Dual Joystick Control On The TI-99/4A
I built the adapter suggested by Gary Cook ("How To Build Your Own TI-99/4A Joystick Adapter," COMPUTE!, August 1983), and it worked well as long as I only used one joystick. However, when I tried two joysticks in a program, I discovered that the joysticks would lock each other up when moved in the same direction until one of the joysticks was disabled.
To correct this, I added diodes (1N914 or equivalent) at each connection except ground. Diodes should be added in the lines going to both joysticks, and they should have the cathode (banded end) toward the joystick switches. After this modification, simultaneous joystick movement was fine.
Enclosed is a modified drawing of the adapter box.
We tested this and it works well. This alteration is unnecessary for those who plan to use only one joystick.