Comments on Brian McClintock's Soundblaster MIDI Interface Design

Richard Steedman [100332,3116]

Having just completed building one of the MIDI interface circuits described in
Brian's article, I would like to point out a couple of things which should help
anyone else undertaking this project.

1.  It is normal to connect pin 2 of MIDI OUT ports to Ground (but not MIDI IN
ports - otherwise earth loops may result).

2.  The interface circuit from Soundblaster card to MIDI works fine, but
concerning the circuit in the opposite direction (i.e. from MIDI to
Soundblaster), I have found that simple opto-isolators such as the 4N35 are
just not fast enough to switch at the MIDI operating speed (31250 baud).  To
get my circuit to work, I replaced the 4N35 with a 6N139 wired up as follows:

   Pin 1    n.c.
   Pin 2    MIDI IN pin 4
   Pin 3    MIDI IN pin 5 via 270 ohm serial resistor
   Pin 4    n.c.
   Pin 5    Ground
   Pin 6    RXD (with 390 ohm pull-up resistor to +5V)
   Pin 7    1 kilo-ohm resistor to Ground
   Pin 8    +5V

Disadvantages of this approach are that the 6N139 is much more expensive than a
simple opto and that it is not pin compatible with Brian's existing circuit. If
you have already assembled the circuit and don't wish to re-wire things, you
may find that a pin-compatible isolator with a Darlington output stage (e.g.
4N32 or 4N33) may be fast enough. I haven't tried this, though.

Good Luck!
