This page and the next pages you can visit from here document the information necessary to build the ZX81 CPU Board V2 which can pair up with the ZX81 Video Board V4 to make a working Euro-ZX81 assembly. Click the links above or below to proceed to the schematic, component placement diagram and partslist pages.

Some Notes

This is the CPU Board V2, which means that it's the third version. (starting from V0) It turned out that I didn't need to update this board with the next revisions of the Video Board (V3 and V4), because there were no changes on this CPU board that warranted a PCB revision. During my updates of the entire Euro-ZX81 design I merely left out a few parts on this CPU Board to allow for more HC/HCT logic to work properly. It has been my strategy with the Euro-ZX81 to have as many HC/HCT ICs as possible, even though some ICs in the Euro-ZX81 need to be LS types. So we get a mix of LS/HC/HCT. I know some people consider this to be a bad idea and bad design, but remember that the original schematics were from Sinclair Research and a few other people who made the updates to upgrade to ZX81 functionality years ago. Basically these original designs, which I used as a basis for the Euro-ZX81, are the reason why there are some LS type ICs necessary in the Euro-ZX81. I don't really mind this combination, I just keep in mind to have as much HC/HCT as possible, and where it isn't possible I'll use LS. The IC types are clearly marked on the partslist page and they should be exactly those types since I can't guarantee that it will work if you use different ICs. Feel free to try any combinations, however, be warned that it could be difficult and there could be other unwanted side effects that are at first perhaps not even so clearly visible.

The parts that can be left out on the CPU Board are C33 and R2. These are not present in the partslist to avoid confusion. D11 should be used only if you want to use a NTSC television.(in the USA and Japan for example) If you leave it out, you will get a PAL video signal.

You can see in the pictures(link below or on Pictures page) that I used pieces of precision IC socket as connectors on the board. They have many advantages, of which I find their relative small size and 0.8mm hole suitability most important. If I would use connectors that needed 1.0 or 1.25mm holes, the pads would have been much larger, so I would have had less space on the board. My single sided design technique is difficult enough as it is, so I always try to save space when I can. Also, these connectors have proven to be very reliable during my past years of prototype building.

ROM Image

The ROM image that I used to burn into the 27C64 EPROM is of course the standard ZX81 ROM. You can download the ROM image from this link.
 

Pictures of the finished CPU Board V2
 

Euro-ZX81 CPU Board V2 Component Side	Euro-ZX81 CPU Board V2 Solder Side

 

Some Further Euro-ZX81 Reading
To read more general info about the technical details of the Euro-ZX81 as a whole, go to the Euro-ZX81 Page.(See Navigation on the left)
It's also a good idea to read the faq document since it provides some additional info and tips.
 
 
 

With questions I can be contacted by email
 
 
 
 

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