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At a speed of 300 baud (bitrate would be more precise) you can only transfer 37,5 bytes per second. Thus 1024 bytes take some 27 seconds to load.
 
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# Sinclair ZX81
{{Infobox computer
{{Infobox computer
| name = Sinclair ZX81
| name         = Sinclair ZX81
| image =  
| image       = [[File:ZX81.jpg|250px]]
| manufacturer = Sinclair Research
| caption      = Sinclair ZX81 – The Affordable Home Computer
| type = Home computer
| manufacturer = Sinclair Research Ltd
| release_date = March 1981
| type         = Home computer
| CPU = Zilog Z80A
| release date = March 1981
| memory = 1 KB RAM (expandable to 16 KB)
| discontinued = 1984
| storage = ROM cartridge, cassette tape
| price        = £69.95 kit / £89.95 ready-built (≈US$199/249 in 1981)
| display = 64 × 48 pixels (monochrome)
| cpu          = Zilog Z80A-compatible (NEC µPD780C) @ 3.25 MHz
| operating_system = Sinclair BASIC
| memory       = 1 KB static RAM (expandable to 16 KB)
| storage     = External cassette tape (300 baud, EAR/MIC jacks)
| display     = Monochrome RF to TV – 32 × 24 text or 64 × 48 block-graphics
| sound        = None (no audio hardware)
| dimensions  = 200 mm × 150 mm × 35 mm (7.9 ″ × 5.9 ″ × 1.4 ″)
| weight      = ≈ 400 g (0.88 lb)
| os          = 4 KB Sinclair BASIC in ROM
| predecessor  = [[Sinclair ZX80]]
| successor    = [[Sinclair ZX Spectrum]]
| codename    = “ZX81 project”
| model        = ZX81
| logo        =
}}
}}


## Overview
The '''Sinclair ZX81''' is an 8-bit home computer launched by '''Sinclair Research''' in March 1981. It was designed as an affordable entry into computing, selling over 1.5 million units worldwide. The ZX81's compact design and low price made it accessible to a broad audience, contributing to the rise of home computing in the UK.
The Sinclair ZX81 is an early home computer designed and manufactured by Sinclair Research. Released in March 1981, it was one of the first computers aimed at the mass market and is notable for its low cost and compact design. The ZX81 was a significant step in the history of personal computing, introducing many users to programming and computer technology.
 
The ZX81 was notable for its simple design and was one of the first computers to be sold for under £100. It provided a platform for programming in BASIC and was widely used in schools and homes for educational purposes.
 
== Architecture ==
* '''CPU''' – Z80A-class microprocessor clocked at '''3.25 MHz'''.
* '''ROM''' – 8 KB mask ROM containing Sinclair BASIC and system routines.
* '''RAM''' – 1 KB of 2114 static RAM, expandable to 16 KB via the rear expansion bus.
* '''Logic''' – Utilizes standard TTL logic chips, with no custom chips.


## Specifications
=== Video generation ===
- **CPU**: Zilog Z80A running at 3.25 MHz
The ZX81 generates video output using the Z80 processor, which produces a sync pulse and fetches character data from RAM. The video output is interleaved with CPU operations, which can lead to display flicker during intensive processing.
- **RAM**: 1 KB standard, expandable to 16 KB
- **Video**: Monochrome display with a resolution of 64 × 48 pixels
- **Input**: Membrane keyboard
- **Storage**: External cassette tape for data storage, optional ROM cartridges
- **Operating System**: Sinclair BASIC, a simple programming language that came pre-installed


## Design
== Storage ==
The ZX81 was designed to be small and efficient. It featured a unique membrane keyboard and a minimalist case design that set it apart from other home computers of its time. The compactness of the ZX81 made it highly portable and accessible for home users.
The ZX81 uses the '''300 baud Kansas City Standard''' for loading and saving programs via cassette tape. The average time to load a program is approximately 27 seconds per kilobyte.


## Legacy
== I/O and Expansion ==
The Sinclair ZX81 is often credited with popularizing home computing and inspiring a generation of programmers and tech enthusiasts. Its affordability and ease of use opened the door to computing for many individuals and families.
* '''Edge connector (40-way)''' – provides access to the Z80 bus, power, and video signals.
* '''9 V DC input''' – unregulated power supply.
* '''RF modulator''' – outputs to channel 36 PAL TV.
Commercial accessories included:
: ''1–16 KB RAM packs, ZX Printer, joystick interfaces, and various expansion modules''.


