My thoughts on FPGA based ZX Spectrum clones

I would like to share my thoughts about FPGA clones of ZX Spectrums.
I was discussing this with some people during the last weeks, and thought to gather the things I wrote and make them public.

First, I really do like the developments that are going on, like the the ZX Spectrum Next, the ZX UNO and its XL brother the ZX GO, and more FPGA implementations.

But even though both the Next and the ZX-GO have some kind of an edge connector, to be honest I don’t think they are what the majority of enthusiasts are looking for.
Please read that again carefully: I say ‘majority of people’ since there its definitely interest in improvements like the Next, but machines with specific new features IMHO actually create new communities with ZX Spectrum roots.
I do want a complete Next myself 😉

But getting to what I call ‘purely ZX Spectrum FPGA based clones’..
(And again: this is what I think, and with some details I might be wrong or misinformed)

I think there will be interest in a ‘fully ZX Spectrum compatible’ FPGA-based board, when it:
1. Can be used with no hassle
2. Has a completely hardware add-on compatible edge connector
3. Are really affordable

Let me explain.

1. No hassle, just plug and play

When it comes to starting to use an FPGA clone, it’s often or maybe in al cases not as simple as one would like I think:

  • A problem with many FPGA based clones at this moment is that they’re most often not as intuitive to use as an original ZX Spectrum: power on, get to BASIC and do what you want.
  • Also documentation is IMHO far from available or ready, which makes it users hard to use it simply as a ZX Spectrum.
  • Then I don’t like boot-delays because the system needs to go through a more general startup-procedure.

Then none of the current implementations as far as I know fit in an unmodified rubber-key case.
You only can easily obtain replicas of rubber-key cases, the other cases are harder to get but if you want to use an original case, the rubber-key one is the easiest choice.

So I think a clone should have just composite at RF, tape in and out at EAR and stereo sound at MIC (like the Harlequin 128) and DC in with an integrated bridge rectifier for safety and PSU compatibility.
I think there would be a LOT of interest in a ‘drop-in replacement’ board.

2. Hardware compatibility

When I was testing the edge connnector of the Next with several add-on, I couldn’t get it working.
The assistance I got during a live stream gave me the feeling that the team not yet put in much effort to test this themselves.
Which also gives me the feeling it doesn’t have much priority, and I wonder it’s actually possible to make it work completely: my experience with the development of the ZX-HD is that if it needs to do a lot on hardware level, a SOC or FPGA has its limitations.

Now with the UNO both the original version and the ZX-GO variant do not get a compatible edge connector.
I personally don’t like a breakout-box like required on the ZX-GO to make it possible to connect add-ons.

I think I’ve seen one other FPGA based clone with edge connector, but I think it was using a separate Z80 and many more parts, so it wasn’t as integrated as the ZX UNO and Next.

When it comes to the emulation: of course it should be accurate, but I have no doubt that is at a high level at this moment.
Though I do wonder how hard it will be to integrate full hardware I/O compatibility on the edge connector.

And that’s where I think a major problem lies: FPGA developers tend to spend a lot of time pushing as many features into the core as possible, and maybe only later start to think of hardware compatibility when it’s requested to use their FPGA implementation on a ZX Spectrum compatible board.
I think many cores started as ‘software emulation with original ZX Spectrum keyboard inputs’..

3. Affordability

I think a FPGA based clone board should be available for under a £100, maybe even less, something like £75.
The reason I say £75 is that since it can be done with only an FPGA and some minor and very cheap discrete parts, the board should cost less than a Harlequin kit, JS128 or something else.
But I know the cost price of one FPGA alone can be around £50, but maybe I’m mistaken.

My conclusions for now then

I think it’s time for someone (or a team) to create a, let’s say, an FPGA based toastrack clone, that fits in a rubber case without the need of customizing the case.
For about £75, which means parts should be £30 or less to make any profit (really, I know).

If anything should be added – like an SD card slot with logic inside the FPGA core, or RGB, VGA, HDMI or anything else, there should be a lot of thought put in of how to add those without needing to modify the case.
If that can’t be done, I’d say: put the additional features in an external interface.
But people should be able to use existing add-ons as well.

Talking about a ‘it can do all’ interface: I’m really curious about the DivGMX which seems to combine things like HDMI with sound over HDMI, DivMMC, TurboSound and much more on an ordinary ZX Spectrum.
But.. I’m not sure if it actually ‘uses’ the connected ZX Spectrum and acts like an interface, or that it ONLY uses the ZX Spectrum ‘as a keyboard’.
There should be one on its way for me to test soon.


3 thoughts on “My thoughts on FPGA based ZX Spectrum clones

  1. What I really want is the SD card interface and HDMI out.
    Extra joystick ports are nice too (usb interface would be fine for space saving)

    I have the Enjoy! Pro 1 and the ZXHD and it works well together, but I think it looks a bit ugly and unstable together. Possibly easier to Damage.

    The DivGMX look tiny although missing joystick ports (unless thats what the usb is for ?)

    If someone could get the functionality of these 2 devices into a smaller board + a nice case I’d be interested.

    Whether its done on FPGA or not.

    A smaller power pack for my +2A would be good too as the current one is massive and takes up space. I think your working on that ?

    • Well, I was discussing this on Facebook and I think the UNO XL or ZX-GO have the best of both worlds:
      They’re fast and proper ZX Spectrum clones, and have SD and advanced video outputs (though not digital yet) available at the back.
      But those additional connectors are put at the edge connector slot in the case, so no full edge connector by default.

      The developers of those are still working on things like HDMI / DVI / DP, so most probably it will come.
      But if someone would want to use lets say a PlusD interface or something, a breakout board should be added to get a full edge connector.
      That breakout board does have proper protection for the FPGA though, which some think the Next lacks, which could cause defects, I’m not sure.

      I need to test the DivGMX to find out what it is capable of.
      If it’s interesting, I would design another version with features that I will discuss in forums.
      But let’s first wait for the tests.

      Yes, I should have the +2A/+3 PSU replacements available soon, but there was way too much work during the last months.
      Fortunately the new production line is now running, so more time is coming!


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