Some Notes and Issues
The final outcome of my efforts is almost perfect in my opinion, with only a small problem remaining in FAST mode, for which there is a solution only for those who wish to work in FAST mode only, for example because they have a ZX-Team LCD interface which handles the LCD display generation independantly of the ZX81, therefore not influenced by the ZX81's FAST and SLOW modes. 
This solution involves the simple removal of a single capacitor in the slow mode circuits. Also, perhaps it would be possible to include a switch for this capacitor, depending if it can work in a stable way of course. I have not tested this, but can imagine it may work fine. 
I don't own an oscilloscope or other similar timing measurement equipment, which accounts for the fact that I am not able to improve these timing problems, simply because I can't see the timing itself which is essential for solving such problems. The ZX80 by Sinclair Research makes use of the impedance and response characteristics of 74LS series ICs in combination with R/C circuits which are used for inserting timing delays. Apparently some of these impedance sensitive circuits are influenced by the extra circuitry in the Euro-ZX81 used to implement the ZX81 SLOW mode feature, in effect to expand the ZX80's display capabilities to match those of the ZX81. This influence results in the beforementioned FAST mode problem. Of course, it is always better to not use any capacitors and resistors to adjust timing, it would be far better to use D-Flipflops with a fixed clock pulse to delay certain signals, as done in the ZX97 for example. Still, I feel this project is very valuable to build because it is based on Sinclair's ZX80 which is in it's essence completely present in the Euro-ZX81. (!)
Also I want to mention that I tried to implement as many HC/HCT technology chips as possible contrary to the original  ZX80 design since these HC/HCT ICs are more stable (providing no inputs are left open) and use very little power current compared to LS logic. 
Those ICs that I used LS types for in my component list are absolutely to be LS ICs because they are connected to said R/C timing circuits which are thrown off when they are used with a HC/HCT IC. (!)
Please note: In normal operation of the Euro-ZX81, there are no problems at all and it performs fine just like a Sinclair ZX81, completely stable. Also: In FAST mode, the small timing problem doesn't disrupt operation, it only causes an unusual display generation with slightly vertically shifted characters on the screen. As soon as you enter SLOW mode again by keyboard command, the problem is gone. So no crashes or anything will occur, and everything works perfectly during normal ZX81 operation. I have used my prototype for years and it has never failed me once to work properly. If it weren't a good design, I wouldn't have taken the trouble to perfect it and make this website after all. :-)
 

Two PCBs
The prototype I built of the final revision is realised on two Euro sized PCBs(10x16cm) that are piggy backed on top of eachother by using a set of connectors. This method does require that the two PCBs are fastened together into an assembly which does not allow for any mechanical movement in the connections between them, so make sure it's a solid assembly when you power it on, if you decide to build it for yourself. Please note that I have never had problems with this, but that is mainly because I kept it in mind from the beginning, so I never had any bad experiences so far with connection stability. I used this piggy back technique to reduce the overall PCB size of the device.

The two PCBs are called:
ZX81 Euro TTL CPU Board V2 or 'ZX81 Euro CPU Board'
ZX81 Euro TTL Video Board V4 or 'ZX81 Euro Video Board'
The ZX81 Euro CPU board contains the Z80 CPU, RAM, EPROM, and most ICs that have many address and data connections.
The ZX81 Euro Video board contains most of the video I/O control logic gates, clock pulse generator, slow mode circuits and memory decoder. The V2 and V4 are pre-release version identifiers, I started to designate at V0 which are my first prototypes.

The final ZX81 prototype which I call the Euro-ZX81 has a horizontal flatcable connector(on the CPU board) which functions as an expansion connector that allows for it to be connected in a larger system which can involve numerous PCBs with different I/O functions. For example, I was able to get my Euro-ZX81 to work very well with the so-called ZX96 assembly of the German ZX-Team, by designing a custom Signal Buffer/Poke Board PCB which on one end connects to the Euro-ZX81 with a very short 50pin flatcable, on the other side it has a 64 pin DIN 41612 connector which directly fits into the ZX-Team's ZX96 backplane PCB. The Euro-ZX81 and the Buffer/Poke board can both be inserted into a 19" rack where the other ZX96 extensions can also take place connected to the backplane PCB. To see what the ZX96 of the ZX-Team looks like, and read about the details, feel free to visit Kai Fischer's excellent homepage on the ZX96, of which Kai himself made a very large design and development contribution together with other members of the ZX-Team.
 

FAQ
For some general building/methods tips and other information about this site, check the FAQ document. Before you decide to build a Euro-ZX81 for yourself, I strongly advise to read my FAQ because it could answer some questions you may have. If not, write to me (link below) and I'll try to answer your questions.
 

Pictures
If you are curious about what the Euro-ZX81 prototype looks like, and perhaps would like to see pictures of many more things related to my prototypes and some other things I have built, feel free to visit my pictures section. (see horizontal navigation bars)
 

Next Section
To read more about the technical details of the Euro-ZX81, proceed to the Euro-ZX81 Page.(See Horizontal Navigation bars)
 
 
 

With questions I can be contacted by email
 
 
 
 
 
 

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