How to crack a Pacman Plus!


For those of you that are unfamiliar with Pacman Plus (I sure was), it was an "official" upgrade to Pacman that was meant to make the game more challenging and more interesting. There's some cool twists-- eating a power pill may or may-not change all the ghosts to be vulnerable, eating the fruit makes the ghosts disappear (but be vulnerable), on higher levels the maze disappears, new graphics, etc.

Bally/Midway didn't want their new version to just be a ROM replacement though, so they built a special upgrade. The upgrade uses a daughtercard that plugs into the Z-80 socket and then uses encrypted ROMs to prevent people from duplicating the upgrade without the special hardware from Bally/Midway.

They really didn't want people to get into this thing. It's encased in a sheet of plexiglass filled with epoxy-potting compound. One way or another I needed in there to decrypt the ROMs to help with a MAME emulation of the hardware...

First off I wanted to know what I was dealing with. This was relatively simple to accomplish with some help from our dog's vet. (Always good to know where you can find an x-ray machine...) The x-ray revealed four chips-- presumably a Z-80, two PALs, and a buffer. The PALs looked to be something similar to a 20x10 type architecture, but from the early 80's. The exact type wouldn't really be important if my plan worked as I hoped.

The x-ray let me trace out most of the circuit (which was verified with Dock Cutlip who'd had a look as his own Pacman Plus module before). The pinout of the main chip seemed to be a stock Z-80, with the databus interrupted and routed through PAL1. Looks like PAL1 scrambles datalines based on 8 possible combinations or passes data directly through to the Z-80. PAL2 looked like a fancy address decoder. It was obviously scrambling data depending on address.

This was very good news for my plan... The PALs of the day didn't have much for internal latches, and the pinouts of the two in the epoxy seemed to suggest combinatorial inputs and outputs only. The "buffer" chip was too big to be a latch (and didn't match up any pinouts) so I figured it was a 74LS244.

Now to get in there and get at the Z-80 so I can read the code... A Craftsman "Robogrip" plier made short work of the plexiglass...

...but, let me say that that epoxy crap is impressive. Hard as hell. "Fine," I thought. I'll dissolve it. Turns out it's impervious to Methyl Ethyl Ketone (although it was really "clean" after trying). "Fine," try again. Meet "Jasco" epoxy paint stripper. That actually let me get off about 1/1000" after about 20 minutes of trying. Bastards. Can't use fuming NHO3 'cause it'd eat the tops off the chips... "Fine," time for the brute-force approach...

Blow torch. Heat evenly, scrape off with a putty-knife. Success!

With the top of the Z-80 and the 74LS244 exposed I was then able to take a Dremmel to the Epoxy From Hell. The stuff ate two carbide cutting bits in a matter of seconds! I switched to a "cut-off" wheel that worked much better. I cut down to the Z-80, exposing the pins. Once there I moved in on the chip and cut through the leadframe. That effectively left me with a "header" into the PCB.

Forty small pieces of wire and a socket later I had a socketed Pacman Plus "blob". Cool. In goes the Fluke 9010A microsystem troubleshooter... I wrote a program on the Fluke to step through all of ROM memory and dump it to the serial port in ASCII. A laptop captured the data which I dropped into Excel and pasted into a text file. Once I had a complete text file I wrote a short 'C' program coverted the text to binary and rebuilt ROMs.

Once the ROMs were rebuilt, I removed the original Pacman Plus board and EPROMs, replaced the ROMs with my (now) decrypted ones, and dropped in a stock Z-80. Fired up the game and... It worked!

Thanks go to TomW, Kev, and Dock for supplying the boards, EPROM images, and support!

Updated: 5/14/2000


2000, Clayton Cowgill clay@yahoo.com