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[Celelibi]

Hello,

 

I am the proud owner of a venerable Casio Graph 25. Which is the french equivalent to an fx-7450G as I understand. My goal was to assess the hackability of the calculator. Meaning I'm looking to answer at least those question:

• What hardware is there in the calculator?
• Is there any specification about it? Especially about the CPU ZX941?
• Can I download the OS to a computer? If so, how?
• Can I execute native code on that calculator? How?
• Can I write and upload my own OS to the device?
• How about the serial protocol? Is there any documentation about it? Has someone tried to fuzz it to search for undocumented commands?

Even just finding some informations about the hardware of this particular calculator proved to be quite difficult. The renaming for the french market didn't help. (BTW, do you know why there are distinct names in France?)

 

So I opened my calculator to check the main chips. I found:

• a Toshiba TC55257DFL-85V, 32K of SRAM, from which only 20K are available to the user
• a NEC D23C4001EJGW-377 of unknown purpose but that seems directly connected to the SRAM.
• a chip under a tinfoil rectangle connected to everything, but mainly to the LCD screen.

I guess under the tinfoil is the microcontroller in the form of an ASIC chip and that the NEC chip handles I/O routing or something. But I'm pretty much stuck there. I have no idea if the OS is stored on a ROM (if so, is it an EEPROM?) or flash, if it can be physically overwritten or not?

 

Has someone ever done some reverse engineering on those old calculators?

I would take any clue, even about related hardware.

 

Best regards

Edited by Celelibi, 04 December 2017 - 01:05 AM.

[frankmar98]

Hi Celelibi, welcome to UCF!

 

Thank you for doing research in reverse engineering of this old calculator.

 

About:

-Download the OS to a computer, execute native code: It may be possible, but today it is undocumented.

-Write your own OS to the calculator: I think is impossible because the ROM memory in 90s used to be physically unoverwritable. This has to be proved.

-The serial protocol is partially documented. I had read somewhere the documentation of the serial protocol, you can search it.

 

You are from France! The best thing about speaking French, is the Planet Casio forum (the biggest Casio calculator forum), France is the largest market for Casio graphing calculators, I possibly even bigger than the rest of the world.

I don't speak French , but I would like to learn it.

 

Regards, Frank Martinez Moreno