How to build your own ZX80/ZX81
Please note that you are NOT allowed to reproduce any of this page elsewhere on the Web without my permission!
This page details the construction of your own ZX80. Follow this link to build an NMI generator which, when used with the ZX80 circuit detailed below and the ZX81 ROM, will produce a FULLY FUNCTIONING ZX81 USING OFF-THE-SHELF COMPONENTS.
by Grant Searle
Last update: 8th June 2009
** Jan 09 - Accurately re-drawn schematic
(vector graphics) - see it
here
(Adobe reader 7 or higher needed)**
** May 09 - Version 2 of the NMI generator (exact timings) to convert the ZX80 into a ZX81
here**
Introduction
Construction
Parts List
Circuit Diagram
ROM images
Keyboard overlay
Pictures of the finished ZX80
Software
Pics of the original ZX80 kit
Links to other sites of interest
For many years now I have been working with the latest technology using high-powered computers. However, nothing nowadays can replace the time when the first home microcomputers hit the streets. I am a keen collector of the home computer era. Follow this link to see my current collection. In those days you HAD to make your programs fit the hardware and memory available. This added to the enjoyment of programming to see something you had written to fit in a few K spring to life. The later ZX81 is very similar to the ZX80 except a lot of the ICs in the 80 has been merged into one chip. As a result, you can learn a lot about the workings of the ZX81 by referring to the ZX80 circuit. I have several computers from the start of the 80's but have always liked the Sinclair ZX80 for the following reasons:
1. It was so tacky :)
2. It did not have a single dedicated IC in sight.
It is because of the second reason this page has come into existence. Here I show you how to build your own piece of history. Virtually all parts for the ZX80 are available from most of the larger electronic companies. The remaining parts (the Z80 processor, normally) can be picked up cheaply from eBay.
I must point out this is not a job for the absolute beginner. You will need to blow your own EPROM image and it will probably be a distinct advantage to have access to an oscilloscope in case it doesn't work. I can assure you the circuit supplied here DOES work without any modifications.
I have, however made the following modifications for my own version:
1. I originally used a 6264 SRAM to make the base memory 8K instead of 1K. I have since
replaced this with a 62256 (a 32K x 8 chip) to bring the memory up to the "full
16K" (the unused address line is tied to a power line).
2. I could not get the ceramic resonator. Instead I used a crystal and used a load
capacitance of 47pF.
The complete computer is built on a large piece of perforated stripboard. All digital connections are made using wire-wrap wire as this is a lot thinner than conventional hook-up wire so the data and address bundles can be tied to form a neat layout. The positioning of the ICs in relation to each other matches the positioning in the original ZX80.
I recommend printing the circuit out on an A3 sheet if possible. The pin numbers are easier to read on my re-drawn schematic than on the original scan.
Begin construction by positioning the IC sockets (see my pictures below for guidance), ensuring there is sufficient space between then to make all of the connections, and position the remaining components.
I would recommend that you adopt a layout similar to Sinclair's (that is what I did).
Here is my re-drawn version of the layout found in the ZX80 assembly manual:
Click on
this image to see a large version (c) Grant Searle.
When you click on the image using Internet Explorer or Firefox, the large
version will be displayed bit it will be scaled to fit the window, and will not
display the image at
it's best. You normally need to click on the image which is then displayed to
zoom in full-size.
The pale-green components are not actually used in the circuit, but there are
spaces for them on the PCB.
Here is my version:
Click on this image to see a large version.
As you can see, the component placement is in a similar manner to the original. The two original RAM chips replaced with a single larger one instead.
Next I recommend wiring the data and address buses for the top half of the circuit. As each wire is soldered it, mark it off on the circuit diagram. This helps avoid any errors resulting from missing connections. Wire in the logic ICs, resistors, capacitors and diodes. As virtually all pins on every IC are to be connected, any errors made will become obvious as more connections are made. For initial testing, there is no need to wire the keyboard into the circuit.
Once all components are soldered and without any ICs inserted into the sockets, connect a continuity tester between the power supply pins. If a short exists then check the underside of the board for any missing track breaks or flakes of copper between tracks. If all is well connect a power supply (current regulated to 500mA if possible). Check the +5V and 0V connections on each IC socket. Turn off the supply and insert the ICs. Turn on the power supply and the television or monitor. If the circuit is working then expect a power consumption of around 310mA. If using a TV then adjust the tuning to get a picture (note: the display of black text on white will require the brightness control of the TV to be turned up). A working circuit should show an inverse "K" in the bottom left of the screen. If all is well then wire in the keyboard if not already done.
This is a full list of discrete components required to build the ZX80 excluding any hardware such as power supplies, nuts and bolts etc.
