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After my HP-25 keyboard got too flaky to put up with, I decided to build a replacement! While I was at it, I decided to super-size it as a desktop machine so I could read it easily with my boomer eyes, and use Cherry MX mechanical keyswitches which should hopefully out-last me.

The hardware is typical of the era but unrelated to HP calculators: Z80 CPU, 32K EEPROM, 32K static RAM, ICM7218 display drivers and simple matrix keyboard.

I started out with great enthusiasm to write my own microcode emulator in Z80 assembly language, but in the end I ported the "famous" nonpareil simulator core by Eric Smith to the Z80 using the z88dk development environment with SDCC compiler. It is a bit slow, but running the CPU at 16MHz is close to original speed.

Some day when I'm really bored I might revive the assembly version just for fun.

The finished product also needs a box.

CPU board schematic

LED/Display board schematic

Photo below: Calculator running, with Cherry MX key switches and "re-legendable" keycaps. Printed the labels on a color laser printer, and after a bit of tweaking I'm pleased with the appearance.

[Image: calc.jpg]

The guts are a stack of two PC boards, all thru-hole, made for almost nothing by JLC-PCB in China. The keyboard/display PCB has two ICM7218 display driver chips on the back driving nice bright 10mm common anode 7-segment displays. Had to use the "unencoded" mode so it correctly displays things like "Error".

The 6-pin connector on the CPU board accommodates a USB/serial adapter (FTDI) for a bit-banged serial port. (Note to self -- never build another Z80 system without a UART, big-banging serial on a Z80 is no fun!).

[Image: boards.jpg]
Cool project, looks like a lot of effort went into it.


Outstanding project!
Very nice work!
Nice project !
Looks really good.

Very impressive, both technically and visually!

I'm impressed. A little scared too Smile

Amazing! When does it go up for sale? Big Grin
Very, very cool project. Thanks for sharing this.

I love the keyboard and labels! Which color Cherry MX switches? If blue or green, they may rival an original HP-25 for that clicky HP feel.

Is this a 25 or a 25C (retains memory when then calculator is shut off, not the actual Z80 boards)?
I would love to build one of these!
Will you be making the boards available for purchase?

Sorry didn’t mean to shout but that would look so coool in the lab.

(04-29-2020 06:51 PM)Geoff Quickfall Wrote: [ -> ]I WANT ONE TO MAKE,!!!!

MEEE TOOO!!! It would be big enough to cover our flight management system completely if I ever take it to work (provided there is any work left in a couple of weeks...).

However I would probably only want to make the keyboard and display part and connect that to some Arduino. My Z80 machine language programming days lie almost 40 years back. I rather learn something new (or read a couple of good books) than to start on the Z80 again.

A realla nice project!

You could add a printer and call it the "HP-26" :-)

Very cool and FUN! It appears as if one of the 3D printer gurus needs a new project. ?
Very cool! I have a Z80@20Mhz "Zeta" single board computer I built several years. ago. It runs CP/M with dual 3.5" floppy drives, as well as an add-on terminal board that also emulates 8 8MB hard drives (the largest supported by CP/M!). http://www.malinov.com/Home/sergeys-projects/zeta-sbc

It would be cool to have it act as a calculator and drive an LED display.

(04-29-2020 07:00 PM)Maximilian Hohmann Wrote: [ -> ]provided there is any work left in a couple of weeks...


Understandably people are eager to have restrictions lifted, but after reading about the devastating second wave of the Spanish Flu at the end of World War One I would exercise extreme caution.

That's hardcore! I bet those mechanical switches must feel very good.
Maybe you can give those cheap HPDL-1414 bubble displays a try? You can get new ones on Aliexpress or somewhere else for about a US dollar each.
Would you be willing to share the source code? I’m very interested in seeing the port to SDCC as I’ve had a look at Eric’s code and it’s simply beyond my ability to port to another environment (more of a hardware guy).
Which key caps did you use? I could do with some of those as well:

[Image: img_20200604_1331272doj0s.jpg]
This is my HP45 desktop calculator. It is basically a prototype board for my classic replacement board, running Tonys emulation code on a PIC, display drivers are original HP ICs and there is a NEC mcirocontroller interfacing between the two.
(04-28-2020 05:37 PM)eshazen Wrote: [ -> ]Photo below: Calculator running, with Cherry MX key switches and "re-legendable" keycaps. Printed the labels on a color laser printer, and after a bit of tweaking I'm pleased with the appearance.
Your key labels have turned out quite well.

I have tried used water slide decals, for text on the front face of keycaps, with limited success. Like you, I prefer light text on a dark background. This is hard to do without a printer that actually has white ink.
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