Most MIDI output circuits that I see use a 220R resistor from the PIC=20 pin to the negative side of the output jack and another 220R resistor=20 from the +5V rail to the positive output pin of the output jack. The=20 opto LED inside the receiving device usually has another 220R=20 resistor in series (Oops protection). I've seen this far to often to doubt that it works reliably. And the=20 numbers justify that feeling: you should see somewhat more than=20 5.3mA, assuming a nominal 1.6V drop across the opto LED and a nominal=20 5V rail. The receiving opto is spec'd at 5mA and works reliably down=20 to about 2mA. You can safely drop the 220R in series with the +5V supply down to=20 100R with no ill effects. I'd still keep the PIC pin with a 220R=20 series resistor (more Oops protection). dwayne At 10:26 AM 9/3/2013, Jacopo Monegato wrote: >Hello there, I am designing a simple MIDI controller with a 16f628.=20 >For those who don't know MIDI communication is done through a 20 mA=20 >current loop. > >The MIDI Standard Document states that you should put a buffer or an=20 >inverter after the micro's TX pin, depending by the capability of=20 >the micro to run with non-inverted/inverted output, so that the=20 >inverter could be a single NPN in case of inverted output. However,=20 >the PIC USART module doesn't have the inverted output mode so it=20 >needs a buffer to operate properly, according to the standard. --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .