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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
| name        =
| image        =
| caption      =
| manufacturer =
| type        =
| release date =
| discontinued =
| price        =
| cpu          =
| memory      =
| storage      =
| display      =
| sound        =
| dimensions  =
| weight      =
| os          =
| predecessor  =
| successor    =
| codename    =
| model        =
| logo        =
}}
```markdown
= Sinclair ZX81 =
{{Infobox computer
{{Infobox computer
| name        = Sinclair ZX81
| name        = Sinclair ZX81
| image        =  
| image        = [[File:ZX81.jpg|250px]]
| caption      =  
| caption      = Sinclair ZX81 – The Affordable Home Computer
| manufacturer = Sinclair Research
| manufacturer = Sinclair Research Ltd
| type        = Home computer
| type        = Home computer
| release date = March 1981
| release date = March 1981
| discontinued = 1984
| discontinued = 1984
| price        = £69.95
| price        = £69.95 kit / £89.95 ready-built (≈US$199/249 in 1981)
| cpu          = Zilog Z80A
| cpu          = Zilog Z80A-compatible (NEC µPD780C) @ 3.25 MHz
| memory      = 1 KB (expandable to 16 KB)
| memory      = 1 KB static RAM (expandable to 16 KB)
| storage      = Cassette tape
| storage      = External cassette tape (300 baud, EAR/MIC jacks)
| display      = 32 x 24 characters (monochrome)
| display      = Monochrome RF to TV – 32 × 24 text or 64 × 48 block-graphics
| sound        = None
| sound        = None (no audio hardware)
| dimensions  = 23 x 14 x 4 cm
| dimensions  = 200 mm × 150 mm × 35 mm (7.9 ″ × 5.9 ″ × 1.4 ″)
| weight      = 1.5 kg
| weight      = ≈ 400 g (0.88 lb)
| os          = Sinclair BASIC
| os          = 4 KB Sinclair BASIC in ROM
| predecessor  = Sinclair ZX80
| predecessor  = [[Sinclair ZX80]]
| successor    = Sinclair ZX Spectrum
| successor    = [[Sinclair ZX Spectrum]]
| codename    =  
| codename    = “ZX81 project”
| model        =  
| model        = ZX81
| logo        =  
| 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.


=== Video generation ===
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.


== 💾 Storage ==
== 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.


== 🔌 I/O and Expansion ==
=== 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.
== 🔧 Known PCB Revisions & Errata ==
 
== 🛠️ General Maintenance ==
 
== 📜 Schematic & PCB ==
 
== 🔧 Common Modifications ==
 
== 🔢 ROM Keyword Map ==


== 🔗 Edge-Connector Pin-out ==
== 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.


== 🗺️ Exact Memory Map & Key ROM Entry Points ==
== 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''.


=== Popular ROM Vectors (ROM v1.2, 1980) ===
== Known PCB Revisions & Errata ==
 
{| class="wikitable styled-table" style="width:70%; text-align:center;"
== 📺 Video Timing Characteristics ==
|+'''ZX81 Logic-Board Issues'''
 
! Issue !! Visible Clues !! Key Fixes / Changes
== 🎬 Trivia / Pop-culture ==
|-
 
| '''Issue 1''' (Mar 1981) || “ZX81 Issue 1” silkscreen; no solder mask. || • Missing pull-up resistors on video output caused weak signals on some TVs. 
== 🗂️ Related Pages ==
• Early models had a tendency to overheat; later revisions included heatsinks.
 
|-
== 🧠 Architecture and Processor ==
| '''Issue 2''' (late 1982) || Added solder mask; improved layout. || • Enhanced power regulation for stability. 
 
• Additional RAM compatibility improvements.
== 💾 Memory and Storage ==
|}
 
== 🖥️ Display and Graphics ==
 
== 🔊 Sound Capabilities ==
 
== 🎮 Input/Output and Expansion ==
 
== 🧰 General Maintenance ==
 
== 🧾 PCB Schematics & Service Guide ==
 
=== 📷 Motherboard Images ===
 
== 📚 Technical Documentation ==
 
== 📷 Gallery ==
 
== 🧪 Related Pages ==


== 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.
|}


== PCB Schematics & Service Manual ==
Regular maintenance, including keyboard cleaning and power supply checks, is essential for longevity.
 
=== Apple Service Manual ===
 
=== Logic Board & Analog Board Schematics ===
 
== Capacitor Replacement Guide ==
 
== Retrobrite ==
 
== Troubleshooting ==
 
== Technical Details ==
 
=== System Architecture at a Glance ===
 
=== Memory Map (Physical) ===
 
=== ROM Layout (256 KB, v5.0) ===
 
=== ADB & PDS Highlights ===
 
=== Floppy / Mass-Storage ===


=== 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]]
|}


=== Sound Path ===
== 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.


=== RAM Upgrade Notes ===
== ROM Keyword Map ==
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>
|}


=== Hardware Easter Egg ===
== 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
|}


==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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