JOYSTICK

11/29/97

Joystick: The classical PC Joystick consists of two 50K variable resistors mounted at right angles such that the resistors indicate the position of a movable control rod that is free to swing about in two dimensions. The joystick also includes two pushbuttons. Joysticks are used primarily for playing computer games.

The Joystick input is generally through a port at Port address 201x. Unlike other PC input devices, the Joystick does not use interrupts. Instead, it consists of four 555 timers or their equivalent whose level change is triggered by a capacitor and the joystick resistor. The signal level change must be detected by software and the time from reset to the signal change must be measured. This process is not very compatible with multitasking operating systems such as Windows NT or Windows 95. Fortunately, high precision is not required for most joystick applications. Joystick precision and readout frequency are dictated by the resistance being measured. Resolutions of a hundred ohms and readout frequencies of 10-20Hz probably represent practical maxima.

Joysticks use a 15 pin female D connector. A few soundcard joystick ports incorporate MIDI signals onto a couple of previously unused pins.

The joystick port can be, and sometimes is, used as an inexpensive voltmeter or ohm meter. While adequate for many hobby or Science Fair purposes, there are better approaches to serious measurement needs.