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---
title: Sinclair ZX 81
categories:
- Computers
types:
- Home Computer
origins:
- United Kingdom
years:
- '1981'
cpus:
- Zilog Z80A
params:
name: ZX 81
image: /media/museum/photos/sinclair_zx81_1s.jpg
company: /companies/sinclair
company_name: Sinclair
specs:
- label: NAME
value: ZX 81
- label: MANUFACTURER
value: Sinclair
- label: TYPE
value: Home Computer
- label: ORIGIN
value: United Kingdom
- label: YEAR
value: march 1981
- label: BUILT IN LANGUAGE
value: Sinclair Basic
- label: KEYBOARD
value: Touch-sensitive keyboard, 40 keys
- label: CPU
value: Zilog Z80A
- label: SPEED
value: 3.5 MHz
- label: RAM
value: 1 KB (901 bytes available, up to 64 KB)
- label: ROM
value: 8 KB
- label: TEXT MODES
value: 32 x 24 (but 2 lines are reserved for system messages and commands)
- label: GRAPHIC MODES
value: 64 x 44
- label: COLORS
value: Black and white
- label: SOUND
value: None
- label: SIZE / WEIGHT
value: 167 x 175 x 40 mm / 350gr
- label: I/O PORTS
value: Z80 Bus, tape, video
- label: POWER SUPPLY
value: 9V DC , 600 mA (centre polarity = +ve)
- label: PRICE
value: 'Kit :£50 (UK, june 1981) / Ready-built : £70 (UK, june 1981)'
tabs:
- photos
- adverts
- hardware
- software
- emulators
- links
- docs
forum_thread: 273
photos:
- title: LEFT SIDE
subtitle: ''
text: |-
From left to right :\
\- UHF TV output,\
\- MIC & EAR (Tape interface),\
\- Power in (9v DC).
image: /media/museum/photos/sinclair_zx81_l-side.jpg
- title: FRONT
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_front.jpg
- title: ZX-81
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_1.jpg
- title: RIGHT SIDE
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_r-side.jpg
- title: REAR
subtitle: ''
text: The only thing here is the expansion port, which can be used for many things...
image: /media/museum/photos/sinclair_zx81_rear.jpg
- title: TOP
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_top.jpg
- title: INSIDE ZX-81
subtitle: ''
text: |-
**(1)** Z-80 microprocessor.\
**(2)** Peripherals and memory expansions connector.\
**(3)** RAM memory chips. One or two chips can be used.\
**(4)** 8 Kb. ROM circuit holding Basic interpreter, Input/Output routines and character generator.\
**(5)** Keyboard connectors.\
**(6)** Three jack connectors for the external power supply and tape recorder.\
**(7)** Specialized circuit named ULA in charge of the whole computer's logical management.\
**(8)** TV aerial RF modulator.\
**(9)** Keyboard. Each key can perform up to six different functions.
image: /media/museum/photos/sinclair_zx81_inside_1.jpg
- title: '"COFFRET INITIATEUR"'
subtitle: ''
text: This "inititateur" box included a ZX-81 + an ABS chicklet keyboard + a BASIC book + 2 tapes with 10 games. It cost 100 Euros in 1984.
image: /media/museum/photos/sinclair_zx81_initiateur_1.jpg
- title: BOX
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_box_1.jpg
- title: 'BOX #2'
subtitle: ''
text: ''
image: /media/museum/photos/sinclair_zx81_box_3.jpg
- title: THE DEF 3000
subtitle: ''
text: This computer was launched in 1983. It was compatible with the ZX 81 and had the same features (except the 2KB RAM). It had very limited success. The photo shows the DEF with the 'color pack' extension (color + enhanced graphic mode)
image: /media/museum/photos/dcf.jpg
- title: THE LAMBDA 8300
subtitle: ''
text: The Lambda 8300 is another ZX81 clone.
image: /media/museum/photos/lambda8300.jpg
- title: ARTIST IMPRESSION
subtitle: ''
text: |-
Sculpture made by italian ArteStudio53 using a ZX-81...
