Part 2:making an interface for Atari Trackball to TI-99/4a

Well, last time, I cleaned up the Atari CX-22 trackball and made it usable. This time I go to making an interface for the trackball to a TI-99/4a.

A little review, the Atari trackball has two modes: the trackball mode (that is rarely used) and the joystick mode. I recently had a little discussion on the Atariage board about the trackball mode.

Here is a big write up in the Atariage 2600 forum on the trackball. The trackball was used a lot more for the 2600 then the 8-bit computers.

interesting side note: since Atari changed the trackball mode in the middle of it’s production only certain trackball adapted programs work on certain trackballs.

Anyway, back to the adapter. As I stated in the last post, I will be using the joystick mode for the TI-99/4a.

Here is a quick summary of the difference between the TI-99/4a joystick and the Atari. The TI-99/4a had only one port that had a Y connector for both joysticks. The Atari had two connectors, one for each joystick. The Atari also had pins for paddle and supplied a +5 from the joystick port.

Here is a link to the Atari joystick layout:

Here is the TI-99/4a joystick layout:

Here is the layout for an Atari to TI-99/4a adapter (Note: I found that the diodes – in this case- are optional): 

Here is a picture of a single Atari joystick to TI-99/4a adapter I created a while back.


I used an old Ethernet cable and two serial connectors to create the adapter. Whenever I play a game on the TI I use an Atari joystick since the original TI joysticks are, in reality, small torture devices created by trolls.

You could connect the trackball straight to the TI-99/4a using this cable if you had a +5 to the trackball sensors and the trackball would work. So all I need to do was supply a +5 to PIN 7 of the Atari and ‘wallah’, it works…well almost.

The parts needed are a serial mail to female cable, a +5 power source (in my case a x4 AA battery holder), a 1N914 diode and a project box to make it all look nice.

First I cut the serial cable then rewired it with clip leads to be sure it would at least work as a Atari to TI-99/4a joystick adapter.


Then tested it with an Atari joystick.

I used this simple program in TI BASIC to test the joystick.


200 CALL KEY(1,A,B)


400 GOTO 100



And it worked perfectly.

After doing a quick tying of the wires together and taping them secure, I now introduce the +5 power adapter plug to PIN 7 of the Atari line to power the sensors on trackball. I used the common ground for the negative.


This did not go well. I started getting all kinds of odd characters on the screen when I ran the test program. More likely an issue with the power adapter not being pure enough.

So, I replaced the power adapter with a 4 AA battery pack. I also cut and redid the wires. I didn’t like the way they looked. As for the power I first started out with using just 3 batteries (4.5 volts) but later found 4 (6 volts) would work OK.


This cleared up all but one problem. When adapter was attached the characters on the screen defaulted to lower case. This really flummoxed me. I ran several test and discovered that the UP line on the TI side was causing the problem. (The UP line is the TI line 3 to Atari line 1). Disconnect the line and the problem went way. So, I went on the Atariage TI board to pick some greater minds then mine and Ksarul suggested I insert a 1N914 diode in the UP line. tried it and that worked! (thanks Ksarul).

Her is a picture of all the wires shrink wrapped. The large lump up front is the diode shrink wrapped. The two green wires are to the battery pack.


I put the whole thing in a project box, Velcro the batteries on top and it works. Doesn’t look too bad either.

Well, that’s it. The trackball works really well with games like Buck Rogers and Centipede.

Next time I will start working on putting together the FlashROM 99 cartridge for the TI-99/4a. Till then.

Note: I initially had a power supply instead of batteries but got some weird problems. Those problems might go away if a diode is put on all the lines but I never tested it. AAA batteries might also work but I had a AA battery holder laying around. Any voltage from 4.5 to 6 seems OK.