Hi, While the MIDI output works, there are two problems with the MIDI input: * the optocoupler load resistor R54 (4.7K) is too high * the optocoupler's current transfer ratio is too low (50%, those recommended for MIDI [1] have current transfer ratios of 2000%)
Even after lowering R54 to 220 ohms, it did not work. The voltage swing is too small (output oscillates between 1V and 3V) and the rise/fall times are too slow (checked on scope with a 49kHz square wave as input, fed from the MIDI output of the board with an oscillator programmed in the FPGA). This results in a large number of transmission errors when connected to a MIDI device. I will order samples of 6N138 optocouplers and test them - those ones should work better. The PCB footprint is not compatible, but I don't think we'll ever find a high-gain optocoupler in a 4-pin package. Sébastien [1] http://www.midi.org/techspecs/electrispec.php _______________________________________________ http://lists.milkymist.org/listinfo.cgi/devel-milkymist.org IRC: #milkym...@freenode Webchat: www.milkymist.org/irc.html Wiki: www.milkymist.org/wiki