From [ArteStudio53](http://www.artestudio53.it/arte/) **(Italy)**
image: /media/museum/photos/sinclair_zx81_specialcase_s1.jpg
adverts:
- title: 'ADVERT #1'
subtitle: ''
text: Picture courtesy of [Steve Browning, www.sbrowning.com/vintage](http://www.sbrowning.com/vintage).
image: /media/museum/pubs/sinclair_zx81_pub_1.jpg
- title: 'ADVERT #2'
subtitle: ''
text: Picture courtesy of [Steve Browning, www.sbrowning.com/vintage](http://www.sbrowning.com/vintage).
image: /media/museum/pubs/sinclair_zx81_pub_2.jpg
- title: 'ADVERT #3'
subtitle: ''
text: Picture courtesy of [Steve Browning, www.sbrowning.com/vintage](http://www.sbrowning.com/vintage).
image: /media/museum/pubs/sinclair_zx81_pub_3.jpg
- title: US ADVERT (APRIL 1982)
subtitle: ''
text: ''
image: null
- title: 'ADVERT #6 (1982)'
subtitle: ''
text: |-
The new sale price is $99.95 instead of 149.95
Advert said:
The ZX81 represents the latest technology in microelectronics. More than 10,000 are sold every week. In fact, the ZX81 is the best selling personal computer in the world.
image: /media/museum/pubs/sinclair_zx81_pub_4.jpg
- title: KIT VERSION (1982)
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_kitpub_1.jpg
- title: MEMOTECH LEAFLET
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_memopak_3.jpg
- title: GERMAN ADVERT
subtitle: ''
text: |-
Einstein's face probably means: Buy a ZX-81 and you will become a new genius...
Karl_Heinz - www.sharpmz.org
image: /media/museum/pubs/sinclair_zx81_germanad_1.jpg
- title: SPANISH ADVERT
subtitle: ''
text: ''
image: null
- title: FRENCH ADVERT (1981)
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_pub_avenir-offre-a-vous.jpg
- title: FRENCH ADVERT (1981)
subtitle: ''
text: '"It has not finished to surprise you"'
image: /media/museum/pubs/sinclair_zx81_pub_pas-fini-etonner.jpg
- title: FRENCH ADVERT (FEBRUARY 1982)
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_pub-french_surprenante.jpg
- title: FRENCH ADVERT (APRIL1982)
subtitle: ''
text: '"The Sinclair Phenomenon"'
image: /media/museum/pubs/sinclair_zx81_pub_phenomene_1.jpg
- title: FRENCH AD (DEC. 1982)
subtitle: ''
text: '"Your personnal computer for 670 F"'
image: /media/museum/pubs/sinclair_zx81_pub_670f.gif
- title: FRENCH ADVERT (SEPT. 83)
subtitle: ''
text: '"The initiator"'
image: /media/museum/pubs/sinclair_zx81_fr-pub_3.jpg
- title: FRENCH AD (MAY 1984)
subtitle: ''
text: A selection for the Initiator... at Sinclair price
image: /media/museum/pubs/sinclair_zx81_pub_selection.jpg
- title: FRENCH ADVERT (JULY 1984)
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_pub_initiateur_1.jpg
- title: FRENCH AD (JULY 1984)
subtitle: ''
text: ''
image: /media/museum/pubs/sinclair_zx81_pub-ordi-5-vitesses.jpg
- title: 'PROMOTIONAL PICTURE #1'
subtitle: ''
text: '"Learn computing today to be ready for tomorrow"'
image: /media/museum/pubs/sinclair_zx81_prm-pic_1.jpg
- title: 'PROMOTIONAL PICTURE #2'
subtitle: ''
text: '"The ZX-81 will soon become a precious work tool"'
image: /media/museum/pubs/sinclair_zx81_prm-pic_2.jpg
hardware:
- title: SOUND CARD
subtitle: brand unknown
text: ''
image: /media/museum/hardware/sinclair_zx81_sound-card.jpg
- title: VOCAL SYNTHESIZER
subtitle: ''
text: ''
image: /media/museum/hardware/sinclair_zx81_vocal-synthesizer.jpg
- title: EPROM STORAGE
subtitle: ''
text: This little black box contains a 2732 EPROM on which you can save & read your data. Maximum amount is 4 KB. Programs stored in the EPROM can then be automaticaly loaded at startup.
image: /media/museum/hardware/sinclair_zx81_eprom2732_1.jpg
- title: EPROM BURNER
subtitle: brand unknown
text: ''
image: /media/museum/hardware/sinclair_zx81_eprom-programmer_1.jpg
- title: '"NO NAME" JOYSTICKS'
subtitle: ''
text: ''
image: /media/museum/hardware/sinclair_zx81_joysticks_1.jpg
- title: CHICKLET KEYBOARD
subtitle: ''
text: |-
An essential option if one used the ZX81 more than 5 minutes per day.
