Galaxian Single Rom Hack

 

by Patrick Peters

Click here for a 39k ZIP file of all the ROM images needed for the following conversions. This include two different versions of Galaxian, the graphics ROM images at 1H and 1K, and the color PROM 32x8 BiPolar 82S123 at location 6L. These images are thanks to Patrick Peters.

  Galaxian Single 27128 EPROM Modification

This document describes how to take a Midway Galaxian PCB (printed circuitry board) and modify it to use a single 27128 EPROM instead of five 2716 EPROMs that live on the satellite PCB. This should increase the reliability of the PCB as there are fewer chips, and the satellite board can be discarded.

Necessary equipment:
wire wrap wire
(1) 28 pin socket.
(1) 27128 EPROM
EPROM image. GALAX_7F.128 is the position 7F 27128 EPROM file for the original version of Galaxian. SGALX_7F.128 is the position 7F 27128 EPROM file for the Super Galaxian version. You can use either of these files.

PCB Modification steps.

  1. Remove the original Galaxian satellite board from sockets 7F and 7L. This board holds the five 24 pin 2716 EPROMs (or ROMs) in positions 7U, 7V, 7W, 7Y, 7Z.

  2. Bend out the following leads on a 28-pin DIP socket: 1, 2, 20, 22, 23, 26, 27, 28.

  3. Install the socket into the Galaxian main logic board at location 7F (the right most socket, as facing the PCB with the edge connector towards the ceiling, that held the satellite board), aligning the inserted socket pin 14 with PCB socket pin 12. Pins 1, 2, 27, 28 will hang over and not plug into the PCB socket.

  4. Connect pins 1, 27 and 28 together with wire-wrap wire, leaving about 1" connected to the socket. Connect the attached wire hanging from pins 1, 27, 28 to +5 volts on the logic PCB. This is best connected at location 7J pin 24 (where there is no socket or chip).

  5. Connect the remaining pins on the new socket with wire-wrap wire as follows:
    1) Tie Pin 2 (A12) to chip 7D Pin 5.
    2) Tie Pin 20 (CE) to Ground (best connected at location 7J pin 12).
    3) Tie Pin 22 (OE) to chip 8E pin 13.
    4) Tie Pin 23 (A11) to chip 7D Pin 3.
    5) Tie Pin 26 (A13) to chip 7D Pin 7.

  6. Use hot-melt glue to secure the wire-wrap wire to the PC board.

  7. Insert the pre-programmed 27128 EPROM into the new 28-pin socket.

Galaxian ROM code explained.
Galaxian uses 10k bytes of program code. Without resorting to fancy address decoding, the game had to be put into a 16k EPROM (a 27128). That's pretty wasteful, but it's the easiest way and ROMs are cheap. Here's the map of the ROM: 

$0000 - 07FF Galaxian 7U
$0800 - 0FFF Galaxian 7V
$1000 - 17FF Galaxian 7W
$1800 - 1FFF Galaxian 7Y
$2000 - 27FF Galaxian 7Z
$2800 - 2FFF Galaxian 7W (filler)
$3000 - 37FF Galaxian 7Y (filler)
$3800 - 3FFF Galaxian 7Z (filler)

This is 16384 bytes, or 16K. You need a 27128 EPROM for this. You'll notice that I've duplicated certain ROMs in some blocks of memory, like in $2800 - 3FFF. I don't know if this is necessary. I had to fill the unused blocks with something. GALAX_7F.128 is the position 7F 27128 EPROM file for the original version of Galaxian. SGALX_7F.128 is the position 7F 27128 EPROM file for the Super Galaxian version. You can use either of these files.

 

Galaxian 2 Game Mod using one 27256 EPROM.

Necessary equipment:
wire wrap wire.
(1) 28 pin sockets.
(1) 27256 EPROM.
(1) SPDT mini-switch.
EPROM image. GALAX_7F.256 contains the position 7F original Galaxian and Super Galaxian ROM images.

Note: If you have done the single 27128 EPROM modification (outlined above), this is the same procedure with some additions.

PCB Modification steps.

  1. Remove the original Galaxian satellite board from sockets 7F and 7L. This board holds the five 24 pin 2716 EPROMs (or ROMs) in positions 7U, 7V, 7W, 7Y, 7Z.

  2. Bend out the following leads on a 28-pin DIP socket: 1, 2, 20, 22, 23, 26, 27, 28.

  3. Install the socket into the Galaxian main logic board at location 7F (the right most socket, as facing the PCB with the edge connector towards the ceiling, that held the satellite board), aligning the inserted socket pin 14 with PCB socket pin 12. Pins 1, 2, 27, 28 will hang over and not plug into the PCB socket.

  4. Connect pins 1 and 28 together with wire-wrap wire, leaving about 1" connected to the socket. Connect the attached wire hanging from pins 1 and 28 to +5 volts on the logic PCB. This is best connected at location 7J pin 24 (where there is no socket or chip).

  5. Attach the SPDT switch to the PCB using silicon or a hot melt gun at a convenient location.

  6. Connect the remaining pins on the new socket with wire-wrap wire as follows:
    1) Tie Pin 2 (A12) to chip 7D Pin 5.
    2) Tie Pin 20 (CE) to Ground (best connected at location 7J pin 12).
    3) Tie Pin 22 (OE) to chip 8E pin 13.
    4) Tie Pin 23 (A11) to chip 7D Pin 3.
    5) Tie Pin 26 (A13) to chip 7D Pin 7.
    6)* Tie Pin 27 (A14) to the center pin of the SPDT switch.
    7)* Tie one outside pin of the SPDT to ground.
    8)* Tie the other outside pin of the SPDT to +5 volts.

    * additional steps to the single 27128 procedure outline previously.

  7. Use hot-melt glue to secure the wire-wrap wire to the PC board.

  8. Insert the pre-programmed 27256 EPROM into the 28-pin socket.

You can now switch between the two different version of Galaxian with the attached switch.

Galaxian ROM code explained.
Galaxian uses 10k bytes of program code. Without resorting to fancy address decoding, the game had to be put into a 16k EPROM (a 27128). That's pretty wasteful, but it's the easiest way and ROMs are cheap. Here's the map of the ROM: 

$0000 - 07FF Galaxian 7U
$0800 - 0FFF Galaxian 7V
$1000 - 17FF Galaxian 7W
$1800 - 1FFF Galaxian 7Y
$2000 - 27FF Galaxian 7Z
$2800 - 2FFF Galaxian 7W (filler)
$3000 - 37FF Galaxian 7Y (filler)
$3800 - 3FFF Galaxian 7Z (filler)

$4000 - 47FF Super Galaxian 7U
$4800 - 4FFF Super Galaxian 7V
$5000 - 57FF Super Galaxian 7W
$5800 - 5FFF Super Galaxian 7Y
$6000 - 67FF Super Galaxian 7Z
$6800 - 6FFF Super Galaxian 7W (filler)
$7000 - 77FF Super Galaxian 7Y (filler)
$7800 - 7FFF Super Galaxian 7Z (filler)

This is 32768 bytes, or 32K. You need a 27256 EPROM for this. You'll notice that I've duplicated certain ROMs in some blocks of memory, like in $2800 - 3FFF. I don't know if this is necessary. I had to fill the unused blocks with something. GALAX_7F.256 contains this code for the 27256 ROM images and contains both the original Galaxians and Super Galaxians.