Sinclair ZX81

The Sinclair ZX81 home computer was released in 1981 by Sinclair Research as a follow up to the popular ZX80. The unit was very popular in both the UK and the US until it was replaced by its greatly upgraded successor, the ZX Spectrum.

The base system contained 1 kB (1024 bytes) of RAM, which was used to hold the screen image, the computer's system variables and any data and programs. Expansion packs were available: 16 kB (which cost as much as the unit itself), 32 kB and 64 kB. The packs were plugged onto the main circuit board expansion bus edge connector, and were renowned for their loose and wobbly connections that could cause a computer crash with just a swift jolt.

As with the ZX80, the processor was a NEC Zilog Z80-compatible, which ran at a clock rate of 3.25 MHz. However, the system board had been redesigned and contained only four or five chips: a Z80A microprocessor, either one 4118 1Kx8 bit or two 2114 1Kx4 bit RAM chips, a 2364 8Kx8 bit ROM chip and a custom logic chip (Ferranti ULA) or ASIC. This was compared to the ZX80's 21 chips.

Video output, as in the ZX80, was to a television set, which could display monochrome text only, 32 characters wide by 24 high. Blocky graphics with a resolution of 64 by 48 pixels could also be displayed by the use of the PLOT command, which creatively selected among a set of 16 graphics characters. The ZX81 did not use ASCII but had its own character set, where character code 0 was space, codes 1-10 were used for blocky graphics, codes 11-63 corresponded to punctuation, numbers and upper case characters, and codes 128-191 were reverse video versions (white on black) of the first 64 characters. Other codes represented BASIC keywords and control codes such as NEWLINE.

Saving and loading programs could be carried out using an ordinary home audio tape recorder and magnetic audio tapes. A spark printer was also released to accompany the ZX81, in which a wire point burnt off the surface of 4-inch-wide aluminised paper to reveal the black paper underneath. However, due to FCC compliance issues, the ZX Printer was not marketed in the US, and the Timex-Sinclair 2040 thermal printer was offered instead.

Although the ZX81 did not have the ability to make sound, it was possible to modulate the interference that the processor caused on the TV to create a very simple musical keyboard.