ZX 81

ZX 81 - 03

The Sinclair ZX 81 is the successor of the ZX 80, and can be considered
as an evolution of it. 

The ZX80 could not handle floating point numbers or cassette files,
but the ZX-81 can.
The ZX-80 had 4k ROM : the ZX-81 has 8K ROM with 30 additional functions and
some instructions to drive the printer.
Thanks to a higher level of integrations (the total number of chips in the
basic system is 4, against the ZX80's 21), the ZX-81 cost £30 less
than the ZX-80.
The plastic case is also different.
Whereas the ZX-80 looked cheap in it lightweight white case, the ZX-81 is
beautifully designed in its black ABS plastic case. 

The keyboard is still formed by an underprinted plastic, but this one is
made of non-reflective material.
Even whith this slight improvement it's quite horrible to use, that's why
a lot of additional keyboards appeared quickly on the market.
Some of the keys have up to 5 functions, right in the Sinclair tradition,
accessed through the "GRAPHICS", "SHIFT" and "FUNCTION" keys. 

Here are the functions added over the ZX-80 : ASN, ACS, ATN COS, EXP,
INKEY$, PI, SGN, SIN SQR, INT, LEN, LN, TAN, VAL, <=, >=, < >, COPY,
Some are quite useful and makes you wonder how it was possible to make
anything on the ZX-80 !? However, one function has disapeared : TL$ which
was used to return a string minus its first character. 

The ZX-81 can be operated in two modes, SLOW and FAST.
The FAST mode which was the only mode available on the ZX-80, only refreshes
the display when the system has finished computing, resulting in a painful
screen flicker !
It is a useful mode when you need to do a lot of calculation whthout the
need to see what's going all the time on the screen.
The SLOW mode, which is indeed quite slooooowwww, behaves like all the other
computers do, refreshing the screen all the time. 

The ZX81 contains only four main chips : the ROM, Z80A CPU, 1K RAM and the
Ferranti custom-made chip ! It is as simple as that.
The machine was assembled by Timex Corporation in their Scottish plant. 

This computer had a very great success in europe in the beginning of the 80's.
It was very cheap and a lot of people who now are working on modern PCs or
Macs, made their first move on a ZX 81 even if it had actually poor
performances ! 

A very great range of peripherals were developped for this computer, among
them : 3.5" floppy disk units, keyboards, high resolution graphic cards,
RS232 or Centronics interfaces, RAM expansions, etc... 
In fact, it was possible to make a pretty good computer from the ZX 81 ! 

It was followed by the ZX-Spectrum and was also sold as the
Timex Sinclair 1000 in the USA. 


Manufacturer Sinclair Name ZX 81
Type Homecomputer Origine UK
Introduction Date 1981 End of production ???
Built in Language Sinclair Basic Keyboard Touch-sensitive keyboard, 40 keys
CPU Zilog Z80A Speed 3.5 Mhz
Coprocessor ??? Amount of Ram 1 kb (901 bytes available, up to 64 KB)
Vram ??? Rom 8 Kb
Text Modes 32 x 24 (but 2 lines are reserved for system messages and commands) Graphic Modes 64 x 44
Colors Black & White Sound None
Size / Weight 167 x 175 x 40 mm / 350gr Built in Media None
I/O Ports Z80 Bus, tape, video OS ???
Power Supply 9V DC , 600 mA (centre polarity = +ve) Introduction Price Kit :£50 (UK, june 1981) / Ready-built : £70 (UK, june 1981)
Sold ??? Serial Number ???
Other Extras a lot of velleman kits, manuals, cassettes, selfmade keyboard with built in cassette recorder,4 expansion modules (2 unopened) Bought Where Temse
Bought When April 13, 2002 Condition OK
Price Paid 25 € Specs of my Model ???
Setup Today ???

Left view Selfmade keyboard

ZX 81-03:

Bought via auction site for 20€ on 13/04/2002 (Temse)

This is really strange stuff.

I got 3 ZX 81 “motherboards”, a lot of velleman kits, some manuals and some cassettes.
It came with a selfmade keyboard with built in cassette recorder.
I also paid 5€ for 4 expansion modules (2 unopened).
Took me about 1 hour to find out how all this worked.