I found some images of DIP pinouts in my “Desktop” desktop folder. Why does one need a desktop folder on their desktop? It’s like a dumping ground for things that sit on my desktop too long but I can’t bring myself to throw away. I guess this is why I always figured it was important to document things as they happen. Otherwise you end up with what you see below. Good luck! Maybe I’ll remember what it was for, could it have been the Mega Drive Sonic 1 flash cart? Snap! That is what it was for and I found images to back it up!

From Replacing 27C040 with AM29F040B.rtf The Am29F040 is a 4 Mbit, 5.0 V-only Flash memory organized as 512K bytes of 8 bits each. The Am29F040 is offered in a 32-pin package as a 4 Megabit (524,288 x 8-Bit) chip.

Atmel AT27C040 4 Mbit (512K x 8) OTP EPROM.

The pin mismatch is the important part. On the AT27C040 PDIP, pin 1 is VPP and pin 31 is A18. On the Am29F040B PDIP, pin 1 is A18 and pin 31 is WE#. For read-only use in this cartridge, I wired Am29F040B pin 1 to socket pin 31 so A18 reached the correct address input, then held WE# high by tying Am29F040B pin 31 to VCC at pin 32. That describes what I did on this cartridge; it is not a universal drop-in replacement for every 27C040 application.

The surviving file-splitting note below is rough historical working text. It is incomplete and should not be treated as a paste-ready procedure.

From SPLITTING FILE.RTF

Start with normal text bin or md file byteswap in USBPrg (GQ-4X or similar) Win Hex thanks for the info, you were correct I used winhex to combine the lower and upper roms using “Tools - File Tools Dissect - bytewise and selecting the 2 halves.

(If not byteswapped first, the first one burnt might be the second rom… it doesn’t really make a difference just swap the ROM chips around in the sockets)

There was also a SwapEndian.zip included in the folder so here it is - SwapEndian.zip - Mystery solved?

Sources