May 1997 Draft of the ASM Guide

An interactive display having an array of independently labelled display elements such as LEDs. The output of a recirculating shift register is supplied as long or short pulses which are scanned across the array. Each element in response emits either long flashes and appears lit or short flashes and appears unlit. A light-pen, when pointed at an element, detects the flash from that element, and causes a bit of the opposite sense to be introduced into the shift register in place of the recirculated bit. The display state of each element can thus be changed, both from lit to unlit or vice versa. The contents of the shift register can also be transferred to exterior equipment or supplied by it, allowing the display to act as an input/out device.