Tape Storage of your Programs
If you plan to use an audio device such as a cassette recorder or your PC soundcard for saving and loading, you can make use of the Euro-ZX81's Tape input and output ports. The 'EAR' port(J3 on the CPU Board) is the ZX81's audio input port, connected to the Earphone output of the cassette tape recorder, and the 'MIC' port(J4 on the CPU Board) is the ZX81's audio output port, connected to the Microphone input of the cassette tape recorder(or PC).
I did use a different value in the circuit diagrams for some parts because with the R/C values as in the original Sinclair schematics I had problems recording programs through the MIC output. After some experimenting I got good results with the values as in the current circuit diagram. 
Also there may be problems with the keyboard being slightly unreliable especially with long keyboard wires (capacitive), and also with loading programs from tape or PC soundcard. These problems could be solved by using a 74LS365A instead of a HC type for U10, which is the input port for both keyboard and tape input. An LS type will be much less sensitive.

Keyboard
For those not familiar with the ZX81 hardware, the keyboard that is used with the ZX81 is called a matrix keyboard. This means that all the keys used are mapped in a matrix position. When a 'row' line is connected to a 'column' line by a keyboard switch, it will be registered by the computer and the action that goes with this keypress will be executed by the computer. The 'row' signals are located in J7(CPU Board) and the 'column' signals in J6(CPU Board).
The row signals of J7 are then devided in two groups of 4 signals that pair up with the 5 column signals of J6 to make a block of 4x5 keys. Two of these blocks then add up to 40 keys total. The matrix keyboard is more clearly illustrated by the following table:
 
 

Block 1	J6-1:	J6-2:	J6-3:	J6-4:	J6-5:	Block 2	J6-5:	J6-4:	J6-3:	J6-2:	J6-1:
J7-4:	1	2	3	4	5	J7-5:	6	7	8	9	0
J7-3:	Q	W	E	R	T	J7-6:	Y	U	I	O	P
J7-2:	A	S	D	F	G	J7-7:	H	J	K	L	Newline
J7-1:	Shift	Z	X	C	V	J7-8:	B	N	M	.	Space

Please note: most keys have several functions. These functions can be selected by the following methods: 
1. Shift + Key 
2. Shift + Enter: changes the cursor into a 'F' which is the 'function mode'. In this mode, most keys have extra functions that are accessed by simply pressing the key after you have changed the cursor to 'F'. After pressing a key, the 'F' cursor becomes a 'K' or 'L' cursor again. 
3. Shift + 9: changes the cursor into a 'G' which is the 'Graphics mode'. Keys pressed with a 'G' Cursor will result either in inverted text or in combination with shift some of them produce graphic symbols. The 'G' cursor will not change back by itself, you will have to press Shift + 9 again in order to change back to normal typing mode.
4. 'K' cursor mode. This mode results in a Keyword when certain keys are pressed. 
5. 'L' cursor mode. This mode results in Letters when you press the letter keys. You can however still use Shift+Key and 'F' + Key  in this mode which in some cases also produces keywords and functions. 
Also note: the ZX81 handles the 'K' - 'L' cursor transitions automatically. For example, when you enter a keyword with the 'K' cursor, it changes to 'L' and you're able to enter variables, mathematical symbols etc. 

Jumpers/Options
JMP2 selects border color, and JMP1 selects character color, however note that JMP1 inverts the setting of JMP2 if changed. 
For example, set both jumpers to 1-2 to make sure you can see the picture on the tv screen because with certain settings can be difficult to see the tv picture. 

Diode D11 (CPU Board) selects the TV standard: do not place it when you want 50Hz (PAL, for example in Europe); Install it to get 60Hz. (NTSC, for example in the US) 

Memory decoder
The ZX81 memory decoder operates as follows for the main memory blocks: 

Memory-map
 

Mem Area:	0000-1FFF	2000-3FFF	4000-5FFF	6000-7FFF	8000-9FFF	A000-BFFF	C000-DFFF	E000-FFFF
Normal:	ROM	RAM	RAM	RAM	RAM	RAM	RAM	RAM
/M1 low:	ROM	RAM	RAM	RAM	RAM	RAM	RAM*	RAM*

* during /M1 low, and certain other signal conditions, DFILE is read from RAM, and any memory access to the C000h-FFFFh area is redirected by the memory decoder to the 4000h-7FFFh RAM memory area. This is necessary to achieve proper display generation and continued stability while the ZX81 is executing programs in the higher RAM areas. 

PCB Layouts ?
The PCB layouts are large PCX 1bit bitmap files. In order to save webspace, I have decided not to put these large files on the website, but instead you can email me and request the files. (be sure to mention which computer and which version you're interested in) I will then respond as soon as possible with the requested file in an attachment. If you have some special way of accessing the internet or have problems with large attachments feel free to discuss it with me by email. I'm sure that something can be arranged in any case.
 

As I mentioned on the ZX81 main page, the Euro-ZX81 consists of two euro sized (10x16cm) PCBs. I have made seperate pages for both boards, discussing the schematic, component placement layout and partslist for both PCBs seperately to avoid any confusion while building them. You can proceed to these sections by clicking on the buttons in the horizontal navigation bars.
 

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)
 
 
 
 
 
 

With questions I can be contacted by email
 
 
 
 
 
 

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