Could be stuck on top of the original keyboard in seconds.
image: /media/museum/hardware/sinclair_zx81_keyboard_2.jpg
- title: DK'TRONICS KEYBOARD
subtitle: ''
text: The Sinclair ZX-81 may be the most popular computer, but that doesn't mean people like its keyboard. Numerous add-on keyboards and enclosures were manufactured for the ZX-81. This one, from DK'tronics, includes a full-size keyboard with numeric keypad, and an enclosure large enough to fit the Sinclair board and other Sinclair peripherals. Sadly, its price (£45) was almost as much as the ZX-81 computer one.
image: /media/museum/hardware/sinclair_zx81_keyboard_1.jpg
- title: MEMOTECH KEYBOARD
subtitle: ''
text: Before manufacturing computers (MTX Series) Memotech sold various add-ons for Sinclair computers.
image: /media/museum/hardware/sinclair_zx81_memotechkeyb_1.jpg
- title: 64 KB
subtitle: from Suntroncis
text: Suntronics T/S 1000 & ZX81 64K Memory Pack (Male Socket on Back, Female Socket on Front).
image: /media/museum/hardware/timex_1000-suntronics-64k_1.jpg
- title: TIMEX SINCLAIR 1016
subtitle: 16k RAM extension
text: ''
image: /media/museum/hardware/timex_sinclair-1000_16kram-1016.jpg
- title: INDESCOMP 16K RAM EXPANSION
subtitle: ''
text: |-
16K RAM expansion module made by a spanish company
Thanks to **www.zonadepruebas.com**
image: /media/museum/hardware/sinclair_zx81_indescomp_16kram_s1.jpg
- title: 16 KB MEMORY EXPANSION
subtitle: brand unknown
text: ''
image: /media/museum/hardware/sinclair_zx81_16kb-ram_expansion_1.jpg
- title: MEMOPAK MODULES
subtitle: ''
text: |-
Advert said:
**MEMOPAK 64K RAM** The 64K RAM extends the memory of ZX-81 to a full 64 KB. The unit is user transparent and accepts such BASIC command as DIM A (9000). It does not inhibit the use of the printer or other add-on units. No additional power supply is required.
**MEMOPAK 32K RAM** The 32K RAM offers your Sinclair a full 32K of directly addressable RAM. Like the 64K Memopak it is neither switched nor paged and enables you to execute sophisticated programs and store large data bases. It is also fully compatible with Sinclair's or Memotech's 16K RAM to give you a full 48K of RAM
**MEMOPAK 16K RAM** The Memopak 16K RAM provides an economical way to increase the capabilities of your Sinclair. And at the same time, it enables you to continue to add on other features with its "piggy back" connectors. It is compatible with the Sinclair 16K or a second Memopak 16K or Memopak 32K to give you 32K or 48K of RAM respectively.
**MEMOPAK High Resolution Graphics** The Memopak HRG contains a 2K EPROM monitor and is fully programmable for high resolution graphics. The HRG provides for up to 192 x 248 pixels resolution.
**MEMOPAK Printer Interface** The Memotech Centronics parallel or RS232 printer interface packs enable your Sinclair to use a wide range of compatible printers. The resident software in the units gives the ASCII set of characters. Both Memopak printer interfaces provide lower case character capabilities. The RS232 interface is also compatible with modems.
