Modding a NES to run Unisystem VS arcade games (6/14)


Step 4: ROM for the CPU (PRG-ROM)

eprom cpu picture The Unisytem VS uses 4 8KB roms for the CPU (PRG-ROM). I did not want to install 4 sockets and hundread of wires so I decided to use a single 32KB eprom. To ease installation even more, I simply soldered on top of the CPU's memory chip (U1). The /CE signal is connected to pin #3 of the 74ls139 (U3) so the ROM is accessible from $8000 to $FFFF.

Again, I used a diagram to compare the ROM and memory pinout. Be careful, this diagram is for a 27128 (16K) eprom instead of a 27256. On a 27256, pin #1 is A14 and on a 27128, pin #1 is VPP.
photo eprom cpu angle pinout compare diagram

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