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The case was black, with a membrane keyboard; the machine's distinctive appearance was the work of industrial designer Rick Dickinson. Video output, as in the ZX80, was to a television set, and saving and loading programs was via an ordinary cassette recorder to compact audio cassettes. As with the ZX80, the processor was a NEC Zilog Z80-compatible, only this time of the slightly higher clock rate of 3.5 MHz. The system board had been redesigned with a custom chip and now had only four chips: a Z80A microprocessor, a custom logic chip (Ferranti ULA) or ASIC, a 4118 1Kx8 bit RAM chip and a 2364 8Kx8 bit ROM chip. The system ROM had grown to 8 KB in size, and the BASIC now supported floating point arithmetic. In the early days, Sinclair offered the ROM as an upgrade for the ZX80.
The base system as supplied (for approximately £70 in the UK or $100 in the US) had 1KB (1024 bytes) of RAM. This RAM was used to hold the computer's system variables, the screen image, and any programs and data. The screen was text only, 32 characters wide by 24 high; limited, blocky graphics were possible via a set of 16 graphics characters. To conserve memory, the screen bytes were stored as minimal length strings: for example, if a screen line was only 12 characters long, it would be stored as only those 12 characters followed by the code for a new line, the rest of the line being automatically assumed to be spaces. As a memory-saving feature, BASIC keywords were stored as 1-byte tokens; if memory grew short, the number of lines displayed on the TV screen would be reduced.
Even with all these space saving measures the little machine's memory did not go very far, so an expansion pack was available with 16K of RAM. By mid- 1982, 32K and 64K expansion packs were available. These plugged onto the main circuit board (and the 16K Memopak could be "stacked" with a 16K or 32K one) and were notorious for wobbling and losing the results of hours of programming. A printer was marketed to accompany the ZX81: This was a thermal printerDirect thermal printers create an image by selectively heating coated paper when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, creating the image. More recently, two-color direct thermal printers ha in which a wire point sparked the dot pattern into 4-inch-wide silvery-grey thermal paper, accompanied by a distinct odor of ozoneOzone (O) is a molecule consisting of three oxygen atoms. At standard temperature and pressure Ozone is a blue gas. Ozone forms a dark blue liquid, below -112 °C, and a dark blue solid, below -193 °C. Ozone is a powerful oxidizing agent, and is unstable,.
There were also an RS-232RS-232 (also referred to as EIA RS-232C or V. 24 is a standard for serial binary data interchange between a DTE ( Data terminal equipment) and a DCE ( Data communication equipment). It is commonly used in personal computer serial ports. History This stand serial interface (at ~$140) and a CentronicsCentronics is: # A manufacturer of dot matrix computer printers # A standard parallel interface contact and signal set for connecting a printer to a computer. This standard is expected to eventually become obsolete in favor of newer, faster standard trans parallel interface (at ~$105) that would allow the ZX81 to communicate to a standard printer, as well as a full-sized external keyboard (at ~$85).
DK'tronics sold a case and keyboard which, with considerable skill, could beused to replace the membrane keyboard and black "doorstop" case.
An improvement of the ZX81 over the ZX80 was that the ZX81 now had two modes of operation. In the ZX80, the video output was generated by the Z80 chip, so when a program actually ran the screen blanked until the program paused again for input. The ZX81 improved on this, so that the ZX81 could run in fast mode like the ZX80, blanking while programs ran, or the slow mode (approximately 1/4 as fast) in which the video refresh was maintained while programs ran in whatever spare machine cycles remained (hence the slow-down in program speed). Another oddity of the ZX81 was that it echoed the signal from the tape recorder to the screen whilst loading and saving programs using cassettes.
The ZX81 did not use ASCIIASCII A merican S tandard C ode for I nformation I nterchange , generally pronounced 'aski', is a character set and a character encoding based on the Roman alphabet as used in modern English and other Western European languages. It is most commonly used b but had its own character set. Character code 0 was space, codes 1-10 were used for blocky graphics, codes 11-63 corresponded to punctuation, numbers and upper case characters. Character codes 128-191 were reverse video versions of the first 64 characters. Other codes represented BASIC keywords and control codes such as NEWLINE. There were no lower case characters.
Because the display was generated primarily by software in the ZX81 ROM, it was possible to override the interrupt service routine and generate the display oneself. Several "hi-res" (meaning, 256x192, rather than 64x48) games did this, notably from a company called Software Farm .
There was a notorious bug causing some ZX81s to give the square root of 0.25 as 1.3591409 rather than 0.5. Sinclair's reputation for poor quality control was due less to the existence of the bug in some machines, and more to the time it took to react once the bug had been reported.
The ZX81 sold in large numbers, until it was replaced by its greatly upgraded successor, the ZX Spectrum.
The Sinclair ZX81 was sold in the U.S. by Sinclair itself (from its facility in Nashua, New Hampshire) and also by Timex as the Timex-Sinclair TS1000.