Memopak 64K Memory map:\
**0...8K:** Sinclair ROM\
**8...16K:** This section of memory switches in or out in 4 KB blocks to leave space for memory mapping, holds its content during cassette loads, allows comunication between programs and can be used to run assembly language routines.\
**16...32K:** This area can be used for BASIC programs and assembly language routines.\
**32...64K:** 32 KB of RAM memory for BASIC variables and large arrays.
image: /media/museum/hardware/sinclair_zx81_memopak_2.jpg
- title: MEMOPAK 32
subtitle: ''
text: ''
image: /media/museum/hardware/sinclair_zx81_memopak32_1.jpg
- title: PANDA 16K
subtitle: ''
text: One of the numerous second-source 16 KB memory expansion modules
image: /media/museum/hardware/sinclair_zx81_pandaram.jpg
- title: 8 INPUTS/OUTPUTS CARD
subtitle: brand unknown
text: ''
image: /media/museum/hardware/sinclair_zx81_i-o-card_1.jpg
- title: 8 INPUTS CARD
subtitle: brand unknown
text: ''
image: /media/museum/hardware/sinclair_zx81_8-inputs--card_1.jpg
- title: MODEM CARD
subtitle: by Timex
text: This is a modem card developed by Timex around 1982, originally for the Timex-Sinclair 1000 system.
image: /media/museum/hardware/timex-sinclair_t1000_modem_1.jpg
- title: ZX THERMAL PRINTER
subtitle: ''
text: |-
This is a small thermal printer (of very poor quality !). It can print all graphical characters of the ZX-81 and can print 9 lines of text per inch. It was possible to do a hard copy of the screen thanks to the "Copy" command.
This tiny thermic printer can also be used with the ZX-Spectrum.
image: /media/museum/hardware/sinclair_zx81_zxprinter_1.jpg
- title: ZX PRINTER'S BOX
subtitle: ''
text: ''
image: /media/museum/hardware/sinclair_zx81_zx-printer_box_1.jpg
screenshots:
- title: SURVIVOR
subtitle: ''
text: ''
image: /media/museum/software/ss_survivor.gif
- title: CRAZY KONG
subtitle: ''
text: ''
image: /media/museum/software/ss_crazykong.gif
- title: NIGHT DRIVER
subtitle: ''
text: ''
image: /media/museum/software/ss_nightdriver.gif
- title: OLYMPIADE
subtitle: ''
text: ''
image: /media/museum/software/ss_olympiade.gif
- title: PAC MAN
subtitle: ''
text: ''
image: /media/museum/software/ss_pacman.gif
- title: RABBITS
subtitle: ''
text: ''
image: /media/museum/software/ss_rabbits.gif
- title: REDANTS
subtitle: ''
text: ''
image: /media/museum/software/ss_redants.gif
- title: RISEHIVE
subtitle: ''
text: ''
image: /media/museum/software/ss_risehive.gif
- title: SHORT CIRCUIT
subtitle: ''
text: ''
image: /media/museum/software/ss_shortcircuit.gif
- title: SNAKE
subtitle: ''
text: ''
image: /media/museum/software/ss_snake.gif
- title: STARTREK
subtitle: ''
text: ''
image: /media/museum/software/ss_startrek.gif
- title: STEINSCH
subtitle: ''
text: ''
image: /media/museum/software/ss_steinsch.gif
- title: SULTAN
subtitle: ''
text: ''
image: /media/museum/software/ss_sultan.gif
- title: COMPUTACALC
subtitle: ''
text: A speadsheet made in England by Silicon Tricks and written in BASIC language...
image: /media/museum/software/sinclair_zx81_soft_computacalc.jpg
emulators:
- title: EightyOne
url: http://www.chuntey.com/eightyone/
platform: Windows
text: Excellent emulator emulating nearly all ZX computers (80, 81, Spectrums, Timex, etc.). As well as emulating the machines themselves, numerous addon cards are supported, including hi-res graphics, custom character sets, sound and colour cards.
- title: VB81
url: http://freestuff.grok.co.uk/vb81/
platform: Windows
text: Sinclair ZX81 Emulator (with Visual Basic sources). Excellent emulator
- title: X-Tender
url: http://www.delhez.demon.nl/
platform: Windows
text: The best ZX emulator for PC compatibles
- title: ZX 81 Homepage
url: http://www.maxon.de/homes/r_kurz/
platform: Windows
text: ''
- title: ZX81
url: http://www.w-4.de/~kurz/
platform: Apple
text: ''
- title: XZ81
url: http://www.zx81.nl/
platform: Unix / Linux
text: And other ZX-81 emulators there...
