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Arcade joysticks use physical switches which are normally used to ground logic circuits in an arcade board set. The IBM PC on the other hand, has a joystick port which expects an analog signal from the joystick. Interfacing between the two is really quite easy once you've done it, but it can be confusing at first. The PC joystick port has 2 inputs for each joystick. Each input is reserved for an individual axis movement. For joystick #1, pin 6 is used for the up/down signal (y axis) and pin 3 is used for the left/right signal (x axis). A PC joystick uses potentiometers to send a variable voltage signal on the pins, indicating the direction and degree of movement. The PC then measures the voltage and produces an X/Y coordinate of where the joystick currently is. On the Y axis, voltage levels vary from 3 volts (centered) to 5 volts for up and 1 volts for down. Most joysticks don't actually pull the joystick voltage down to 0 because a positive signal is required for the PC to determine a joystick is present. On the X axis, the same voltage rules apply, with center being about 3 volts, left being 5 volts and right being 1 volt. Translating joystick switches to a variable voltage can be done easily using resistors. The schematic below receives +5v from the joystick port and directs it back into the appropriate axis pin at the correct voltage level. If the joystick is pushed up, all 5 volts are directed back. If the joystick is centered, only 3 volts are returned. Please note that the exact resistor and voltage levels are not important. There simply must be a distinct voltage for up, center and down which decreases as the stick moves downward.
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Second Chance Arcade E-Mail: todd1814@yahoo.com |
