Spectra IV pinball. By Lee Davison.

The Spectra IV pinball machine was manufactured in 1978 by The Valley Company. It is one of only two models of rotating table solid state pinball machines. Over 2,500 of these machines were made.

Up to four players can play a game, each sitting at one side of the table, and the whole playfield can be rotated to face each player in turn.

At the heart of the machine is a 6503 CPU. This is a 6502 CPU with a 12 bit address bus in a 28 pin package. The top 3K out of the 4K address range is taken up by the program ROM(s), originally three individual 1K ROMs but often replaced by a single 4K device.

Page zero of the address space is filled with 256 bytes of non volatile RAM and also all the output devices for the solenoids, lamps and displays. This is arranged such that when an address is written to both the RAM and the register of any addressed output device are both written but when an address is read only the RAM responds. As a result all output registers can be treated as read/write and values like the player scores are saved in memory and displayed by a single write action.

Also, as this RAM is non volatile the state of the output registers can be restores simply by reading and writing back each device address on powerup.

The only other device is a 6532 RIOT which provides 128 bytes of volatile RAM for the stack, a timer that is used to double the interrupt rate and the remaining I/O for the sound chip, an SN76477 from TI, and all the switch inputs.

The firmware for this machine has been dumped and can be used in emulators so even if you don't own a real machine you can play something virtually the same. There is a problem with the dumped image that causes strange things to happen when scores cross a thousands boundary especially for players two and three in a multiplayer game.

In an effort to shed some light on the cause of this behavior I have disassembled and mostly commented the firmware and in doing so discovered the cause just past LAB_0AAC where it looks like a JSR became corrupted and turned into an LDY and ASL.

The disassembled source can be found here .

This disassembly is not quite complete and while it reassembles correctly the comments still need some work. If you can help add anything to this please email me.

Reassembled ROMs with the most likely intended original code can be found here .

Last page update: 18th April, 2012.

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