Re: [sdiy] MIDI processor - power from midi in?

[Roman]

If I may comment recent posts with worries about this.

Taking power from MIDI line is done since about the first days if its existence. According to MIDI spec, ground is always present at pin 2 of the DIN socket of transmitter, +5 via 220 ohm is at pin 4. That's a lot of juice for modern micros. Some well known commercial MIDI gadgets use MIDI power. There's no isolation or ground loop issue involved, because MIDI powered device acts like extension cord. There's no third leg of the cable, so no isolation is needed. The only problem begins, when such box has more than 1 MIDI input. Then it should take power only from 1 input, and the other be isolated to comlpy with MIDI. Having multiple MIDI outs with common ground is no problem at all. It's like in any other gear.

Now some comments to the schematic
I'd skip the 3V zener diode. Let it run from almost 5V, and the MIDI out will use more like normal voltage. I usted to make MIDI boards powered from 3V (dual AAA battery) and I quickly moved to 4.5V because smaller voltage caused problems with some devices (mainly soundcards). If only the Attiny can stand 5V, I see no reason for 3V zener. If your total supply current is lower than 5-7mA, I'd also skip the D1, D4 and put bigger C1. You then only take power from DIN-pin-4 via 220 resistor at the transmitter. Then even at 5mA you have less dropout than with 2*220 and diodes.
With almost 5V available, I'd then change R1-R2 to 220ohm to comply with MIDI.

I hope it helps

Roman

---- Wiadomość Oryginalna ----
Od: David Brown <davebr at earthlink.net>
Do: diy synth <synth-diy at dropmix.xs4all.nl>
Data: Wed, 20 Jul 2005 22:10:28 -0700
Temat: Re: [sdiy] MIDI processor - power from midi in?

> Thanks for all the responses.  I've completed a breadboard circuit 
> and it works off of the two keyboards I have tried.  Now I'll have to 
> wire up a more permanent circuit.
> 
> Schematic and the beginning of a web page at
> http://modularsynthesis.com/processor/processor.htm
> 
> Dave 
> 
>