a2gpio/README.md

Apple II Dual-6522 GPIO Card

This is a fairly straightforward implementation of a GPIO interface for the Apple II, based on a pair of 6522 VIAs. It is an extension of a design published in c't Magazine in 1985, with the control lines on each 6522 also brought out to header pins, the data bus buffered with a 74LS245, and the ɸ2 signal needed by the 6522s generated by a pair of flip-flops in a 74LS74 in accordance with a technote published by Apple on the subject. Most components are surface-mount to reduce needed board area, though the 6522s were kept in DIP form to allow use of either NOS (new old stock) 6522s from whatever suppliers or the currently available WDC W65C22S.