Component Type Qty No IC1 Z80A 1 IC2 2732 4K EPROM 1 IC3/4 6264 8K SRAM 1 IC5 74LS373 1 IC6-8 74LS157 3 IC9 74LS165 1 IC10 74LS365 1 IC11,12 74LS00 2 IC13 74LS04 1 IC14,15 74LS05 2 IC16 74LS10 1 IC17 74LS32 1 IC18,19 74LS74 2 IC20 74LS86 1 IC21 74LS93 1 IC22 7805 1 D3-10 1N4148 8 X1 6.5MHz Xtal or 1 ceramic resonator C1-6,12,13 47nF 8 C8 22uF 1 C9,11,14, 15 47pF 4 C10 1uF 1 R29 180R 1 R35 1M 1 R1,3-11,18, 22,23,27, 30,34 1K 16 R32 330R 1 R21 220K 1 R13-16,? 47K 6 R2,25 470R 2 R19 2k2 1 SW1-40 SPST Push 40 Wire wrap cable for interconenctions 1 reel (25m) 3.5mm Jack Socket 2 Phono socket 1 Vero Board 1 40 Pin DIL socket 1 28 Pin DIL socket 1 20 Pin DIL socket 1 16 Pin DIL socket 5 14 Pin DIL socket 11
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I used this diagram to build my version of the ZX80. As a result, I know it is correct, apart from the following errors on the original schematic:
- Thanks to Ismael Pérez for finding many of the errors !
 |
Notes about the circuit diagram MREQ is actually obtained from the output of the NOT gate, IC13,pin 2 (/MREQ is the input). SYNC (IC19 pin 5) goes to the microphone socket (via R15) and to the modulator (via R32). /SYNC (IC19 pin 6) is not actually used and goes to R36 (unused!) on the real ZX80 PCB. Errors on the original scanned version have been corrected on my re-drawn version. |
I have painstakingly redrawn this circuit using vector-based graphics so it
can be scaled with NO pixellation!
I have kept my version as faithful to the original circuit as is possible, but I
have corrected the errors on the original schematic that I have mentioned above and I have added
the missing pin numbers (4,10 and 13) on IC19 near the bottom-right of the
scanned circuit. Apart from that, everything on my redrawn circuit is very
accurately placed. I resized the scanned version to A3 and over-layed my new
version on top of the scanned version while I was developing it. For most of
the diagram, my placement is well within 0.5mm of the original when scaled to be
the same overall size of the original.
Adobe reader 7.0 or higher is required to open the PDF - you will get an error
message if you use an older version.
Click here to get this re-drawn version (PDF format)
Click here to download the original scanned ZX80 circuit
The ZX80 ROM is 4K long. I actually burnt it into an 8K EPROM as that was the smallest I had in my spares box.
Click here to download the ROM in binary format
You can also use the ZX81 rom image (8K):
Click here to download the ROM in binary format
For the ZX81 ROM to work you must take A12 to the appropriate address line on the ROM and ground the /OE pin.
I actually now use a 16KB ROM with BOTH images blown into it with a switch on the A13 line to determine whether to run the ZX81 or ZX80 image.
The overlay here is sized to fit over a set of miniature tactile pushbuttons set at 0.3 inches between centres. You can either resize as necessary or cut out the keys to place on standard keys.
I have spent a lot of time on these overlays to make them as faithful to the originals as possible.
Click
on the image to view a 600DPI version (c) Grant Searle!
When you click on the image using Internet Explorer or Firefox, the large version will be displayed scaled to fit the window, and will not show them at their best. You normally need to click on the image which is then displayed to zoom in full-size.
Higher resolutions are available (as I drew them using vector graphics so I can rescale as much as required). Please let me know if you want them.
For best results, these need to be printed onto glossy photo paper.
Pictures of my finished version (ZX80 only)
Pictures of my ZX80 with NMI option (making it into a ZX81)
Will be here shortly!
ZX80
Follow this link to go to my ZX80 software page, featuring a flicker-free space invaders game for you to type in!
ZX81
A software archive for the ZX81 is available at ftp.nvg.unit.no. (No archives on
the Net for the ZX80 at the moment. If I can, I will move my ZX80 tapes into this area).
These files are in the ".P" format as used by XTENDER, the ZX81 emulator. To use
these on a real ZX81, you can build a VERY simple cable connected to the parallel port and
use this program by Wilf
Rigter to send the files to the ZX81.
Links to other sites of interest
My pages
Build your
own ZX80 - my page showing
you how to build this old micro
|__
ZX80 to
ZX81 conversion - build the NMI generator needed to convert the ZX80 circuit
into a ZX81
|__
ZX80
software - Type in a Space Invaders game into the ZX80
Build your own Jupiter Ace - my page showing
you how to build this old micro
Build your own UK101 - my page showing you how to build
a greatly simplified version of this old micro
Pong - Pictures of
my build of the Atari classic arcade game
My
Machines - My collection of classic 80's micros
Other user's pages
ZX81 Rules - Wilf Rigter's
excellently detailed pages for ZX81 circuitry. He was a great help with the
ZX80/81 video details
Rodney Knapp's
homebuilt micros - lots of information on the ZX81 computer which he has
built
Paul Robson's ZX80 and ZX81
Emulator - excellent freeware emulator
The German ZX-TEAM
- Sinclair enthusiasts page by Peter Liebert-Adelt
"The" software
archive for the ZX81 - at ftp.nvg.unit.no
XTender homepage - another excellent
emulator (shareware) by Carlo Delhez
I hope this page has been useful.
Grant.
To contact me, my current eMail address can be found here. Please note that this address may change to avoid spam.
