This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
If you've compared the pin-outs and info for Apple II and PC joysticks, then you know there are some important differences: The Apple II stick uses a 9-pin plug vs. the PC's 15-pin plug. (Older Apple II sticks may use a 16-pin plug which fits in an IC socket.) The Apple II stick's X, Y controller potentiometers are a bit larger. The buttons are wired differently. You can use an Apple-to-PC adaptor (such as the one supplied with the Epyx A2/PC joystick) to handle plug conversion; or, you can replace the entire cable with one from an old PC stick. The PC's joystick interface will work with the Apple2 150k pots; but, in some applications, you may notice a tendency to max out early in the stick swing. You can correct this by connecting a 300k resistor across each pot (from the center to the end with a wire going to it). The difference in button wiring is the main reason an Apple-to-PC conversion involves opening the joystick and making changes. (The Apple stick has a slightly more complex, less flexible circuit. Apple2-to-PC is not as easy as PC-to-Apple2.) Basically, you need to change the Apple stick's button wiring so that it looks like the PC stick's button wiring. The mods mentioned above are not difficult, especially if you swap in a PC cable. If you want to be able to use the stick on an Apple II, then some kind of switching will be required. Apple II Joystick(9-pin male connector) (Old 16-pin IC-style plug) --------------- +5V ------- 1 --------------- Button 0 ------- 2 --------------- X-axis ------- 6 --------------- Y-axis ------- 10 --------------- Button 1 ------- 3 --------------- Ground ------ 8PC Joystick (15-pin male connector)--------------- +5V  -------------- Button 0  -------------- X-axis  -------------- Y-axis  -------------- Button 1  and/or  GroundBoth sticks tie one end of each X, Y potentiometer to +5 and send the center (wiper) to the an output. (Or the wiper may go to +5V and an end to the output; it doesn't much matter.) The standard Apple II pot is 150K Ohms; most PC sticks use 100k Ohm pots. The buttons are wired differently. On the Apple II stick (see below), each button switch goes to +5V. The other end goes to GND through a resistor (one resistor for each button). A button's Output is from the junction of the switch and its resistor. When the button switch is not closed, its Output is near 0V (=logic 0). Pressing a button sends +5V to the output (= logic 1).+5V | | X Button Switch | |_____Button output to Apple (Press => "1") | Z Z 680 Ohm resistor Z | GNDAs shown below, a PC stick button Output is normally an unconnected wire. Most likely, inside the computer, a PC or compatible Game Port has this line tied to a 1k-3k resistor going to +5V. So, the line will normally be at something close to +5V (= logic 1). Pressing the button grounds the line and pulls it down near to 0V (= logic 0)._____Button output to PC (Press => "0") | | X Button Switch | | GNDApple2-to-PC Joystick Conversion: Step-by-Step DOING THE CONVERSION To convert an Apple2 joystick for PC use you will need a cable from an old PC stick (or a 6-wire cable and 15-pin male connector). You can find junk PC sticks with good cables at flea markets and lots of other places. You will also need two 330k resistors. 1. First, open the Apple joystick case and mark each wire going to the cable. The best way is to use small self-stick labels. Label each wire by function (e.g. "+5", "X", "GND", etc.). You can use an Ohm meter to, for example, verify that the wire you think is Button 0 really goes to pin 7 on the Apple 9-pin connector (or pin 2 on the old 16-pin IC style connector). Pinouts for both kinds of A2 sticks are shown below: Apple II Joystick (9-pin male)---- +5V ---- Button 0 ---- X-axis ---- Y-axis ---- Button 1 ---- GroundApple II Joystick (16-pin IC-style plug)---- +5V ---- Button 0 ---- X-axis --- Y-axis ---- Button 1 ---- GroundThe +5V wire is easy to find. It will go to each pot and to one side of each Button switch. The Apple2 Ground wire goes to the 'bottom' end of each fixed resistor. You do not need to label it; because it will be removed. 2. Once the wires are labeled, cut each about 1 inch from the point it goes into the cable. Remove the cable. (Keep the cable; it may come in handy for some later Apple2 project.) Now, is the time to rewire the Buttons. 3. Remove (snip or unsolder) the two fixed resistors. If the Button 0 or Button 1 lead becomes disconnected from its switch during removal of a resistor, reconnect the lead. Check to see that, now, the Button 0 wire is the only one going to one side of the Button 0 switch. The same goes for the Button 1 wire. Snip off or unsolder the +5 leads going to the other side of each button switch at the non-switch end. If a wire runs from one switch to the other, leave it alone. If not, connect a wire from switch to switch. This is the "common" side of the switches. You want to end up with a single wire going to the common side of the switches and separate Button 0 and Button 1 wires going to the other side:____Button 0 wire | | X B0 Switch | | ------- COMMON Wire | | X B1 Switch | |____Button 1 wireLabel the COMMON wire as "GROUND" 4. The 330k resistors will help bring the outputs of the Apple2 X and Y 150k Ohm pots closer to the 0-100k range PC prefers. Connect a 330k resistor 'across' each pot-- i.e. from the center post to the post going to a +5 lead. 5. Label each of the leads coming from the PC cable. If it is still connected to a joystick, the following pic will help identify each lead: PC Joystick (15-pin male connector)--------------- +5V  -------------- Button 0  -------------- X-axis  -------------- Y-axis  -------------- Button 1  and  ------ GroundIf the cable is still connected, snip the the leads once they are all labeled. If both Ground ( and ) leads are present, twist them together and treat like a single Ground lead. FINISHING UP 6. You have six labeled wires in the Apple2 joystick case: +5, GROUND, B0, B1, X, and Y. The same six leads are labeled on the PC cable. Splice each Apple2 wire to the corresponding PC cable wire. Use heat-shrinkable tubing to cover each connection. 7. Seat the new cable in the joystick case, arrange leads to avoid mounting posts, etc., and close up the case. Viola! CHECKS If you have an Ohm meter here are some checks you can do: X (Horizontal) Check- check R between cable pins 1 and 3. As you move stick left to right R should go from 0 to about 100k. Y (Vertical) Check- check R between cable pins 1 and 6. As you move stick up to down R should go from 0 to about 100k. Button 0 Check- (Button 0 is the main, "Fire" button.) cable pins 2 and 4 or 5. It should be very high and go to 0 when Button 0 is pressed. Button 1 Check- check R between cable pins 7 and 4 or 5. It should be very high and go to 0 when Button 1 is pressed. TRYOUT Plug in the stick and try it with a game. Some games (such as Elite Plus) will claim no joystick is present if the stick is badly out of adjustment. If this happens, try the stick on a game which is less picky and includes pre-play stick adjustment. Once adjusted, your 'new' stick should work fine with all PC wares.