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Back in the early to mid 1980s, 1981-1983, I vaguely recall seeing an advert in a (I think) UK based computer periodical monthly (such as Your Computer, or Computer and Video Games), for a memory peripheral that offered "paging" which would allow the Sinclair ZX811 to have up to 128 kB (it might have been 256 kB) of memory.

I think that the same company offered a proper keyboard as well, and the company name may have been DK Robotics2 (or something very similar to that). Or, it might have been the same company that offered an alternate [character set?] ROM for the ZX81, whose name I really can't remember3. The links provided by DroidW show that it wasn't dk'tronics after all.

I seem to remember it being a full page advertisement, with a photo showing the device, which was a large (uncased4, bare) vertical PCB full of ICs (behind the ZX81), which (IIRC) was plugged into a horizontal expansion board, which was also an uncased bare PCB that went into the back of the ZX81. There may have been the ability of adding a number of boards (i.e. further RAM boards to get the 256 kB, as well as the usual IO Boards, Hi-Res boards, etc.) into the multiple slots on the expansion board, but now my memory is getting a bit unreliable, w.r.t. that part. The full-page advert also explained how the paging was achieved.

I am starting to wonder that, as the board was shown not encased in a plastic shell, maybe it was a prototype. I think that I only saw the ad once. This was not an ubiquitous product, only mail order, not seen in local shops or chains. The ad may also have been in a more "serious", or business type, magazine, like PCW (Personal Computer World), and not in Your Computer or C&VG, as I first thought.

I've looked through my few remaining old magazines and can't find it, unfortunately.

I know that there is a lot of oublié, maybes and mights in this question, but does anyone recall this peripheral, and/or have a photo of the advertisement?


1 It may have also been for the ZX Spectrum - I forget which exactly, but I'm pretty sure it was for only the ZX81

2 dk'tronics seems to be the keyboard producing company, although the Wikipedia page makes no mention of the expanded memory kit.

3 It turns out to be the same company, dk'tronics. Thanks to DroidW, for indirectly providing the link to the advert for the keyboard and the graphics ROM.

4 As the PCBs were definitely uncased, this rules out both the Memotech and Basicare Micro System vertically/horizontally (respectively) stacked solutions.

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  • 1
    FWIW, it was reading this question that jogged my memory, and made me remember the peripheral described above: How can you run a program that is bigger than RAM? Commented May 16, 2019 at 13:37
  • From memory, there were 16kB and 64kB extensions available and, working as programmer, I didn't feel that I could afford even the 16kB version :-) IIRC teh contacts were notoriously flaky and had tendency to lose contact at crucial moments.
    – Mawg
    Commented May 17, 2019 at 6:42
  • 1
    @Mawg - Yes, a blob of bluetak was needed between the rear of the ZX81 and the front of the RAM pack IIRC. Commented May 17, 2019 at 6:49
  • You should probably change the title since it's clear from your comments that you're not looking for the largest peripheral - you want someone to find the exact one you remember.
    – pipe
    Commented May 17, 2019 at 13:59
  • @pipe - yes, I can see what you mean, I am probably coming across as being rather picky :-) Although, if someone did manage to find it, then it would be (I assume) the largest commercial memory upgrade that was available. Commented May 17, 2019 at 14:30

5 Answers 5

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You might be willing to check this hardware list from World of Spectrum. It contains useful information and magazine advertisements of each hardware piece.

Specifically, DK’Tronics memory expansion kits and DK’Tronics keyboard match you DK’Tronics memories, but the memory expansion kits weren’t as big as indicated in your question: “only” 16 KiB or 64 KiB.

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  • Thanks for the link. It was definitely 128k, so it might not have been dk'tronics after all... :-( Commented May 16, 2019 at 13:10
  • 1
    I don't recall seeing anything bigger than 16k what would we need 128k for? Also it ran red hot and was prone to wobbling at critical times.
    – Ian Turton
    Commented May 16, 2019 at 17:10
  • That link to the hardware list seems to have died, I can't find it in the archive section either. Commented Jan 20 at 2:14
  • Archive.org have a copy at web.archive.org/web/20050104080357/http://…, but not every page linked off it has been snapshotted.
    – Kaz
    Commented Jan 21 at 15:09
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There were various companies producing these of which DK'Tronics was one.

For example Ceedata, Bridge Software, Indescomp and Memotech,

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  • Excellent links, thanks. Unfortunately the ads you've highlighted aren't for the product that I have in mind. However, I'll have a peruse of the magazines to try to find it. Commented May 16, 2019 at 13:48
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You may be thinking of the BASICare Micro System, an advert for which can be seen here:

http://www.zx81stuff.org.uk/zx81/publisher/BASICare/mag/SinclairUser/Issue008/Pages/SinclairUser00800004.jpg

In theory, you could use it to expand the RAM of your ZX81 up to a megabyte, although there would be very little use of it past 16K.

