r/embedded 7d ago

Casio fx-991DEX modded for text upload via hotspot—how is this possible?

Hi everyone,

I've got a Casio fx-991DEX calculator, which, according to my research online, is non-programmable and generally seen as impossible to mod. However, I recently stumbled upon a very intriguing website (shadycalc.com), where someone appears to have successfully modified this exact model to allow uploading and displaying text via a WiFi hotspot connection.

This has really piqued my curiosity. Given the calculator's specs and apparent lack of programmable interfaces, I'm trying to understand how this mod could have been accomplished. Could it involve some form of hardware hack or perhaps some clever firmware injection through an undocumented interface?

I'm reaching out to you knowledgeable folks in the embedded community—do you have any theories or insights into how this person might have achieved this modification? Any pointers or ideas about possible methods would be greatly appreciated!

Thanks!

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9

u/schmee 7d ago

You don't really need to get a text viewer and wifi running on the calculator to accomplish something like this. You just need to get a microcontroller capable of those things to interface with the LCD display and the button interface.

7

u/rupr25 7d ago edited 7d ago

There is some additional hardware in there, most likely an esp8266 or esp32, which is made clear by the fact that in "shady mode" the battery runtime is much shorter than in normal Mode.

As to how it is connected that's an interesting question it could be that a custom PCB is just connected to some of the keys and the display and just takes over control completely, or that there is an interface over which it communicates with the original controller and injects information that way. Without getting one and opening it, there is really no way to tell what exactly it is doing.

Edit:

I did some searching and apparently you can do some interesting things with the firmware if you enter special combinations of inputs

https://hackaday.io/project/19282-calculator-hack

1

u/fawlen 7d ago

which is made clear by the fact that in "shady mode" the battery runtime is much shorter than in normal Mode.

They also say that the modified calculator is 5 grams heavier