This article is currently an experimental machine translation and may contain errors. If anything is unclear, please refer to the original Chinese version. I am continuously working to improve the translation.
Back in high school, our school required the Casio 991 CNX for competitions. But it had too few features—many math operations simply weren’t possible. And smartphones were strictly banned.
So I turned to the “ancient” world of scientific calculators, and discovered a real bargain: the HP 39gs.
The HP 39gs now sells for nearly 80 RMB on Taobao, but I got mine secondhand on Xianyu for just 50 RMB. At one point, you could even find them for as low as 20–30 RMB.
The HP 39gs itself isn’t particularly powerful—maybe only slightly better than Casio’s model with graphing and programming functions.
But here’s the fun part: the HP 39gs and the CAS-enabled HP 48gii differ by little more than keycap labels and a single Flash chip. After modding, the calculator essentially becomes a stripped-down version of Matlab—way more fun and capable.
Here’s the final result:
Calculator
Calculator
Modding steps:
- Start prying open the plastic panel around the screen from the gap beside the display. Remove two hidden screws, then snap open the casing.
- Take a photo to record the wiring, then desolder the cable connecting the back cover to the mainboard.
- Use a hot air station (though a soldering iron works too) to remove the original NOR Flash, and replace it with an SST39VF1601-70-4I-EKE. You can ask the Taobao seller to pre-flash the firmware for you. (Download the firmware here.) If you don’t get it pre-flashed, you’ll need to use JTAG to program it—a bit more involved. See the tutorial here.
- Reconnect the back cover wires, reassemble the device, and power it on for testing.
- Print the 48gii key layout onto an A4-sized adhesive label sheet, cut it out, and stick it onto the corresponding keys.
- Done!
There are actually some more advanced mods I never had time to explore back in high school:
- Overclocking: https://www.cncalc.org/thread-11970-1-1.html
- Upgrade to the even more powerful 50g: https://www.cncalc.org/thread-24766-1-1.html
But let’s be honest—HP 48gii is a machine from 2003. It’s ancient by today’s standards.
Still, while it’s no match for modern computing devices in speed (though definitely faster than the 991—lol), the functionality of this once-high-end model far exceeds any high school requirement. The real joy came from tinkering and exploring.
After starting college, this little device retired gracefully into a drawer, gathering dust. But it remains my first ever hardware mod—my little journey into the world of DIY electronics.
This article is licensed under the CC BY-NC-SA 4.0 license.
Author: lyc8503, Article link: https://blog.lyc8503.net/en/post/hp39gs-modify/
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