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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.

cheers

Tony


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

Hello,

Very impressive, both technically and visually!

Nad


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

I'm impressed. A little scared too Smile


Pauli


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

I WANT ONE TO MAKE,!!!!

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

Hello!

(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!

Regards
Max


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" :-)

Jake


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.


[attachment=8385]
[attachment=8386]


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...

Hello,

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.

Nad


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.