Close
0%
0%

CDP 1802 board build

CDP 1802 board

keithKeith
Following
Liked Like project

Just one more thing

To make the experience fit your profile, pick a username and tell us what interests you.

Pick an awesome username
hackaday.io/
Your profile's URL: hackaday.io/username. Max 25 alphanumeric characters.
Pick a few interests
Projects that share your interests
People that share your interests

We found and based on your interests.

Choose more interests.

OK, I'm done! Skip
Similar projects worth following
1.2k views
4 comments
6 followers
View Gallery
CDP 1802 processor board. The first CMOS CPU, not that fast but very low power which secured its use in several early space probes. The only home computer it was used in was the COMX-35. So it makes sense to build a copy of that.

I like my projects to be physically small and light, so I would rather not have the integrated keyboard. I would rather enter software from a terminal emulator. So I would like to make the following modifications:

  • Board and firmware to use a USB module as the user interface.
  • Firmware to use * and / instead of × and ÷ for multiplication and division.

I have built it but omitted the tape, audio and keyboard. I'd like to see if it boots up with a cursor first.

A set of keys and keytops is fairly expensive, and I'd prefer to interface a cheap PS/2 type keyboard to it. Or have some way to operate the keyboard matrix remotely.

COMIX-35 (4).ods

Bits that are fitted or not

spreadsheet - 23.71 kB - 01/19/2026 at 04:00

Download

  • Construction

    Keith • 06/19/2022 at 16:21 • 0 comments

    2022-06-08

    PCB arrives. I have noticed that the legend on the PCB indicates '74LS245' while the parts list specifies 74HC parts (or 74HCU for the crystal oscillator circuit). The original COMX used mix of 4000 and 74LS  series parts. I'm going use CMOS parts with LS switching thresholds, because I have a large stock of 74HCT14, 74HCT86 and 74ACT574 chips. 

    2022-06-10

    Parts arrive. They supplied a 74HC04 instead of the recommended 74HCU04.

    2022-06-12

    Fitted many parts. Soldered the logic and RAM chips directly, because they are too cheap to warrant sockets these days and sockets introduce unreliability. I have fitted sockets for the DIP40 chips.

    2022-06-18

    Board currently looks like this:

    Lacking most of the passive components. I had many 100n decoupling capacitors available. I have the chips for the sockets, but won't fit them until all other components are fitted, to avoid exposure to static discharge.

    I plan to modify the project to input characters from a USB module instead of the keyboard, so I can enter programs by pasting text into a terminal emulator.

  • Component sourcing

    Keith • 05/19/2022 at 18:30 • 0 comments

    COMIX-35 Assembly Resources are already well documented.

    CDP1802 still widely available. Other chips less so:

    CDP1869

    CDP1870

    CDP1871

    2022-06-01 

    I found two sources for the PCB.

    On eBay, https://www.ebay.co.uk/itm/115300569917 for £22.79 + £7.20 postage = £29.99.

    If was a third cheaper from a UK supplier, https://www.dcselectronics.co.uk/comix-35.html for £18.50 and chipset for $31.20 (£24.95), so I order it from there.

    I found a kit of ICs on eBay.com https://www.ebay.com/itm/114536103330 for $24. It was cheaper and less fuss than sourcing them separately. I paid with my card, as this has a slightly better currency exchange rate than eBay.

    ETA 14-17 June for the chips, 11th June for the PCB.

    2022-06-08

    PCB arrives. I have noticed that the legend on the PCB indicates '74LS245' while the parts list specifies 74HC parts (or 74HCU for the crystal oscillator circuit). The original COMX used mix of 4000 and 74LS  series parts. I'm going use CMOS parts with LS switching thresholds, because I have a large stock of 74HCT14, 74HCT86 and 74ACT574 chips. 

    2022-06-10

    Parts arrive. They supplied a 74HC04 instead of the recommended 74HCU04.

    2022-06-12

    Fitted many parts. Soldered the logic and RAM chips directly, because they are too cheap to warrant sockets these days and sockets introduce unreliability. I have fitted sockets for the DIP40 chips.

    2022-08-19

    Received low-height crystals from https://www.ebay.co.uk/itm/265237934281 which will make the board look a bit nicer and less vulnerable to knocks.

    2022-08-25

    Received crystal insulating washers.

View all 2 project logs

Enjoy this project?

Share

Discussions

zpekic wrote 06/19/2025 at 15:12 • point

It would be fun to see if any of the Pecom-32/64 "software library" runs on COMX too (and what are the actual differences between the two) https://retrospec.sgn.net/users/tomcat/yu/Pecom_list.php

  Are you sure? yes | no

Keith wrote 06/25/2025 at 01:28 • point

I'd have to check out the hardware differences.

  Are you sure? yes | no

wrote 06/17/2025 at 03:44 • point

Just curious how this turned out. Also, where'd you get the key caps and switches?

  Are you sure? yes | no

Keith wrote 06/25/2025 at 01:12 • point

I didn't. The completed pic is from the creator's pages. I have not complete mine yet. And I would also like to replace the keyboard with a USB input port. Whatever software is around, I don't want to type it in! 

  Are you sure? yes | no

Does this project spark your interest?

Become a member to follow this project and never miss any updates

Going up?

About Us Contact Hackaday.io Give Feedback Terms of Use Privacy Policy Hackaday API

© 2026 Hackaday

Yes, delete it Cancel

Report project as inappropriate

You are about to report the project "CDP 1802 board build", please tell us the reason.

Send message

Your application has been submitted.

Remove Member

Are you sure you want to remove yourself as a member for this project?

Project owner will be notified upon removal.