

                       ==============================
                         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.
