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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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{{Infobox computer
{{Infobox computer
| name = Sinclair ZX81
| name         = Sinclair ZX81
| image = Sinclair_ZX81.jpg
| image       = [[File:ZX81.jpg|250px]]
| caption = Sinclair ZX81
| caption     = Sinclair ZX81 – The Affordable Home Computer
| developer = Sinclair Research
| manufacturer = Sinclair Research Ltd
| type = Home computer
| type         = Home computer
| release_date = March 1981
| release date = March 1981
| cpu = Zilog Z80A
| discontinued = 1984
| memory = 1 KB (expandable to 64 KB)
| price        = £69.95 kit / £89.95 ready-built (≈US$199/249 in 1981)
| storage = Cassette tape
| cpu         = Zilog Z80A-compatible (NEC µPD780C) @ 3.25 MHz
| display = Monochrome, 32 x 24 characters
| memory       = 1 KB static RAM (expandable to 16 KB)
| os = Sinclair BASIC
| storage     = External cassette tape (300 baud, EAR/MIC jacks)
| connectivity = RF output, Expansion port
| display     = Monochrome RF to TV – 32 × 24 text or 64 × 48 block-graphics
| power_supply = 9V DC
| sound        = None (no audio hardware)
| dimensions = 23.5 cm x 13.5 cm x 3.5 cm
| dimensions   = 200 mm × 150 mm × 35 mm (7.9 ″ × 5.9 ″ × 1.4 ″)
| weight = 0.5 kg
| weight       = ≈ 400 g (0.88 lb)
| successor = Sinclair ZX81+
| os          = 4 KB Sinclair BASIC in ROM
| related = [[Sinclair ZX80]], [[Sinclair ZX Spectrum]]
| predecessor  = [[Sinclair ZX80]]
| successor    = [[Sinclair ZX Spectrum]]
| codename    = “ZX81 project”
| model        = ZX81
| logo        =
}}
}}


== 🧠 Architecture ==
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 based on a Zilog Z80A microprocessor, running at a clock speed of 3.25 MHz. It features a minimalistic design with a compact keyboard and a simple architecture that contributed to its low cost.
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.


=== Video generation ===
=== Video generation ===
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.


The ZX81 utilizes a video output system that generates a monochrome display with a resolution of 256 x 192 pixels. The video signal is produced by the Z80A processor, which handles character generation and screen refresh.
== Storage ==
 
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.
== 💾 Storage ==
 
The primary storage medium for the ZX81 is cassette tape, which is used to load and save programs. The system supports a basic file format for storing data, and users often relied on external cassette recorders for this purpose.
 
== 🔌 I/O and Expansion ==
 
The ZX81 includes a basic RF output for connecting to a television, as well as an expansion port that allows for additional peripherals, such as memory expansions and interface devices.
 
== 🔧 Known PCB Revisions & Errata ==
 
Several PCB revisions of the ZX81 exist, each addressing various hardware issues and improving reliability. Notable revisions include changes to the power supply circuit and modifications to the keyboard matrix.
 
== 🛠️ General Maintenance ==
 
Regular maintenance of the ZX81 includes cleaning the keyboard and ensuring the connections are secure. Users should also check for any signs of corrosion on the PCB and clean the contacts of the expansion port.
 
== 📜 Schematic & PCB ==
 
The schematic diagrams for the ZX81 are available for enthusiasts and repair technicians, providing detailed information about the circuit design and component layout.
 
== 🔧 Common Modifications ==
 
Common modifications for the ZX81 include memory upgrades, keyboard enhancements, and modifications to improve video output quality. These modifications can enhance the overall performance and usability of the system.
 
== 🔢 ROM Keyword Map ==
 
The ROM keyword map for the ZX81 provides a comprehensive list of commands and functions available in Sinclair BASIC, allowing users to program effectively on the platform.
 
== 🔗 Edge-Connector Pin-out ==
 
The edge-connector pin-out details the configuration of the expansion port, which is essential for connecting peripherals and expansion devices to the ZX81.
 
== 🗺️ Exact Memory Map & Key ROM Entry Points ==
 
The memory map outlines the allocation of memory addresses within the ZX81, detailing where ROM, RAM, and I/O registers are located.
 
=== Popular ROM Vectors (ROM v1.2, 1980) ===
 
The popular ROM vectors for version 1.2 include key entry points for various system functions, providing insight into the internal workings of the ZX81.
 
== 📺 Video Timing Characteristics ==
 
The video timing characteristics of the ZX81 define the refresh rates and synchronization signals necessary for proper display operation on connected monitors and televisions.
 
== 🎬 Trivia / Pop-culture ==
 
The Sinclair ZX81 is often regarded as one of the first affordable home computers, making it a significant milestone in the history of computing. It has appeared in various media and is celebrated by retro computing enthusiasts.
 
