On Mar 21, 23:04, Allison J Parent wrote: No offence, but if I can't get a drop-in replacement, I'd rather leave the original as-is, and build my little expansion board. The idea was to disable internal RAM (and perhaps ROM) and put a decoder on the expansion connector (the old PETs have an internal expansion connector). The expansion would carry up to 32K RAM, and one or more EPROMs holding different versions of BASIC (the original doesn't work properly with IEEE-488 disks and some other devices). Anyway, here's the pinout: MCS6550 1024 x 4 SRAM A0 1 22 Vss (0V) A1 2 21 CS1 A2 3 20 CS2 A3 4 19 ~CS3 A4 5 18 ~CS4 A5 6 17 Vdd (+5V) phi2 7 16 D3 A6 8 15 D2 A7 9 14 D1 A8 10 13 D0 A9 11 12 R/~W -- Pete Peter Turnbull Dept. of Computer Science University of York