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  • Yes, I saw this advert, it is similar to the Memotech system, although this one is vertical, whereas Memotech is horizontal. Unfortunately, this is not the one - The one I have in mind was a bareboard, at least the ad showed a bare board, vaguely similar to the AGF joystick interface, except that it was much larger (and possibly had the ability to stack the vertical cards behind each other in banks - although as I mentioned above, I maybe mis-remembering the banks of cards/boards). Commented May 17, 2019 at 5:34
  • Link to Memotech ad. I am starting to wonder that as the board (that I saw in the ad that I'm thinking of) was not cased, that maybe it was a prototype. I think that I only saw the ad once. This was not an ubiquitous product, only mail order, not seen in local shops or chains. The ad may also have been in a more "serious", or business type, magazine, like PCW (Personal Computer World), and not in Your Computer or C&VG, as I first thought. Commented May 17, 2019 at 5:46
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    Another one
    – DroidW
    Commented May 17, 2019 at 8:58
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I am not 100% convinced that this is the device that I saw - it certainly isn't the same advert that I saw (as it does not have a photo of the expansion board, nor the 128 KB board). The company's name does not ring a bell either.

However, ignoring the photo of the 32 kB board, what is described in the lower half of this advert, by Audio Computers, certainly fits the description:

  • 128 kB (expandable to 256 kB and maybe beyond)
  • Multiple memory boards
  • Large memory boards (both in physical size and capacity)
  • An expansion board (described as a "motherboard" in the ad) for additional boards

An abridged summary:

item £
64K bytes Ram expansion 79
128K bytes Ram expansion 139
128K bytes Ram array for further expansion 110
MOTHERBOARD: 5 extra 2 x 23 way slots with extra 5V regulator and heatsink. 10
PARALLEL BOARD: Contains EPROM Programmer, standard ASC11 Printer Driver, 3 x 8 bit parallel ports 12

Note: It isn't totally clear whether the 128 KB boards work with the "motherboard" for the ZX81 - I am assuming that they do.

From Your Computer 1982/03, page 22

Audio Computers advert in Your Computer 1982/03

Looking at the photo, you can see that the 32 kB memory board is bare and uncased and is actually screwed into the back of the ZX81 for stability..!

The same issue also has a RAM pack review/shoot-out for the ZX81, on page 16. The review of the Audio Computers 32 KB RAM pack mentions the instability issue (w.r.t. the screw):

The edge connector on the Audio Computers 32K R A M pack made it extremely difficult to use. The only way I could find to keep it steady was to pull the top of the RAM pack forwards and bind it to the top of the ZX-81 with a 6in. piece of insulation tape. This meant that the edge connector was held tightly against the ZX-81's expansion port. It also meant that the RAM pack could not even be used just after switching on the power and was certainly the worst case I encountered.

There is another (earlier) Audio Computers advert that I have found (Your Computer 1982/01, page 22 and Your Computer 1981/10, page 68), which differs slightly:

Audio Computers advert in Your Computer 1981/10

It might be interesting to note that the MC6665 is used:

How do we do it?

We make the latest device in the field available to everyone ̶ the Motorola MC 6665 L20 or 64K bit on a single chip, which consumes a mere 10mA at 5V to retain your data or programs. We put eight of this on to a board the size of a cigarette packet to give you eight times the actual power of your Microcomputer.


Perhaps there is a later advert (that I've not yet found) that shows a photo the whole 128 KB board, with the expansion board plugged into the back of the ZX81 - just like the one I saw. The ad that I remember was also a lot more ZX81-centric - I don't recall mention of the 6502, or other micros (i.e. Atom, Pet, UK 101, etc.).

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@greenonline, I can't find it but think I recall the advert you are thinking of. Was a set of modules Daisy chained behind a ZX81.. offering number of models including memory and hires graphics etc.

Don't think you have found it yet, but recall it was not often I saw it even though I used to read the same magazines. I remember thinking, why would you spend large sums of money to expand a low cost computer instead of starting with an Apple II?

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  • Possibly, but you aren't thinking about the Memotech products are you? I don't immediately remember the high res module, but there may have been, although I'm worried that could be a faux-memory. WRT to expansion, it was easier, in some cases, to spend incrementally, rather than spent a lot all in one go. Commented May 17, 2019 at 9:15
  • There was also the QuickSilva motherboard and hi-res module (and other modules: Character ROM board, Sound board and 3k static RAM board) which plugged into that vertically. Commented May 18, 2019 at 8:11

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