RetroTechCollection - User contributions [en]

Sinclair ZX81

2026-04-23T08:47:58Z

ZXSpectrum128: 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.

{{Infobox computer
| name = Sinclair ZX81
| image = [[File:ZX81.jpg|250px]]
| caption = Sinclair ZX81 – The Affordable Home Computer
| manufacturer = Sinclair Research Ltd
| type = Home computer
| release date = March 1981
| discontinued = 1984
| price = £69.95 kit / £89.95 ready-built (≈US$199/249 in 1981)
| cpu = Zilog Z80A-compatible (NEC µPD780C) @ 3.25 MHz
| 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 =
}}

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 ==
* '''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 ===
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 ==
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 ==
* '''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 ==
{| 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.
|}

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

== 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]]
|}

== 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.

== 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>
|}

== 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: Sinclair Computers]]

Sinclair ZX Spectrum

2026-04-23T08:12:26Z

ZXSpectrum128: Correct speed of tape loading on the ZX Spectrum is approximately 1500 baud, the wrong speed given on this wiki's website was 300 baud which is the speed a ZX81 will use for tape loading.

{{Infobox computer
| name = Sinclair ZX Spectrum
| image = [[File:ZX_Spectrum.jpg|250px]]
| caption = Sinclair ZX Spectrum – The Iconic Home Computer
| manufacturer = Sinclair Research Ltd
| type = Home computer
| release date = April 1982
| discontinued = 1992
| price = £125 (1982)
| cpu = Zilog Z80A @ 3.5 MHz
| memory = 16 KB, 48 KB, or 128 KB RAM (varies by model)
| storage = External cassette tape (300 baud, EAR/MIC jacks)
| display = 256×192 pixels, 32 characters × 24 lines
| sound = Beeper (monophonic)
| dimensions = 23 cm × 14 cm × 4 cm (9.1" × 5.5" × 1.6")
| weight = 1.1 kg (2.4 lb)
| os = Sinclair BASIC in ROM
| predecessor = [[Sinclair ZX81]]
| successor = [[Sinclair ZX Spectrum+]]
| codename = ZX Spectrum
| model = ZX Spectrum, ZX Spectrum+
| logo = [[File:Sinclair ZX Spectrum-02b.svg|250px]]
}}

The '''Sinclair ZX Spectrum''' is an 8-bit home computer released by '''Sinclair Research Ltd''' in April 1982. It became a defining product of the early home computing era in the UK, known for its distinctive rubber keyboard and vibrant color graphics. The ZX Spectrum sold over 5 million units and is credited with popularizing home computing and gaming in the UK.

== Architecture ==
* '''CPU''' – Zilog Z80A microprocessor clocked at '''3.5 MHz'''.
* '''ROM''' – 16 KB or 48 KB ROM containing Sinclair BASIC and system routines.
* '''RAM''' – Available in 16 KB, 48 KB, or 128 KB configurations, expandable via external RAM packs.
* '''Logic''' – Utilizes a combination of discrete logic and custom chips for video output and keyboard scanning.

=== Video generation ===
The ZX Spectrum generates video using a technique called "bit-banging," where the Z80 CPU directly controls the display output. The screen resolution is '''256×192 pixels''' with a palette of 15 colors, plus black. The unique attribute system allows for color attributes to be assigned to 8×8 pixel blocks, resulting in colorful graphics but also potential color clash.

== Storage ==
Data is loaded and saved via a standard cassette tape interface operating at approximately '''1500 baud'''. The average loading time for a game is approximately 3-5 minutes, depending on the size of the program.

== I/O and Expansion ==
* '''Edge connector (50-way)''' – provides access to the full Z80 bus, power, and video signals.
* '''9 V DC input''' – unregulated power supply.
* '''Composite video output''' – for connecting to monitors and TVs.
Commercial peripherals included:
: ''ZX Printer, joystick interfaces, and various RAM expansion packs.''

== Known PCB Revisions & Errata ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''ZX Spectrum PCB Revisions'''
! Issue !! Visible Clues !! Key Fixes / Changes
|-
| '''Issue 1''' (1982) || Original board layout with no solder mask. || • Early boards had issues with RAM stability; later revisions improved power distribution.
|-
| '''Issue 2''' (1983) || Added solder mask and improved trace routing. || • Enhanced video output stability and reduced color clash issues.
|}

== General Maintenance ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Issue !! Cause !! Remedy
|-
| Flickering display || Aging capacitors in the power supply || Replace with high-quality electrolytic capacitors.
|-
| Unresponsive keys || Worn rubber contacts on the keyboard || Replace keyboard or use conductive paint to restore contact.
|-
| Loading errors || Dirty cassette connectors || Clean connectors and ensure proper tape alignment.
|}

Regular maintenance, including cleaning the keyboard and ensuring proper connections, can help prolong the life of the ZX Spectrum.

== Schematic & PCB ==
The complete schematic for the ZX Spectrum is available for enthusiasts and repair technicians.
{| class="wikitable"
|+
! [[File:ZX_Spectrum_Schematic.png|thumb|center|500px|ZX Spectrum circuit diagram]]
! [[File:ZX_Spectrum_PCB_Top.jpg|thumb|center|260px|Top view of the ZX Spectrum PCB]]
|}

== Common Modifications ==
* '''RAM upgrade''' – Adding external RAM packs to increase memory capacity.
* '''Composite video mod''' – For improved video output quality on modern displays.
* '''Keyboard replacement''' – Upgrading to a full-travel keyboard for better typing experience.
* '''Turbo mode modification''' – Overclocking the Z80 for faster performance (requires careful tuning).

== ROM Keyword Map ==
Each key on the ZX Spectrum keyboard has multiple functions depending on the mode:
{| class="wikitable styled-table" style="text-align:center;"
|+ Example ZX Spectrum 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>
|}

== Edge-Connector Pin-out ==
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:75%; text-align:center;"
|+'''ZX Spectrum 50-way Expansion Bus (viewed from rear, component side up)'''
! Pin !! Signal !! Description !! Pin !! Signal !! Description
|-
| 1 || '''A15''' || Address bus MSB || 26 || '''D4''' || Data bit 4
|-
| 2 || '''A14''' || || 27 || '''D5'''
|
|-
| 3 || '''A13''' || || 28 || '''D6'''
|
|-
| 4 || '''A12''' || || 29 || '''D7'''
|-
| 5 || '''A11''' || || 30 || '''BUSACK''' || Bus acknowledge
|-
| 6 || '''A10''' || || 31 || '''BUSRQ''' || Bus request
|-
| 7 || '''A9''' || || 32 || '''WAIT''' || Insert wait-states
|-
| 8 || '''A8''' || || 33 || '''HALT'''
|}

[[Category:Sinclair Computers]]