nostr:npub13levfmuj9uu5gy4mhh0tg5nkujqujkyky8220rd0dkwpm4p950eqss6saf Ha, I also just checked that the enables are grounded 🙂. The schematics seem to indicate that the MSB output bit 7 only gives a signal "SN" (https://github.com/warmech/lisa-hardware/blob/main/CPU%20Card/Schematics/Video.pdf), so I assume this is only used to generate the "magical" Lisa serial number – there are three different versions of the PROM on bitsavers (SN_*.BIN at http://bitsavers.informatik.uni-stuttgart.de/bits/Apple/Lisa/firmware/).