## Downloads
== Known PCB Revisions & Errata ==
<pdf> File:sinclair_zx81_manual.pdf </pdf>
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''ZX81 Logic-Board Issues'''
! Issue !! Visible Clues !! Key Fixes / Changes
|-
| '''Issue 1''' (Mar 1981) || “ZX81 Issue 1” silkscreen; no solder mask. || • Missing pull-up resistors on video output caused weak signals on some TVs. 
• Early models had a tendency to overheat; later revisions included heatsinks.
|-
| '''Issue 2''' (late 1982) || Added solder mask; improved layout. || • Enhanced power regulation for stability. 
• Additional RAM compatibility improvements.
|}


Download the Sinclair ZX81 User Manual Here: [[:File:sinclair_zx81_manual.pdf|sinclair_zx81_manual.pdf]]
== General Maintenance ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Issue !! Cause !! Remedy
|-
| Flickering display || Poor power supply regulation || Replace with a regulated power supply.
|-
| Unresponsive keys || Wear on membrane keyboard || Replace keyboard or clean contacts.
|-
| Overheating || Lack of ventilation || Ensure proper airflow or add ventilation holes.
|}


[[Category:User Manuals]]
Regular maintenance, including keyboard cleaning and power supply checks, is essential for longevity.
[[Category:Sinclair Computers]]
[[Category:Retro Home Computers]]


<pdf> File:sinclair_zx81_service_manual.pdf </pdf>
== Schematic & PCB ==
The complete PCB schematic is available for reference. 
{| class="wikitable"
|+
! [[File:ZX81_Schematic.png|thumb|center|500px|Sinclair ZX81 circuit diagram]]
! [[File:Sinclair ZX81 PCB Top.JPG|center|thumb|260x260px|Original Issue 1 board – note layout changes]]
|}


Download the Sinclair ZX81 Service Manual Here: [[:File:sinclair_zx81_service_manual.pdf|sinclair_zx81_service_manual.pdf]]
== Common Modifications ==
* '''16 KB RAM upgrade''' – allows for more complex programs and reduces memory limitations.
* '''Composite video output mod''' – improves video quality for modern displays.
* '''Full-travel keyboard mod''' – replaces the membrane keyboard with a more tactile option.


[[Category:Service Manuals]]
== ROM Keyword Map ==
[[Category:Sinclair Computers]]
Each key on the ZX81 keyboard has multiple functions depending on the mode selected:
[[Category:Retro Home Computers]]
{| class="wikitable styled-table" style="text-align:center;"
|+ Example ZX81 keyboard layer
|-
! Key !! <code>F</code> mode !! <code>K</code> mode !! <code>L</code> mode
|-
| '''P''' || <code>PRINT</code> || <code>P</code> || <code>(</code>
|-
| '''R''' || <code>RETURN</code> || <code>R</code> || <code>)</code>
|-
| '''G''' || <code>GOTO</code> || <code>G</code> || <code>></code>
|}


## Conclusion
== Edge-Connector Pin-out ==
The Sinclair ZX81 remains a significant part of computing history. Its innovative design and low price point helped to democratize access to technology in the early 1980s, making it a beloved device among retro computing enthusiasts today.
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:75%; text-align:center;"
|+'''ZX81 40-way Expansion Bus (viewed from rear, component side up)'''
! Pin !! Signal !! Description !! Pin !! Signal !! Description
|-
| 1 || '''A15''' || Address bus MSB || 21 || '''D4''' || Data bit 4
|-
| 2 || '''A14''' || || 22 || '''D5'''
|-
| 3 || '''A13''' || || 23 || '''D6'''
|-
| 4 || '''A12''' || || 24 || '''D7'''
|-
| 5 || '''A11''' || || 25 || '''BUSACK''' || Bus acknowledge
|-
| 6 || '''A10''' || || 26 || '''BUSRQ''' || Bus request
|-
| 7 || '''A9''' || || 27 || '''WAIT''' || Insert wait-states
|-
| 8 || '''A8''' || || 28 || '''HALT
|}


==See Also==
[[Category: Sinclair Computers]]
* [[Sinclair ZX81]]

