Kaypro PC/EXP Info

A number years ago I found this old machine stashed down the back of a family members’ garage. He had kept it for many years intending to restore it as a retirement project. Sadly he passed away before he got the chance.

It’s actually one of the most interesting vintage machines I have, and it seems that no copies of the original manual survive, so I’ve dedicated this page for things I’ve learned about it. I will of course update it as I learn anything new.

There are a number of varieties of this machine out there. I am unsure what model name corresponds to what feature set, etc. Mine appears to be one of the earliest configurations, with a single speed 4.77MHz V20 CPU card.

It was found in the UK with a 230V power supply, however upon dismantling it I’m skeptical that it’s in its original case because the screw holes on the mainboard don’t line up with the chassis, and it also looks slightly different to U.S. models too. Who knows. An interesting story behind that no doubt.

My CPU and I/O cards. My CPU card has no speed switch, commonly seen on other models however the I/O card is the more deluxe variety with floppy controller and RAM, and is fitted with 576KB of RAM, as it was found.

Dip switches

To date I’ve only figured out 3.

SW1 (On CPU board):
1: Open: 8087 Present. Closed: 8087 not present
2: Don’t know
3: Don’t know
4: (Number of floppy drives)
5: (Number of floppy drives)

4/5 Closed: 1 floppy drive
4 Closed, 5 Open: 2 floppy drives
4 Open, 5 Closed: 3 floppy drives
4/5 Open: 4 floppy drives

I’d guess either switches 2 or 3 have something to do with BIOS video modes. I use mine with VGA (has its own BIOS) so perhaps that’s why changing them doesn’t appear to have any effect.


These are XT class machines. An XT keyboard is required. AT and PS/2 keyboards will not work with a passive adapter. To use a non XT keyboard you’ll need one of these.

Serial port

… is rather interesting. It’s a DTE, but, err, it has a female DE-9 connector?

I wonder if this is because this port pre-dates standardisation of RS-232 on DE-9 connectors (DB-25 would have been standarised at that time) so perhaps Kaypro freestyled it.

The pinout is completely different to a standard RS-232 DE-9, so, an adapter cable is needed.

Kaypro pinout:

  • 1: DSR
  • 2: TXD
  • 3: CTS
  • 4: RXD
  • 5: GND
  • 6: DTR
  • 7: DCD
  • 8: RTS
  • 9: RI

Running with XT-IDE

At the time I found the machine I purchased an XT-IDE card for it, which didn’t work. I got an error saying “Error: 40h!” trying to boot from C:

I spent quite a bit of time studying this problem, going as far as doing a full logic analysis on the IDE/ISA buses with a proper logic analyser. It seems that the handlers in the XT-IDE BIOS for reading from disk weren’t being called. I don’t know why this was (got rather tired of debugging it).

The BIOS I was using was built 5 years ago, supplied from a link from the card seller (lo-tech.co.uk). Instead I build a new BIOS from the latest source tree (2020) which now seems to work on the Kaypro, for reasons I don’t understand, and would rather not have to spend the time understanding to be honest.


I’m only aware of one major version of Phoenix BIOS 2.03 (mine is 2.03K – which I suspect means it doesn’t have the dual speed option).

The 286 Upgrade

It is common knowledge that the peculiar layout of these machines was to allow easy upgrading of the CPU card to a newer generation of CPU. Neat idea, but it didn’t catch on.

Exceptionally uncommon: The Kaypro 286 CPU card

What is not so common is the upgrade its self. The card pictured above was sold with Kaypro 286 machines, and also to PC/EXP owners as an upgrade. The Kaypro PC/EXP mainboard (if you could call it that) typically had a couple of 16-bit slots fitted to accommodate this card.

Mine is the only surviving example of this card I’m aware of. Getting it running is on my long list of things to do. If you’ve also got one of these, drop a comment!

If you have one, please get in touch

I’d like to see any photos of other units, and in particular I’d like to collect the various different BIOS versions for it!

7 thoughts on “Kaypro PC/EXP Info

  1. I have a Kaypro PC. It’s a “triple speed” version. There appears to be an added RTC sandwiched between the bios and it’s socket Additionally, the CPU card in mine has a completely different layout, with the RAM and floppy controller integrated.

  2. I just picked up a Kaypro PC w/ RGB monitor and manual. I have not gotten a keyboard set up yet and see that it has some bad memory. But thats as far as I’ve gotten. Mine has a 5″ and 3.25″ drive as well as a HD. I’ll keep you posted and I’ll have to scan the manual for you. Wish it had more info in it!

      1. Where would you like me to send it? Can I ask you a question. In the manual it states it uses an AT style keyboard. I picked up an AT 2 PS/2 converter and am not having much luck. Do you recommend any product for connecting a keyboard to this without having an 80s one? I was able to get past the memory load on mine and the hard drive works. It loads ups drive C. I can’t get past the date and time entry now 🙂 all the best – dave

  3. I managed 3 of the V20 based Kaypros in the 80’s for a company. They decommissioned them in the 90’s – I might still have some cards from from an employee who only wanted the case.
    I wrote a couple of assembly utilities to use the extra 128K of RAM. The first linked the extra RAM into MS-DOS’ memory chain so programs could use it. (The trick is to add blocks for the upper memory and mark the unusable area as used.) The second allowed loading network drivers in the extra memory. (Allocate unused conventional memory, the attempt to load the driver/TSRs indicated on the command line.)

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