Midway 8080 Single ROM Hack

This modification allows you to replace all of the program ROMs on a Midway 8080 black & white game board (ie. Space Invaders) with a single 2764 or 27128 EPROM (or implement a simple switchable multigame).

WARNING:
Use this information at your own risk. Pull out the schematics and verify what I say before making any modifications. It's not my fault if you blow something up!

The Midway 8080 pcbs, like most Midway pcbs, are known for ROM socket issues, corroded legs on masked ROMs, and using tri-voltage EPROMss instead of standard EPROMs. To ameliorate these problem,I hacked my board to use a single 27128 instead of the 5 2716s.

For this example, I'll use Space Invaders deluxe, since it uses more ROMs than most games. First, a little background... the Space Invaders Deluxe ROM memory map is as follows:
ROM 0 0000-07FF (invdelux.h)
ROM 1 0000-07FF (invdelux.g)
ROM 2 1000-17FF (invdelux.f)
ROM 3 1800-1FFF (invdelux.e)
ROM 4 4000-47FF (invdelux.d)
empty 4800-4FFF (blank)
empty 5000-57FF (blank)
empty 5800-5FFF (blank)


To organize the ROM space into a single bank, we'll use A14 on the PCB as A13 on the ROM so the 4000-5FFF bank remaps down to 2000-3FFF, and ~A13 is used as the ROM select line.

To perform the modification:

1. Concatenate the 5 ROM files and 3 blanks into a single image and burn it into a 27128
2. Replace the 24-pin (or 28-pin) ROM socket at 1H
3. Restrap the PCB to use 2716-series ROMs (strapping positions are shown in the schematics package)
   • Strap S1 (24-pin socket pin 22 = A9) to A9
   • Strap S2 (24-pin socket pin 21 = A11) to A11
   • Strap S3 (24-pin socket pin 19 = A10) to A10
   • Strap S5 (24-pin socket pin 20 = ~CE) to ground (COM)
   • Strap S6 (ROM size strapping) pins a, b, and c to ground (COM)
4. Take a 28-pin socket and bend out pins 1, 2, 26, 27, and 28
5. Solder socket pins 1, 27, and 28 together leaving a pigtail of wire (Vpp, ~Pgm, Vcc)
6. Insert the socket into 1H and solder the pigtail to the +5V rail
7. Wire socket pin 2 (A12) to the A12 pin on the S6 strapping block
8. Wire socket pin 26 (A13) to the A14 pin on the S6 strapping block
9. Insert the burned 27128 into the stacked sockets

If you've done everything correctly, your Space Invaders Deluxe will now be working more reliably.


Space Invaders (and other 8k games)
If you're only modding the board for Space invaders and other games that use 4 or fewer 2716s originally., then the assembled ROMs will fit in a 2764 and A13 on the ROM is not needed and may be left floating. In this case, Step 8 above may be skipped.