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A cheap MIDI connector box
for the Sound Blaster card
v1.0
by
Adam Mirowski
mir@chorus.fr
==============================
Sound Blaster has a "built-in MIDI interface", but "all you need
is a MIDI Connector Box (optionally available) in order to connect
your SB to MIDI instruments or keyboards", says the add on the
box. The list price for that gadget is as high as $79.95 and
anyway it is not available here in France. Also, the MIDI
interface is not compatible with the MPU-401 standard (and de
facto norm) and is only one-way-at-a-time. So the box is not worth
the money, except if you try to set up it by yourself. For $5 you
can get the basic functionalities and for $10 everything which
comes with the original one.
Parts
=====
Qty Name Type Tandy ref
---------------------------------------------------
1 Optocoupler 4N25 276-9294
3 Resistors 220 ohm 271-015
1 Resistor 1 Kohm
1 AND gate 74LS08 276-9276
2 DIN plugs Female 5 pins 274-9110
1 Canon DB plug Male 15 pins Not sold
[1] Diode 1N914
1 Veroboard (*) 5cm by 5cm
8 Wires Not shielded
1 Box to put all that stuff inside
[] - didn't use it myself.
(*) I am not sure how it is called in English. It is a piece of
epoxide with a raster of holes spaced at 2.54mm. On one side
there are preprinted copper tracks along each line of holes
(in one direction only). You cut off unused portions of the
tracks and join the other with straps, so as to get a
primitive form of a printed circuit board. You then insert the
electronic parts and solder them.
Sound Blaster MIDI/Game connector REAL cabling
==============================================
+------------------------------------------+
! Pin Signal !
!------------------------------------------!
! 1 +5V !
! 2 X button for joystick A !
! 3 X potentiometer for joystick A !
! 4 Ground !
! 5 Ground !
! 6 Y potentiometer for joystick A !
! 7 Y button for joystick A !
! 8 +5V !
! 9 +5V !
! 10 X button for joystick B !
! 11 X potentiometer for joystick B !
! 12 MIDI out !
! 13 Y potentiometer for joystick B !
! 14 Y button for joystick B !
! 15 MIDI in !
+------------------------------------------+
The interesting pins are 12, 15, 4 (or 5) and 9 (or 8 or 1).
BTW, if your joystick Y cable assumes that pin 12 is ground (as it
is on a normal game port), it will not work with the SB.
Interface cabling
=================
The cabling is roughly the same as in the MIDI 1.0 Specification,
except that I used a different optocoupler and "AND" gates instead
of inverters (simply because Tandy sold the former with docs :-).
Numbers between parentheses are pin numbers. ICs have a standard
pin numbering scheme. Other parts have pin numbers written on
them. For resistors, it's simply to make difference between the
two ends.
6 5 4
+--!----!----!--+
Usually ! ! IC seen from the top
a hole --> > !
! !
+--!----!----!--+
1 2 3
"MIDI-in", "MIDI-out" design the female MIDI plugs. "Game/MIDI"
designs the 15 pin female connector on the SB.
FROM PIN TO PIN
MIDI input part
---------------
MIDI-in (4) Resistor1 220ohm (1)
Resistor1 220ohm (2) Optocoupler (1)
Optocoupler (2) MIDI-in (5)
[Optocoupler (1) Diode -]
[Optocoupler (2) Diode +]
Games/MIDI (9) Resistor4 1Kohm (1)
Resistor4 1Kohm (2) Optocoupler (5)
Games/MIDI (15) Optocoupler (5)
Optocoupler (4) Game/MIDI (4)
MIDI output part
----------------
Games/MIDI (12) AND gate (1)
AND gate (1) AND gate (2)
AND gate (3) Resistor2 220ohm (1)
Resistor2 220ohm (2) MIDI-out (5)
MIDI-out (4) Resistor3 220ohm (1)
Resistor3 220ohm (2) Game/MIDI (9)
Games/MIDI (9) AND gate (14)
Games/MIDI (4) AND gate (7)
Final remarks
=============
When using a "veroboard", don't forget to cut tracks under the IC
and the optocoupler, or they will be short-circuited.
You will probably also want to setup two MIDI cords, to attach the
interface to an instrument. To stay under $5, you could use male
plugs instead of female ones for the interface and plug them
directly into the instrument.
When preparing the cords, be careful about not swapping the wires.
The pin 4 from one plug must be wired to pin 4 from the other end,
and the pin 5 must be attached to pin 5. None of the other pins
must be wired, nor the overall cable shielded. There is no ground
in MIDI cords.
If you want more outputs, you have to slightly extend the MIDI
output part. I would rather have only one AND gate input attached
to the SB MIDI-out pin and only one MIDI-out plug per AND gate
output. If Creative Labs offer 5 MIDI outs in the original
Connector Box, it is probably because they use simple inverters
(or buffers) and there are 6 per IC. As I previously said, I
bought AND gates only because they had data sheets attached,
whereas inverters/buffers had not...
Direct your remarks to mir@chorus.fr, or to
M. Adam Mirowski
Chorus systemes
6, avenue Gustave Eiffel
F-78182 Saint-Quentin-en-Yvelines CEDEX
FRANCE
My phone number is +33 (1) 30-64-82-00
I obviously won't take any responsibility for the damages you
could do to your Sound Blaster or to your MIDI equipment while
using this interface.
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