The IBM design used a capacitance screen with a metal pencil for input, providing a directly read X and Y coordinate. The basic system was similar to modern touch screen systems like the iPhone, but not sensitive enough to detect a finger and instead required a wired conductor to complete the circuit. The pencil replaced the light pens because it was much faster to decode; a light pen had to wait for a pulse of light when a vector was being redrawn and then work its way through the list of vectors to find out which one was selected.
