Some revisions of the Apple II (maybe just revision 0?) use the Signetics 8T28 quad three-state buffer. Two of them sit between the 6502 data bus and the rest of the computer. These are still available (as of July 2025) as new old stock, but Signetics was acquired by Philips in 1975, which in turn spun out its semiconductor manufacturing to NXP in 2006. Signetics' 8Txx logic family has long since been EOL'd.

This board uses a 74AHCT125 and a 74AHCT126 wired together as a pin-compatible replacement. AHCT parts have multiple manufacturers available through DigiKey as of July 2025.

Later Apple II revisions (and the II+) replaced two 8T28s with a single National DP8304B. The pinout was similar to the still-easily-found 74LS245, but with the pins shifted around a bit. This project addresses that substitution.