The ROMulan PET RAMulator Is a One-Stop RAM and ROM Replacement for Your Commodore PET

Clever board packs 32kB of static RAM and 16 banks of ROMs — including two useful diagnostic tools for tracking down hardware issues.

Gareth Halfacree
3 years agoRetro Tech

Developer Gavin Andrews has launched an open-source add-on for the Commodore PET family, and other MOS 6502-based microcomputers, designed to replace the read-only and random-access memories (ROM and RAM) — upgrading their specifications and bringing otherwise-dead systems back to life.

"During the restoration of my PET 2001-8 I needed a way to replace the defective 6550 RAM and flaky 6540 ROMs," Andrews explains of the project's origins, "so I built a replacement board and added a few other features along the way. The board contains images for popular test programs so if your PET has working display logic and decoding for video then this board should help you on your way."

Dubbed the ROMulan PET RAMulator, the board includes 32kB of static RAM and a 4Mb ROM storage area holding 16 banks of ROM images. Many of these images run various versions of Commodore BASIC, including those with patches originally written for the VICE emulator, while two hold a pair of testing programs: PETTESTER and VOSI Tester, which assist in checking PET hardware for a variety of failures and bugs.

Another key feature of the board is what Andrews describes as "EAzy NOPulation," a jumper which causes the board to present $EA NOP — No OPeration — on all reads. "This will cause the processor to perform a read instruction to each address in turn as the PC [Program Counter] increases," Andrews notes, "i.e. the Address Bus will increment continually from $0000 to $FFFF. The addresses are presented to the motherboard and can help diagnosis."

The ROMulan PET RAMulator is now available from Andrews' Tindie store, priced at $67.72; source code and design files are available on GitHub under the GNU General Public License 2.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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