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. Ciao, -- FA Io lo dico sempre: l'Italia è troppo stretta e lunga. _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/mailman/listinfo/linux-audio-dev
