Bus Sniffing Leads To New Display For Vintage Casio

Despite his best efforts to repair the LCD on his Casio FX-702P, it soon became clear to [Andrew Menadue] that it was a dead-end. Rather than toss this relatively valuable device in the trash, he wondered if would be possible to replace the LCD with a more modern display. Knowing that reverse engineering the LCD panel itself would be quite a challenge, he decided instead to focus his efforts on decoding the communications between the calculator’s processor and display controller.

With his logic analyzer connected to the Casio’s four bit bus [Andrew] was able to capture a sequence of bytes during startup that looked promising, although it didn’t quite make sense at first. He had to reverse the order of each nibble, pair them back up into bytes, and then consult the FX-702P’s character map as the device doesn’t use ASCII. This allowed him to decode the message “READY”, and proved the concept was viable.

Of course a calculator with a logic analyzer permanently attached to it isn’t exactly ideal, so he started work on something a bit more compact. Armed with plenty of display controller data dumps, [Andrew] wrote some code for a STM32 “Blue Pill” ARM Cortex M3 microcontroller that would sniff and decode the data in near real-time. In the video after the break you can see there’s a slight delay between when he pushes a button and when the corresponding character comes up on the LCD below, but it’s certainly usable.

Unfortunately, the hardware he’s created for this hack is just slightly too large to fit inside the calculator proper. The new LCD is also nowhere near the size and shape that would be required to replace the original one. But none of that really matters. While [Andrew] says he could certainly make the electronics smaller, the goal was never to restore the calculator to like-new condition. Sometimes it’s more about the journey than the destination.

10 thoughts on “Bus Sniffing Leads To New Display For Vintage Casio

  1. Back in the day the surplus houses were full of small japanese display modules with wacky non ASCII character sets…. still even if they were around it would probably have been just as much effort to match one up

    1. It’s because Obama made the EPA put meth precursors in the gas, which only cooks up in a large engine like a bus, trying to get our kids addicted to public transport, instead of individual V8 muscle like sweet baby J intended.

  2. I would avoid any buss sniffing until we have the corona virus under control. Speaking of which, I was looking through some of my old stuff and I think I found an old fashion cure for the virus. I found a can of corona dope. I am pretty sure if you whiff enough of this, you won’t get the virus… or anything else.

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