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HP-25 DIY with Z80 - Printable Version

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HP-25 DIY with Z80 - eshazen - 04-28-2020 05:37 PM

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]

RE: HP-25 DIY with Z80 - teenix - 04-29-2020 12:10 AM

Cool project, looks like a lot of effort went into it.



RE: HP-25 DIY with Z80 - Steve Simpkin - 04-29-2020 12:34 AM

Outstanding project!

RE: HP-25 DIY with Z80 - Dwight Sturrock - 04-29-2020 01:44 AM

Very nice work!

RE: HP-25 DIY with Z80 - Graan - 04-29-2020 05:43 AM

Nice project !
Looks really good.

RE: HP-25 DIY with Z80 - Nad - 04-29-2020 05:44 AM


Very impressive, both technically and visually!


RE: HP-25 DIY with Z80 - Paul Dale - 04-29-2020 10:36 AM

I'm impressed. A little scared too Smile


RE: HP-25 DIY with Z80 - Moggul - 04-29-2020 10:53 AM

Amazing! When does it go up for sale? Big Grin

RE: HP-25 DIY with Z80 - rprosperi - 04-29-2020 12:30 PM

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)?

RE: HP-25 DIY with Z80 - twoweims - 04-29-2020 03:38 PM

I would love to build one of these!
Will you be making the boards available for purchase?

RE: HP-25 DIY with Z80 - Geoff Quickfall - 04-29-2020 06:51 PM


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

RE: HP-25 DIY with Z80 - Maximilian Hohmann - 04-29-2020 07:00 PM


(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!


RE: HP-25 DIY with Z80 - Jake Schwartz - 04-30-2020 02:36 PM

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


RE: HP-25 DIY with Z80 - jjohnson873 - 04-30-2020 04:31 PM

Very cool and FUN! It appears as if one of the 3D printer gurus needs a new project. ?

RE: HP-25 DIY with Z80 - Craig Bladow - 04-30-2020 06:01 PM

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.


RE: HP-25 DIY with Z80 - Nad - 05-01-2020 03:04 AM

(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.


RE: HP-25 DIY with Z80 - andylithia - 05-02-2020 08:41 AM

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.

RE: HP-25 DIY with Z80 - GreyUser - 06-04-2020 05:01 AM

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).

RE: HP-25 DIY with Z80 - Harald - 06-04-2020 11:35 AM

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.

RE: HP-25 DIY with Z80 - sa-penguin - 06-04-2020 11:50 AM

(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.