Latest revision as of 09:47, 23 April 2026

Sinclair ZX81
Sinclair ZX81 – The Affordable Home Computer
Specifications
ManufacturerSinclair Research Ltd
TypeHome computer
ReleasedMarch 1981
Discontinued1984
Intro price£69.95 kit / £89.95 ready-built (≈US$199/249 in 1981)
CPUZilog Z80A-compatible (NEC µPD780C) @ 3.25 MHz
Memory1 KB static RAM (expandable to 16 KB)
StorageExternal cassette tape (300 baud, EAR/MIC jacks)
DisplayMonochrome RF to TV – 32 × 24 text or 64 × 48 block-graphics
SoundNone (no audio hardware)
Dimensions200 mm × 150 mm × 35 mm (7.9 ″ × 5.9 ″ × 1.4 ″)
Weight≈ 400 g (0.88 lb)
OS / Firmware4 KB Sinclair BASIC in ROM
PredecessorSinclair ZX80
SuccessorSinclair ZX Spectrum
Codename“ZX81 project”
Model no.ZX81

The Sinclair ZX81 is an 8-bit home computer launched by Sinclair Research in March 1981. It was designed as an affordable entry into computing, selling over 1.5 million units worldwide. The ZX81's compact design and low price made it accessible to a broad audience, contributing to the rise of home computing in the UK.

The ZX81 was notable for its simple design and was one of the first computers to be sold for under £100. It provided a platform for programming in BASIC and was widely used in schools and homes for educational purposes.

Architecture

[edit | edit source]
  • CPU – Z80A-class microprocessor clocked at 3.25 MHz.
  • ROM – 8 KB mask ROM containing Sinclair BASIC and system routines.
  • RAM – 1 KB of 2114 static RAM, expandable to 16 KB via the rear expansion bus.
  • Logic – Utilizes standard TTL logic chips, with no custom chips.

Video generation

[edit | edit source]

The ZX81 generates video output using the Z80 processor, which produces a sync pulse and fetches character data from RAM. The video output is interleaved with CPU operations, which can lead to display flicker during intensive processing.

Storage

[edit | edit source]

The ZX81 uses the 300 baud Kansas City Standard for loading and saving programs via cassette tape. The average time to load a program is approximately 27 seconds per kilobyte.

I/O and Expansion

[edit | edit source]
  • Edge connector (40-way) – provides access to the Z80 bus, power, and video signals.
  • 9 V DC input – unregulated power supply.
  • RF modulator – outputs to channel 36 PAL TV.

Commercial accessories included:

1–16 KB RAM packs, ZX Printer, joystick interfaces, and various expansion modules.

Known PCB Revisions & Errata

[edit | edit source]
ZX81 Logic-Board Issues
Issue Visible Clues Key Fixes / Changes
Issue 1 (Mar 1981) “ZX81 Issue 1” silkscreen; no solder mask. • Missing pull-up resistors on video output caused weak signals on some TVs.

• Early models had a tendency to overheat; later revisions included heatsinks.

Issue 2 (late 1982) Added solder mask; improved layout. • Enhanced power regulation for stability.

• Additional RAM compatibility improvements.

General Maintenance

[edit | edit source]
Issue Cause Remedy
Flickering display Poor power supply regulation Replace with a regulated power supply.
Unresponsive keys Wear on membrane keyboard Replace keyboard or clean contacts.
Overheating Lack of ventilation Ensure proper airflow or add ventilation holes.

Regular maintenance, including keyboard cleaning and power supply checks, is essential for longevity.

Schematic & PCB

[edit | edit source]

The complete PCB schematic is available for reference.

Sinclair ZX81 circuit diagram
Original Issue 1 board – note layout changes

Common Modifications

[edit | edit source]
  • 16 KB RAM upgrade – allows for more complex programs and reduces memory limitations.
  • Composite video output mod – improves video quality for modern displays.
  • Full-travel keyboard mod – replaces the membrane keyboard with a more tactile option.

ROM Keyword Map

[edit | edit source]

Each key on the ZX81 keyboard has multiple functions depending on the mode selected:

Example ZX81 keyboard layer
Key F mode K mode L mode
P PRINT P (
R RETURN R )
G GOTO G >

Edge-Connector Pin-out

[edit | edit source]
ZX81 40-way Expansion Bus (viewed from rear, component side up)
Pin Signal Description Pin Signal Description
1 A15 Address bus MSB 21 D4 Data bit 4
2 A14 22 D5
3 A13 23 D6
4 A12 24 D7
5 A11 25 BUSACK Bus acknowledge
6 A10 26 BUSRQ Bus request
7 A9 27 WAIT Insert wait-states
8 A8 28 HALT
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