Running Moon Cresta on Galaxians

Sean Riddle and Clive Bittlestone

The Moon Cresta PCB is very similar to the Galaxians PCB; in fact, the schematics even list the same location numbers. There are 4 differences that need to be taken care of:

The CPU ROM on Moon Cresta is 16K, versus 10K for Galaxians. A new ROM daughterboard needs to be built.

The graphics ROM on Moon Cresta is 8K, versus 4K for Galaxians. A ROM daughterboard must be built.

Moon Cresta very slightly alters some address decoding. This needs a circuit to select which game will be played.

The Moon Cresta PCB routes the video sync signals to inputs. This modification is documented in the Moon Cresta schematics. It does not affect Galaxians, except that the second player controls are used, most likely causing problems for cocktail machines.

I chose to place all the CPU ROM code into one chip. The daughterboard I built can hold 27C040 EPROMs, which gives plenty of room (up to thirty-two 16K games!) I'm currently using a 27C010, since I've only got 5 different ROM files (leaving 3 slots empty).

The PCB ROM sockets are wired for 23xx series EPROMs, so we have to move a few signals around. My daughterboard is very similar to the Galaxians daughterboard- it plugs into EPROM sockets 7H and 7L (to get both chip select inputs) and uses one 7400 NAND IC for address decoding. It has inputs for ROM selection.

Since the graphics ROM address bus is 16 bits wide, I chose to use 2 8-bit EPROMs, but I used much larger capacity EPROMs to hold the graphics for multiple games. Along with the two 27512 EPROMs, the graphics ROM daughterboard has a 7400 NAND IC and a 74157 IC for address decoding. Again, the graphics EPROMs were 23xx series, so some signals have to be moved.

Three signals must be jumpered from the motherboard to the graphics ROM daughterboard for Moon Cresta.

Galaxians decodes RAM and I/O at $4000, while Moon Cresta uses $8000. Moon Cresta also changes the signal that enables the non-maskable interrupt. Both of these must be switched to select either Galaxians or Moon Cresta. One 74157 is used to rearrange the signals appropriately. I also enable the Moon Cresta graphics ROM chip select signal here, in case Galaxians uses it.

Part of the schematics for the Gremlin version of Moon Cresta show some modifications to the PCB. A 7404 inverter IC is piggy-backed onto IC 6H, and two signals are routed to the player 2 control inputs. Also, the slam switch is wired directly to reset.