- title: ZX-81 Emulator 2.1
url: http://www.hh.schule.de/hhs/mjaap/ftp/zx81_v21.zip
platform: Atari
text: ''
- title: Java ZX81 Emulator
url: http://home.clara.net/rsergeant/
platform: Other platform
text: Java emulator
- title: Timex/Sinclair 1000 Emulator (Java)
url: http://www.vavasour.ca/jeff/trs80.html#ts1000
platform: Other platform
text: With this JAVA emulator, you'll be able to run a Timex/Sinclair 1000 in your browser window
- title: zx81emu
url: http://www.tlienhard.com/ap800.shtml
platform: Other platform
text: ZX-81 Emulator for the SonyEricsson P800 Smart Phone
- title: MESS
url: http://www.mess.org
platform: Multiplatform
text: MESS emulates the ZX-81, Lambda 8300, Power 3000 and other clones !
links:
- title: Achromic's ZX81 Site
url: http://zx81.achromic.co.uk/
text: A small, ZX81 fan site. With games and links to some emulators
- title: DOCUMENTATION
url: http://www.8bit.ht.st/
text: The Lil Old Sinclair Computer Technical Information Repository
- title: DOCUMENTATION
url: http://web.ukonline.co.uk/sinclair.zx81/
text: ZX-81 on-line manual. Great work !
- title: Jame's Sinclair ZX-81 Page
url: http://james.istop.com/zx81/
text: Some technical infos and links
- title: Planet Sinclair
url: http://www.nvg.ntnu.no/sinclair/computers/zx81/zx81.htm
text: Planet Sinclair has very interesting information on the ZX-81
- title: Sinclair ZX Hardware Website
url: http://www.xs4all.nl/~echorod/zx/index.htm
text: Features designs to build your own Sinclair computer
- title: The ZX81 Hardware and Software Collection
url: http://zx81.atspace.com/
text: Online museum for ZX81 hardware, software and literature
- title: TS1000.US
url: http://www.ts1000.us/
text: Documentation, software, hardware tips and more
- title: Zebra Systems, Inc.
url: http://www.zebrasystems.com/zebrasystems/zx81/index.html
text: Website where you can buy Timex/Sinclair products including ZX-81 kits !
- title: ZX81 Collection
url: http://www.zx81stuff.org.uk/
text: Online collection of ZX81 tapes, books and hardware
- title: ZX-81 forum
url: http://zx81.tlienhard.com/
text: A new ZX-81 forum. cool.
- title: ZX81 Home Page
url: http://www.honneamise.u-net.com/zx81/
text: FAQ and ZX81 Charts.
- title: ZX-81 The Archive
url: http://www.imarshall.karoo.net/zx81/zx81technical.htm
text: Lot of info here
- title: ZX-81 webring
url: http://www.zx81.de/webring/index.htm
text: Very useful to explore other ZX-81 related sites
- title: ZX-Team Homepage
url: http://www.zx81.de/
text: Quite a reference for the ZX-80, ZX-81, TS1000-1500, PC8300, Lambda and Jupiter Ace !
- title: ZX-TEAM-homepage & ZX-81 Web-Ring
url: http://www.zx81.de/
text: Logically useful
- title: Web Wiz CAPTCHA
url: http://www.webwizcaptcha.com
text: Sinclair computers
docs:
- title: SINCLAIR ZX-81 BASIC PROGRAMMING
url: ftp://ftp.worldofspectrum.org/pub/sinclair/books/ZX81BASICProgramming.pdf
cover: /media/museum/docs/couvs/sinclair_zx81_basic-programming.jpg
text: |-
[**SINCLAIR ZX-81 BASIC PROGRAMMING**](ftp://ftp.worldofspectrum.org/pub/sinclair/books/ZX81BASICProgramming.pdf)written by Sinclair Research Limited / Steven Vickers
 compiled/archived by Robin Stuart\
 type of document : PDF\
 size : 5,2 MB\
 source : [www.worldofspectrum.org](http://www.worldofspectrum.org/)
**... READ THE ONLINE VERSION BY CLICKING BELOW :**
- title: TIMEX SINCLAIR 1000 - USER MANUAL
url: http://www.ts1000.us/usermanuals.htm
cover: /media/museum/docs/couvs/timex-sinclair_1000_user-manual.jpg
text: |-
[**TIMEX SINCLAIR 1000 - USER MANUAL**](http://www.ts1000.us/usermanuals.htm)written by Timex / Sinclair / Steven Vickers
 compiled/archived by Bill Hicks\
 type of document : PDF\
 size : 3.5 MB\
 source : [www.ts1000.us](http://www.ts1000.us/)
**... READ THE ONLINE VERSION BY CLICKING BELOW :**
- title: THE COMPLETE TIMEX TS1000 & SINCLAIR ZX81 ROM DISASSEMBLY
url: null
cover: /media/museum/docs/couvs/sinclair_zx81_rom-disasembly.jpg
