputs use optoisolators so the MIDI output isn't
necessarily expected to drive much more than a couple
LEDs.  

If there are devices that use 3 connections (i.e. one
of them is a power pin for external devices), then you
could also run into problems since some MIDI cables
only use 2 of the pins.  This is because only 2 pins
are "required" for MIDI transmission.

I would carefully check the spec sheets of your
devices.  It is oversimplifying the matter to say that
older devices draw more current than modern devices. 
There are too many other factors.  Though I realize
that Colin is generalizing here, and for the most part
the statement is probably accurate.  I also agree that
there would likely be a resistor on that data line (to
protect the output transistor/gate) and it will limit
the available output current.

Jim, US Air Force 
Ramstein AB, Germany
Usually just a lurker.


  

> The typical midi output has 3 connections - +5v, 0v
> and data.
> Both the data and +5v lines are usually protected by
> small (~200 ohm)
> resistors.
> So, you can draw a small amount of current from the
> +5v line to power
> something, but the more you draw, the more voltage
> drop you will get due
> to the resistor.
> You may find that some devices have larger resistors
> on the +5v line, so
> line powered devices wont get enough voltage.
> Modern devices can be powered easily from the midi
> line, as they draw tiny
> amounts of current, but older devices may be
> marginal.
> 
> Colin f
> 
> 
> 
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