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Unlike the ALTAI Joystick Interface, the IJK interface avoided corruption of the sound by using the strobe and Ack lines provided on the Printer Port.

The code may be modified to set both top bits low and therefore permit the combined detection of either joystick movements.

Note all drivers use a common label list that is found here generic_joystick_drivers

The IJK joystick connects to the parallel port and uses the strobe line in a clever way so that it could be prevented from corrupting sound. It was supported in most IJK games afaik. Unfortunately though it is quite rare.

When the Strobe bit was set to Output(In DDRB) and low(PB4) then it enabled the interface.

B7 Select Right Joystick(Sockets were clearly identified)
B6 Select Left Joystick(Unlike Altai setting 11 will disable joys)
B5 Low whenever IJK joystick attached so could detect IJK Interface
B4 Up(Inverted)
B3 Down(Inverted)
B2 Fire(Inverted)
B1 Left(Inverted)
B0 Right(Inverted)

It used very low power radio transistors to avoid overloading the bits on the unpowered Printer Port.

The following code provides a standard output for detection of joystick movements in your own code. The bits corresponding to joystick movement use a look up table to make them comply with the Generic Format.

     GenericIJKBits
      .byt 0,2,1,3,32,34,33,0,8,10,9,0,40,42,41,0
      .byt 16,18,17,0,48,50,49,0,0,0,0,0,0,0,0,0

     GenericReadIJK
         ;Ensure Printer Strobe is set to Output
         LDA #%10110111
         STA via_ddrb
         ;Set Strobe Low
         LDA #%00000000
         STA via_portb
         ;Set Top two bits of PortA to Output and rest as Input
         LDA #%11000000
         STA via_ddra
         ;Select Left Joystick
         LDA #%01111111
         STA via_porta
         ;Read back Left Joystick state
         LDA via_porta
         ;Mask out unused bits
         AND #%00011111
         ;Invert Bits
         EOR #%00011111
         ;Index table to conform to Generic Format
         TAX
         LDA GenericIJKBits,X
         STA joy_Left
         ;Select Right Joystick
         LDA #%10111111
         STA via_porta
         ;Read back Right Joystick state and rejig bits
         LDA via_porta
         AND #%00011111
         EOR #%00011111
         TAX
         LDA GenericIJKBits,X
         STA joy_Right
         ;Restore VIA PortA state
         LDA #%11111111
         STA via_ddra
         RTS

This provides the following bits to joystick movements.

The following routine may be used to detect the presence of the IJK joystick interface.

Please note it has not been tested against having other Joystick interfaces plugged into the printer port instead.

         LDA #%11000000
         STA via_ddra
         STA via_porta
         LDA via_porta
         AND #%00100000
         BNE ijkPresent

The above code has not been tested.

Thanks to Fabrice Frances for the autodetection technique.

Diagrams by “Silicebit”.