Page:Gametronics Proceedings.djvu/183

 Electrically, the joypad emulates two potentiometers with a common wiper (see Figure #1), and that is precisely the symbol we have adopted for it. The simplistsimplest [sic] method for using the pad involves placing two sets of diodes across the orthogonal axes and impressing an alternating voltage across them, picking off X and Y information on each half cycle. The method outlined in Figure #1 is slightly more complex, using two analog gates rather than diodes to switch each orthogonal axis on and off. It has, however, the advantage of using only a single ended D.C. supply. To explain the operation of the circuit, a two phase D. C. clock (Note: It is not necessary that the pulse widths of the clock be equal) is used to switch a reference supply "V" alternately across the X and Y axis of the pad, simultaneously switching the wiper into X and Y detection circuits. The output X and Y pulses have amplitudes as a function of the wiper position. These pulses may be smoothed and used as control voltages (See Figure #2) or by properly selecting the pulse widths, the pulses can be used directly to control some external circuitry or device such as a T.V. game, computer cursor or machine tool.

In conclusion, there is now available to the designer a simple, inexpensive input device, rugged enough to withstand the abuse of consumer applications. The door is now open to applications in interactive graphics, character recognition and general control. 1 (ELLIS 1) T. O. Ellis and W. L. Sibley, "On the Development of Equitable Graphic 1/0", IEEE Transactions on Human Factors in Electronics, Vol. HFE-8, No. 1, March 1967.

Fletcher, W. E., "ON-Line Input of Graphical Data", presented at the 1963 Digital Equipment Users Society Meeting, Livermore, California.

Note Lisa's control button.