Mon Oct 8 20:04:39 EDT 2012


Midi physical connection[1][2].

1-Way current loop driven from 5V (current source) to the output of
an inverter (current sink) connected through a couple of 220k
resistors limit the current.  The opto-coupler LED is included in this

While the voltage on current source is the UART voltage if the current
loop is open, when closed it will produce a current that is the
inverse of the UART signal, which will again be inverted by the
optocoupler's output transistor/resistor network, to be fed into the
receiving UART.

I cut a HOSA 3ft MIDI cable in half and find 4 wires + shield:

(1) NC   b lack
(2) GND   cable shield
(3) NC    yellow
(4) CSINK white
(5) CSRC  red

Looking at the MIDI output of the BCR-2000 in S-1 mode shows indeed a
5V serial signal on CSRC/red that would go straight into the serial

To make a non-isolated MIDI work, connect the shield from the midi
cable to ground of the receiver circuit, and power the receiver with

[1] http://www.midi.org/techspecs/electrispec.php
[2] http://pinouts.ru/Home/MidiOut_pinout.shtml