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The Sinclair ZX81

ZX81.gif Starting in 1981, the ZX81 replaced the ZX80.

The ZX81 had 8 kB of ROM with approx. 30 additional functions and instructions and could handle floating point numbers. Many standard TTL ICs were integrated into one custom chip, so that the total number of chips was not more than 4. Therefore the ZX81 was even cheaper than the ZX80 was: Only £70 for the ready-built system and £50 for the kit. The case was smaller than the ZX80's and black. The keyboard was still a printed plastic membrane and horrible to use. Due to aging the narrow bent plastic connection leads are nearly almost cracking – making whole rows and columns of keys non-funcional. A nuisance!

Basic used the single-key token entry system like the ZX80 and later the ZX Spectrum. Some keys yielded up to 5 tokens, using the shift and function keys for selection. Otherwise the keyboard would have been nearly impossible to use. Most additional words dealed with floating point functions or the printer.

The ZX81 could be run in two modes, SLOW and FAST. In FAST mode the ZX81 only created a TV display when the system was waiting for keyboard input. While the program was running, the video signal was not maintained. This mode was the original ZX80's screen refresh method. In SLOW mode, the screen was always properly displayed, but this meant that the CPU could not execute the user programme in this time, which could only be executed in a small time slot per frame. So this mode was really slow, approx. only 1/4 of the FAST speed (and even much less if you changed the screen refresh from 50 to 60 Hz like TS1000), but sufficient for entering text and Basic programmes.

The ZX81 contained only four highly integrated chips: the ROM, a Z80A CPU, 1 kB of RAM and the Ferranti ULA, the custom-designed chip, which replaced all the others of the ZX80's 21 chips! The first models had 5 chips, because the 1 kB RAM was split into two 4 bit wide 2114 ICs.

Due to it's low price, the ZX81 was a very great success in Europe in the beginning of the 80's. From March '81 to December it sold over 250,000 times and by August '83 over 1/2 a million units were shipped. Lots of peripherals were developped for this little machine, first of all RAM expansion packs, but also proper keyboards, floppy disc interfaces and so on.

The machine was assembled by Timex Corporation in their Scottish plant and thus rapidly was also sold as the Timex Sinclair TS1000 in the USA. But sales of the TS1000 in the USA started later than the introduction of the ZX Spectrum in the UK, which was introduced in April 1982 in the UK market, featuring colours and rubber key tops.


based on: www.old-computers.com

Name              ZX81
Manufacturer Sinclair Research Ltd., Cambridge, UK
Production March 1981 (UK) - 1982 (?)
Built-in language Sinclair Basic
Keyboard Membrane keyboard, 40 keys
CPU Zilog Z80A or NEC D780C-1, 3.25 MHz
RAM 1 kB, up to 901 bytes available (upgradable to 56 kB)
ROM 8 kB
Text modes 32 chars x 24 lines, 64 non-ascii characters
. 22 lines available for PRINT, lower 2 lines for INPUT
Graphic modes 64 cols x 48 rows using 10 block graphics characters
Colours Monochrome
Sound None
Size, weight 16.7 (W) x 17.5 (D) x 4.0 (H) cm, 350 g
I/O ports TV, Z80 bus, tape (approx. 300 bps)
Price Kit: £50,00 (1981,UK)
. Assembled: £70,00 (1981,UK)


Good emulators need to mimic the hardware down to the cpu cycle, because games and demos generate high-res effects synchronized to the video display.

Xtender is reportedly a very good emulator for DOS: www.delhez.demon.nl/.

NO$ZX81 is for DOS down to 8086 and Windows: www.work.de/nocash/.


Name Letzte Änderung Länge 
Ads/ 2019-08-20 05:24
Current Selling Prices/ 2019-08-20 05:24 107 
Images/ 2019-08-20 05:24 29 
PCBs/ 2019-08-20 05:24
ROMs/ 2020-08-27 14:08 19 
Service and Assembly Manual/ 2020-08-27 14:08 10 
Spare parts/ 2019-08-20 05:24 11 
Tech specs/ 2020-08-27 14:08
ZX81 Emulator Cartridge for ZX Spectrum/ 2020-08-27 14:08 21 
US Patent US272349D - ZX81 Computer.pdf 2011-05-20 22:55 42953 
ZX81 Basic Programming.pdf 2005-12-01 11:05 5339756 
ZX81 FAQ [2000-01-30].txt 2002-02-23 12:14 11113 

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