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

The ZX-80 could not handle floating point numbers or cassette files, but the ZX-81 could. The ZX-80 had 4k ROM compared to 8K in ZX-81. In addition ZX-81 had 30 additional functions and some instructions to drive the printer. Thanks to a higher level of integrations 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 with 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$, PI, SGN, SIN SQR, INT, LEN, LN, TAN, VAL, <=, >=, < >, COPY, DIM A$, FAST, FOR ... TO ... STEP, LLIST, LLIST n, LPRINT, PAUSE, PLOT, PRINT AT, PRINT TAB, SCROLL, SLOW, UNPLOT. Some are quite useful and make you wonder how it was possible to make anything on the ZX-80!? However, one function has disappeared: 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 without the need to see what is going on 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 was 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 though its performance was actually quite poor.

A very great range of peripherals were developed 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.