digital joystick interface

The BBC and Electron computers pro- duced by Acorn have a joystick port to which only analogue joysticks can be connected. For many purposes, a digital joystick, i.e., one with four con- tacts, is much more suitable. The interface suggested here enables a digital joystick to be used with the two computers mentioned. The joystick port is provided with a voltage of 1.8 V when the analogue joystick is set to the left or top pos- itions, 0 V with the joystick in the right or bottom positions, and 0.9 V with the joystick in neutral. The 1.8 V is the reference voltage of the analogue-to- digital converter in the computer. As can be seen from the circuit diagram in figure 1, the various voltages can simply be provided by four sets of contacts or switches. Each of the sets of contacts controls an electronic switch. The 0.9 V for neutral is obtained from a potential divider. The electronic switches are required because the contacts in the joystick have a common connection and can, therefor...