Editing Sinclair ZX80

{{Infobox computer
| name         = Sinclair ZX80
| image        = [[File:Sinclair_ZX80.jpg|250px]]
| caption      = A fully–assembled ZX80 with 9 V PSU and UHF TV lead
| manufacturer = Science of Cambridge Ltd (later Sinclair Research Ltd)
| type         = Home computer / hobbyist kit
| release date = 29&nbsp;January&nbsp;1980
| discontinued = March&nbsp;1981 (superseded by ZX81)
| price        = £79 .95 kit / £99 .95 ready-built (≈US$199/249 in 1980)
| cpu          = Zilog Z80A-compatible (NEC µPD780C-1) @ 3.25 MHz
| memory       = 1 KB static RAM on-board (expandable to 16 KB, 48 KB with mods)
| 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   = 195 mm × 175 mm × 50 mm (7.7 ″ × 6.9 ″ × 2.0 ″)
| weight       = ≈ 340 g (0.75 lb)
| os           = 4 KB Sinclair Integer BASIC in ROM
| predecessor  = [[Science of Cambridge MK14]]
| successor    = [[Sinclair ZX81]]
| codename     = “ZX80 project”
| model        = ZX80
}}

The '''Sinclair ZX80''' is an 8-bit home computer marketed by Sir Clive Sinclair’s '''Science of Cambridge''' (soon renamed '''Sinclair Research''') as *“the first personal computer for under one hundred pounds.”* Released in January 1980, it ignited the UK micro-boom and sold about 100 000 units worldwide despite numerous limitations – most famously its blanking video that disappears while BASIC is running code.

== 🧠 Architecture ==
* '''CPU''' – Z80A-class microprocessor clocked at '''3.25 MHz''' (half the UK colour-burst for simple TV timing).  
* '''ROM''' – 4 KB mask ROM containing Sinclair Integer BASIC, line-editor and I/O routines.  
* '''RAM''' – 1 KB of 2114 static RAM; addressable to 48 KB via the rear expansion bus (commonly 16 KB DRAM pack).  
* '''Logic''' – Built entirely from 18 inexpensive 74-series TTL ICs; no custom chips – video, keyboard scan and cassette I/O are all done in firmware.

=== Video generation ===
The Z80 itself bit-bangs UHF video: during the '''FAST''' display loop it outputs a sync pulse then immediately fetches the next character row from RAM, interleaving code and picture. When the CPU is diverted to BASIC interpretation the timing breaks, so the TV loses sync – producing the ZX80’s trademark flicker. (The later ZX81 added a SLOW mode with hardware assistance to cure this.)

== 💾 Storage ==
Load / SAVE uses the '''300 baud Kansas City Standard''' with simple square waves on 3.5 mm EAR/MIC sockets. Program listings average ~4 seconds per KB.

== 🔌 I/O and Expansion ==
* '''Edge connector (44-way)''' – exposes full Z80 bus, power and video lines.  
* '''9 V DC input''' – un-regulated (internally regulated to 5 V).  
* '''RF modulator''' – channel 36 PAL TV.  
Commercial add-ons included:
: *1–3 KB SRAM packs, 16 KB DRAM pack*  
: *ZX Printer, joystick adapters, floppy-drive interfaces, composite video mods.*

== 🛠️ General Maintenance ==
{| class="wikitable" style="width:90%;"
! Issue !! Cause !! Remedy
|-
| Unstable picture / snow || Ageing electrolytic C5 (47 µF) on video rail || Replace with low-ESR 105 °C part
|-
| Reboots when touched || Loose 7805 regulator tab shorts to case || Insulate tab / fit TO-220 spacer
|-
| RAM pack “wobble” resets || Leverage on edge bus ⇒ poor contact || Fit retention bracket or right-angle header
|-
| Overheating || No ventilation slots (black “stripes” are cosmetic) || Drill discreet vents or run uncased board
|}

Full recapping, regulator heatsinking and replacement of the brittle membrane keyboard are popular reliability upgrades.

== 📜 Schematic & PCB ==
The complete single-layer PCB schematic is freely available (public-domain scan of Sinclair service sheet 322-10).  
[[File:ZX80_Schematic.png|thumb|center|500px|Sinclair ZX80 circuit diagram]]  
[[File:ZX80_PCB_Top.jpg|thumb|center|260px|Original Issue 1 board – note hand-drawn track work]]

== 🔧 Common Modifications ==
* '''8 KB ZX81 ROM upgrade''' – drops-in to give floating-point BASIC, SAVE/LOAD VERIFY and better editing (still no SLOW mode).  
* '''Composite video mod''' – bypass RF can for crisp LCD/monitor output.  
* '''Full-travel keyboard''' – external matrix wired to KB pins 8-15.  
* '''Internal 16 KB SRAM''' – replaces external pack, eliminating “wobble.”  
* '''Turbo crystal (3.58 MHz)''' – simple Xtal swap for ≈10 % speed gain (requires retune TV).

== 🔢 ROM Keyword Map ==
Each key performs multiple functions depending on mode :  
{| class="wikitable" style="text-align:center;"
|+ Example ZX80 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>
|}

== 🗂️ Related Pages ==
* [[Sinclair ZX81]] – successor with improved video logic.  
* [[Sinclair ZX Spectrum]] – coloured 1982 evolution of the Sinclair line.  
* [[Sinclair Computers General Maintenance]]

[[Category:Sinclair Computers]]