Part 1:making an interface for Atari Trackball to TI-99/4a

This is a two part project. My goal is to interface an Atari trackball to a TI-99/4a.  Part one will be a tear apart and cleaning of an Atari CX-22 Trackball.

Well to make an interface for an Atari Trackball to a TI-99/4a I will need an Atari Trackball. I picked this one up locally for about $15.00.


It is a CX-22 trackball. There were actually two version  and two releases of the Atari trackball: the two versions were the CX-22 which had an XL style (pictured above) and the CX-80 which had a black ball and triangle shaped buttons and was more styled for the older 8-bits. Each version also had two releases, an earlier release that was not compatible with the ST in trackball mode and later release that was.

All had a joystick mode, which emulated a standard Atari joystick, and the above mention trackball mode. I haven’t found any programs that actually used the trackball mode except some hacks. For this project the trackball mode will be ignored.

The trackballs were never very popular. As I said, very few games took advantage of the trackball mode. In fact, I haven’t been able to find any games that utilized the trackball mode except hacks for the 2600 and 8-bit line. In reality though all games could be used in the joystick mode but not all played well with the trackball. Missile Command and Centipede were probably the best games to use with the trackball but even they only utilized the joystick mode.

Back to the trackball I got for $15.00. It worked, but the movement was very erratic and the buttons were not very responsive. So, I opened it up and this was what was inside.


A you can see a major cleaning was in order. Bug guts, rust and just plain dirt needed to be cleaned out for this device to work properly.

First thing I did was remove everything and clean the insides thoroughly with soap and water.


The next thing was to get the switches to work properly. The switches were standard pop metal switches with a piece of tape over the metal. The tape was pealing and the internal contacts were so corroded that they were no longer getting good contact. Here is the mess of a switch I pulled out.


Even with this poor picture you can see how bad off the switch was.

I removed the tape and cleaned the contacts with alcohol and a light sandpaper. I then re-taped the pop metal button on the circuit board and tested. It tested good.

Next I cleaned and sandpapered the rollers then sprayed the bearings with WD-40. Smooth as glass.

A little discussion of how the trackball works. Refer back to the top picture and you can see that the rollers have a big black wheel attached to them. when the ball moves over the roller the wheel turns. In the wheel are little holes. On one side of the wheel is a small light source. On the other side is a photovoltaic sensor. Each time the wheel turns a flash of light goes through one of the holes to sensor. the sensor registers one movement in that direction. One wheel is for up-down and one wheel is for left-right. This is also how the early ball mouse worked, except upside down.

After the cleaning I put the trackball back together and it looked like it did before, except this time it works as it was meant to.

I tested it with some games on my Sears Video Arcade II (2600 clone) and my 800XL. works like a charm. Best game was Buck Rogers. Worst; Pac Man.

Next Post will be the actual construction in the interface to the TI-99/4a.