[LAD] [semi-OT] midi snakes using CAT5?

fons at kokkinizita.net fons at kokkinizita.net
Sun Nov 1 20:06:04 UTC 2009

On Sun, Nov 01, 2009 at 07:23:27PM +0100, Jörn Nettingsmeier wrote:

> any midi has to carry 5v iirc. and midi is not balanced, so there is no
> signal+ and signal-. would be cool if there was - then 100m would be no
> problem :)

Midi does not carry a 5V supply. You need 5V to (or more) to 
create a midi output, but that voltage is required in the
equipment providing the output only.

A midi input is floating (the input of an optocoupler).

Pin 4 is the '+' side, pin 5 is '-'. Usually one of the
two has a series resistor (220R or so) to limit the current
if accidentally connected to a low-impedance voltage source,
and there may be a diode parallel to the optocoupler to
protect against reversed voltage.

For output, pin 4 is normally connected via a 220R
series resistor to +5V, and pin 5 is the output of an
open-collector logic gate able to sink at least 10 mA,
again with 220R in series. That is the standard way to
make a midi out. But since the input is floating you
could reverse the active/passive parts, e.g. use an
active high open collector output on pin 4 and connect
pin 5 to ground (both again via 220R). You could even
make it active in both states and balanced. But little
is gained by doing that. Some midi receivers may rely
on the 'standard' way to steal some power, but I don't
think the standard provides for this.



Io lo dico sempre: l'Italia è troppo stretta e lunga.

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