== 🗂️ Related Pages ==
 
* [[Sinclair ZX80]]
* [[Sinclair ZX Spectrum]]
* [[Sinclair BASIC]]
 
== 🧠 Architecture and Processor ==


The architecture of the ZX81 is designed to be simple yet effective, utilizing the Z80A processor to manage all computing tasks.
== I/O and Expansion ==
* '''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''.


== 💾 Memory and Storage ==
== Known PCB Revisions & Errata ==
 
{| class="wikitable styled-table" style="width:70%; text-align:center;"
The ZX81's memory architecture allows for expansion up to 64 KB, significantly enhancing its capabilities for running more complex programs.
|+'''ZX81 Logic-Board Issues'''
 
! Issue !! Visible Clues !! Key Fixes / Changes
== 🖥️ Display and Graphics ==
|-
 
| '''Issue 1''' (Mar 1981) || “ZX81 Issue 1” silkscreen; no solder mask. || • Missing pull-up resistors on video output caused weak signals on some TVs.
The display capabilities of the ZX81 are limited to monochrome graphics, but it can effectively render text and simple shapes.
• Early models had a tendency to overheat; later revisions included heatsinks.
 
|-
== 🔊 Sound Capabilities ==
| '''Issue 2''' (late 1982) || Added solder mask; improved layout. || • Enhanced power regulation for stability.
 
• Additional RAM compatibility improvements.
The ZX81 does not have dedicated sound hardware, but users can create sound through the speaker by manipulating the processor's output.
|}
 
== 🎮 Input/Output and Expansion ==
 
The I/O capabilities of the ZX81 are limited, but the expansion port allows for additional hardware to be connected, enhancing its functionality.
 
== 🧰 General Maintenance ==
 
General maintenance practices include regular cleaning and checking for hardware issues to ensure the ZX81 operates effectively.
 
== 🧾 PCB Schematics & Service Guide ==
 
Detailed PCB schematics and service guides are available for those looking to repair or modify their ZX81 systems.
 
=== 📷 Motherboard Images ===
 
Images of the ZX81 motherboard are available for reference, showcasing the layout and components used in the system.
 
== 📚 Technical Documentation ==
 
Technical documentation for the ZX81 includes user manuals, programming guides, and hardware specifications.
 
== 📷 Gallery ==
 
A gallery of images showcasing the Sinclair ZX81, including various models and modifications, can be found here.
 
== 🧪 Related Pages ==
 
* [[Retro computing]]
* [[Home computers]]
* [[Zilog Z80]]


== General Maintenance ==
== 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.
|}


Regular maintenance is crucial for preserving the functionality of the ZX81, including cleaning and checking connections.
Regular maintenance, including keyboard cleaning and power supply checks, is essential for longevity.
 
== PCB Schematics & Service Manual ==
 
The service manual provides detailed information on the ZX81's PCB layout and troubleshooting tips.
 
=== Apple Service Manual ===
 
The Apple service manual includes information relevant to similar computing devices and their maintenance.
 
=== Logic Board & Analog Board Schematics ===
 
Schematics for the logic and analog boards provide insight into the design and function of the ZX81's hardware.
 
== Capacitor Replacement Guide ==
 
A guide for replacing capacitors in the ZX81 to maintain performance and reliability.
 
== Retrobrite ==
 
Retrobrite techniques can be applied to restore the appearance of the ZX81's plastic casing.
 
== Troubleshooting ==
 
Common troubleshooting tips for the ZX81 can help users resolve issues that may arise during operation.
 
== Technical Details ==
 
Detailed technical specifications and features of the ZX81 are outlined here.
 
=== System Architecture at a Glance ===
 
An overview of the ZX81's system architecture, highlighting key components and their functions.
 
=== Memory Map (Physical) ===
 
The physical memory map provides a detailed breakdown of memory allocation and usage.
 
=== ROM Layout (256 KB, v5.0) ===
 
The ROM layout outlines the organization of the ZX81's firmware and available commands.
 
=== ADB & PDS Highlights ===
 
Highlights of the ADB (Apple Desktop Bus) and PDS (Peripheral Data Storage) features relevant to the ZX81.
 
=== Floppy / Mass-Storage ===
 
Information on floppy and mass-storage options available for the ZX81.


=== Key I/O ICs ===
== 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]]
|}


A list of key input/output integrated circuits used in
== 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.


==See Also==
== ROM Keyword Map ==
* [[Sinclair ZX81]]
Each key on the ZX81 keyboard has multiple functions depending on the mode selected: 
{| 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>
|}


{{Navbox-AppleVintage}}
== Edge-Connector Pin-out ==
<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
|}


[[Category:Retro Home Computers]]
[[Category: Sinclair Computers]]

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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