viernes, 7 de julio de 2017


Reading the works of Grant Searle, I have decided to start with the project to build a full computer to understand in depth his functionality. What can be more proper that a ZX80 totally based in discrete chips? 

In my spare time, I have started to work on it and you can find the results in the following lines.

I have copyied the schematics from the original zx80, and have made some modifications on it.

- I have added the extra circuitery to generate /NMI signal (Grant Searle Schematics), making possible to have two machines in one, by selecting it through a switch.
- Original 1K RAM in two chips have been replaced by one SRAM chip of  32KB.
- Original 4K ROM have been replaced by a EEPROM of 32KB. It necesary to store both ROMs (ZX80/ZX81) and it is cheaper than other smaller EPROMs.
-  RF modulator have been removed and replaced by a circuit to generate the standard back porch, missing in the original ZX80 PAL signal.


The final result is nor a exact copy of the original as the case of Grant, but the board match accurately in the original case, and I very happy with the result.


Below you can find the necesary information to build your own zx80. I think the most funny way to obtain your clon is to do it from zero, but if you wanna a clon and need the PCB or you have not enough knowledge or experience to assembly it, I have some spare PCBs and componentes, and I can sell them or assembly the clon for you. Please feel free to contact me and I'm sure we can reach an agreenment. ;)

Current version: 1.1



JP1: RAMOE, Not relevant but must be present in one position
              1-2 - Inverse Video
              2-3 - Standard Video
S2: ZX80/81 Mode selection (If you use external selection from the expansion board this jumper have to be removed)
              1-2 - ZX81 mode
              2-3 - ZX80 mode.

S4: Function of the Pin23B in the expansion bus. 
In original ZX80 pin 23B of the expansion bus is spare, but in a ZX81 PIN23B is use as /ROMCS signal.
If you want to use external zx81 mode selection form the expansion board you have to use position 1-2. In this case (in zx81 mode) cards replacing the internal ROM will not works.
If you prefer to use this kind of cards, you have to wire the zx81 selector (S2) directly to an accesible switch and set S4 in 2-3 position (remember to remove JP1 in the expansion board)
              1-2 - ZX81/ZX80 mode selection in PIN 23B
              2-3 - /ROMOE in PIN 23B

Back porch Grant Searle solution:

              JP3 = 2-3
              JP5 = OFF
              JP6 = 1-2

Back porch generation José Leandro solution:
              Adjust image with R37 & R33
              JP3 = 1-2
              JP5 = ON
              JP6 = 2-3

Without Backporch generation (origina ZX80 video)
              JP3 = 1-2
              JP5 = OFF
              JP6 = 1-2



This board has double functionality, first leaves enough room to connect zx81 cards  without trip with the video connector, and by other hand adds some extra functionality to the main board.

Layout add-on
Eagle Files
GERBER files
Schematics in PDF format

Expansion board let you to supply power to the computer with a miniUSB charger. If you plan to use external cards using 9V you will must to connect the computer to a 9V PS through the standard conector.
If you use the miniUSB supply, you could use the ON/OFF switch present in the board (S1) to turn on the computer.
S2 is a reset button, and it will works in any case.
JP7 Lets you to select ZX80/81 mode from the board if the position of the main jumper S4 is 1-2 and JP1 in the expansion board is ON.

JP1 Lets you aisle pin23B in the computer from the JP7 selector. It must be OFF if computer jumper S4 is 2-3.

Clon working together with the ZXBlast

CONTACT: wilcoavs at gmail dot com

18 comentarios:

  1. Este comentario ha sido eliminado por el autor.

  2. Hi wilco2009

    Excellent work :-)

    Thanks for publishing all the details :-)


  3. Hello,

    I am very interested in your clone - do you still have parts to make one please ?


  4. Yes, I still having parts.
    Kit price without case is 65€ + shipping.
    Case is sold by a colleague. I can ask him for the price of a case and ship everything together if you want.

    1. Alejandro, that is some nice work! I am trying to print the ZX80 case from the files by marcus8008, but all of my software finds issues with the shell bottom. Would your colleague be willing to share his file or its source?

    2. Please, send me an email to te above address, and I will ask him to contact you about that.

    3. Hi, I'm the designer of the case, I will publish the files to print yourself one in a future, in thingiverse website. but not now, I'm selling/printing it for several people, and for respect to they, I'm not publish until all of they have one, so I calculate that about xmas I will make it public.

    4. Hello Álvaro, can you make cases for people in the UK please as several people are interested to buy your case, thanks.

  5. Yes please that's really great - could you assemble kit for a charge ?

  6. Please, send me an email to wilcoavs at gmail dot com to close the details.

  7. Hola. Tienes disponible la pcb y los componentes para poder comprar el kit?

    1. Placas tengo y creo que tengo todos los componentes, aunque tendría que hacer recuento.
      Si te interesa el kit lo vendo a 50€ envío a España incluido.
      Si eres de fuera de España tendría que ver el coste del envío.