The wired interface appears to be a voltage divider so each button returns a voltage.
Power and ground to the control panel is 3 volts (could be a low battery). The normal return voltage on the two data lines is 1 volts.
Pressing the "Fire" button changed PB1 to 2 volts.
Pressing "UP" and "Down" changes PB1 to 1.2 and 1.3 volts
Pressing "Right" and "Left" changes PB2 to 1.2 and 1.3 Volts.
Here is the control circuit board. I am working on the schematic.
Here is the schematic diagram. The resistors were measured in circuit so they are off a little bit. The 32K resistors might be labeled "513" as in 51K.
I rebuilt a solid state relay board to interface to the Nerf missile launcher. I used this same board years ago to control a phone to light message waiting lights....
However when I tapped into the control board two resistors were destroyed. R33 and R34 are gone. I need to come up with a better way to interface to the control board.