HP-48S faulty firmware resurrection

I got this HP-48S for a few bucks. It was so cheap because it wasn't working. When I turned it on it showet just some random picture.

Picture after power-on.

So I started fixing. First I tried to replace the resonator, no change. Then I swapped RAM with no success. So the last option was the faulty ROM. After some research I found out that the ROM is uPD23C2000GC. So I desoldered it and made an adapter for programer and dumped the content of ROM. You can explore it here, it is file ROM_orig.BIN. It is firmware version J as can be seen at the end of the file.

Original ROM ready for dump.

I compared the dumped content with correct firmware and it revealed that on some addresses there are swapped MSB bits.

Correct FW on left and dumped FW on right

Then I connected a socket for new ROM, actuly it will be a Flash. I chose SST39SF040 in PLCC package.

Socket preparation.

Socket with Flash in place.

I progrmmed the Flash with firmware version J and voila, it is working. Now I can program any firmvare version for this calculator (ROM_A to ROM_J) and it works.

Firmware version "J"

Firmware version "A"