text: |-
**THE COMPLETE TIMEX TS1000 & SINCLAIR ZX81 ROM DISASSEMBLY**\
written by Dr. Ian Logan & Dr. Frank O'Hara
 compiled/archived by Bill Hicks\
 type of document : PDF\
 size : 5 MB\
 source : [www.ts1000.us](http://www.ts1000.us/)
**... READ THE ONLINE VERSION BY CLICKING BELOW :**
- title: TIMEX SINCLAIR 2020 COMPUTER PROGRAM RECORDER
url: null
cover: /media/museum/docs/couvs/ts2020_tape-recorder_manual.jpg
text: |-
**TIMEX SINCLAIR 2020 COMPUTER PROGRAM RECORDER**\
written by Timex Computer Corporation
 compiled/archived by Bill Hicks / www.ts1000.us\
 type of document : Online reader
**... READ THE ONLINE VERSION BY CLICKING BELOW :**
related:
- /museum/czerweny-cz-1000
- /museum/czerweny-cz-1000-plus
- /museum/lambda-electronics-ltd-pc-8300
- /museum/microace
- /museum/sinclair-zx-80
- /museum/timex-sinclair-1000
- /museum/tonel-pc
oc_id: 263
source:
url: https://web.archive.org/web/20230528020450/http://www.old-computers.com/museum/computer.asp?c=263
date: '2023-05-28'
---
The Sinclair ZX 81 was the successor of the [ZX 80](/museum/sinclair-zx-80/), and can be regarded as an evolution of it.
The ZX80 could not handle floating point numbers or cassette data files, but the ZX-81 could. The ZX-80 had 4k ROM : the ZX-81 had 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 was 4, against the ZX80's 21), the ZX-81 cost £30 less than the ZX-80. The plastic case was also different. Whereas the ZX-80 looked cheap in it lightweight white case, the ZX-81 was beautifully designed in its black ABS plastic case.
The keyboard was still formed by an underprinted plastic, but this one was made of non-reflective material. Even with this slight improvement it was quite horrible to use, that's why a lot of additional keyboards appeared quickly on the market. Some of the keys sported up to 5 functions, right in the Sinclair tradition, accessible 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, &lt;=, >=, &lt; >, COPY, DIM A$, FAST, FOR ... TO ... STEP, LLIST, LLIST n, LPRINT, PAUSE, PLOT, PRINT AT, PRINT TAB, SCROLL, SLOW, UNPLOT. Some are quite useful and make you wonder how it was possible to make anything on the ZX-80 !? However, one function disappeared: TL$ which was used to return a string minus its first character.
The ZX-81 could be operated in two modes, SLOW and FAST. The FAST mode which was the only mode available on the ZX-80, only refreshed the display when the system had completed computing, resulting in a painful screen flicker! It was a useful mode when you had to do a lot of calculation without the need to see what's going on all the time on the screen. The SLOW mode, which was indeed quite slooooowwww, behaved like all other computers did, refreshing the screen all the time.
The ZX81 contained 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 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!
The range of expansion options was quite extraordinary. For instance, Memotech created a whole range of modules which would piggy-back, one after another (and using velcro pads to secure to each other and the to the ZX81, as there was no mechanical locking on the edge connector).
So, for instance, you could have additional memory (up to 128k!), high-resolution graphics, disc controllers, joystick controllers, serial ports, and so on.
Popular add-ons also included replacement keyboards - one was a series of mechanical keys which stuck over the membrane keyboard, others involved re-housing the ZX81 inside a new, larger case with a "proper" keyboard.
In fact, the options were - seemingly - endless. Sadly the power supply wasn't particularly stable, so "glitches" were common, resulting in a system reset and the loss of hours of painstaking coding!
It was followed by the [ZX-Spectrum](/museum/sinclair-zx-spectrum/) and was also sold as the [Timex Sinclair 1000](/museum/timex-sinclair-1000/) in the USA.
**Contributors:** Silas Denyer