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HP-25 DIY with Z80
04-28-2020, 05:37 PM
Post: #1
HP-25 DIY with Z80
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]
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04-29-2020, 12:10 AM
Post: #2
RE: HP-25 DIY with Z80
Cool project, looks like a lot of effort went into it.

cheers

Tony
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04-29-2020, 12:34 AM
Post: #3
RE: HP-25 DIY with Z80
Outstanding project!
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04-29-2020, 01:44 AM
Post: #4
RE: HP-25 DIY with Z80
Very nice work!
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04-29-2020, 05:43 AM
Post: #5
RE: HP-25 DIY with Z80
Nice project !
Looks really good.
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04-29-2020, 05:44 AM
Post: #6
RE: HP-25 DIY with Z80
Hello,

Very impressive, both technically and visually!

Nad

Everything will be alright in the end. If it's not alright, then it's not the end.
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04-29-2020, 10:36 AM
Post: #7
RE: HP-25 DIY with Z80
I'm impressed. A little scared too Smile


Pauli
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04-29-2020, 10:53 AM
Post: #8
RE: HP-25 DIY with Z80
Amazing! When does it go up for sale? Big Grin


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04-29-2020, 12:30 PM
Post: #9
RE: HP-25 DIY with Z80
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)?

--Bob Prosperi
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04-29-2020, 03:38 PM
Post: #10
RE: HP-25 DIY with Z80
I would love to build one of these!
Will you be making the boards available for purchase?
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04-29-2020, 06:51 PM
Post: #11
RE: HP-25 DIY with Z80
I WANT ONE TO MAKE,!!!!

Sorry didn’t mean to shout but that would look so coool in the lab.
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04-29-2020, 07:00 PM
Post: #12
RE: HP-25 DIY with Z80
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
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04-30-2020, 02:36 PM
Post: #13
RE: HP-25 DIY with Z80
You could add a printer and call it the "HP-26" :-)

Jake
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04-30-2020, 04:31 PM
Post: #14
RE: HP-25 DIY with Z80
Very cool and FUN! It appears as if one of the 3D printer gurus needs a new project. ?
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04-30-2020, 06:01 PM (This post was last modified: 04-30-2020 06:02 PM by Craig Bladow.)
Post: #15
RE: HP-25 DIY with Z80
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.


   
   

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05-01-2020, 03:04 AM
Post: #16
RE: HP-25 DIY with Z80
(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

Everything will be alright in the end. If it's not alright, then it's not the end.
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05-02-2020, 08:41 AM (This post was last modified: 05-02-2020 08:43 AM by andylithia.)
Post: #17
RE: HP-25 DIY with Z80
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.

Vintage HP handheld user
Lithcore.cn
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