versatile interface

[Cyrus Roton <croton@ridgecrest.ca.us>]

GAME PORT, VERSATILE INTERFACE

A multitude of interface cards exist for the Apple II But, few 
people realize that the Apple II game port can be a very 
versatile interface.

A "paddle" input is internally connected to a trigger chip and 
a .022 MFD capacitor. The paddle is a variable resistor and a 
voltage source. In use, the capacitor is discharged, a timer 
(counter) is reset, and then the capacitor is allowed to charge 
by current passing through the external resistor while the 
counter is incrementing. When the charge on the capacitor reaches 
3.2 volts, the trigger circuit stops the counter. The number 
stored in the counter at that time gives an indication of the 
time required for the capacitor to charge up to the trigger 
voltage, and that time relates the value of the external resistor 
(paddle) as follows;

V = (input voltage)*(1-EXP(-t/R*C))