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Created page with "{{Infobox computer | name = Acorn Atom | image = 250px | caption = Acorn Atom | manufacturer = Acorn Computers Ltd | type = Home Computer | release date = March 1980 | discontinued = 1982 | price = Kit: £120, Assembled: £170 (1980) | cpu = MOS Technology 6502 @ 1 MHz | memory = 2 KB RAM (expandable to 12 KB), 8 KB ROM | storage = Cassette tape interface, optional floppy disk | display =..."
 
Replace memory map with Acorn's own (Technical Manual p. 20); fix adaptor rating, composite output, VIA position and board-issue claims; remove uncited edge-connector and RAM fault claims; add machine photo; copyedit
 
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{{Infobox computer
{{Infobox computer
| name        = Acorn Atom
| name        = Acorn Atom
| image        = [[File:Acorn_Atom.jpg|250px]]
| image        = [[File:Acorn Atom.jpg|260px|frameless]]
| caption      = Acorn Atom
| caption      = Acorn Atom (photo: Simon Inns, CC BY 2.0)
| manufacturer = Acorn Computers Ltd
| manufacturer = Acorn Computers Ltd
| type        = Home Computer
| type        = Home computer
| release date = March 1980
| release date = March 1980<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/AtomComputers.html "Acorn Atom Computers"] and [https://chrisacorns.computinghistory.org.uk/Computers/Atom.html "Acorn Atom"], Chris's Acorns, archived by The Centre for Computing History. Release in March 1980; kit £120, assembled £170, fully expanded over £200; based on the System 3; a basic machine with motherboard 202,000 Issue 4; Acorn mains unit labelled 8 V 1.8 A DC; Atom Disc Pack and Econet via Eurocards.</ref>
| discontinued = 1982
| discontinued = 1983<ref name="wp">[https://en.wikipedia.org/wiki/Acorn_Atom "Acorn Atom"], Wikipedia (retrieved 2026-10-01). Source for the 1983 end of production, the case designer, the author of Atom BASIC, the nine graphics modes, the case dimensions and the regulators running hot.</ref>
| price        = Kit: £120, Assembled: £170 (1980)
| price        = Kit £120, assembled £170 (1980)<ref name="chris" />
| cpu          = MOS Technology 6502 @ 1 MHz
| cpu          = 6502 at 1 MHz (IC22)
| memory      = 2 KB RAM (expandable to 12 KB), 8 KB ROM
| memory      = 2 KB RAM, expandable on the board to 12 KB; 8 KB system ROM plus sockets for two 4 KB ROMs
| storage      = Cassette tape interface, optional floppy disk
| storage      = Cassette (CUTS, 300 bit/s); Atom Disc Pack optional
| display      = 256×192 monochrome graphics, 32×16 text mode
| display      = MC6847 VDG (IC31): 32 × 16 text, graphics up to 256 × 192; UHF and composite video
| sound        = Internal speaker (software-driven)
| sound        = Loudspeaker driven from the 8255
| dimensions  = 381 mm × 241 mm × 64 mm
| dimensions  = 381 × 241 × 64 mm<ref name="wp" />
| weight      = 1 kg
| os          = Atom operating system and BASIC in ROM
| os          = Acorn MOS with Atom BASIC
| predecessor  = [[Acorn System 3]]
| predecessor  = [[Acorn System 3]]
| successor    = [[BBC Micro Model A/B]]
| successor    = [[BBC Micro Model A/B]]
| codename    =
| model        = Main PCB 202,000
| model        =  
}}
}}


The '''Acorn Atom''' was a home computer manufactured by Acorn Computers Ltd from 1980 to 1982. Designed around the MOS Technology 6502 processor, the Atom served as Acorn's entry into the home computer market and established the technical foundation for the subsequent BBC Micro series.
The '''Acorn Atom''' is a 6502-based home computer that Acorn Computers sold from March 1980, either as a kit at £120 or assembled at £170.<ref name="chris" /> It is based on the [[Acorn System 3]], in a new case with an integral keyboard, and it accepts the same Acorn Eurocards.<ref name="chris" /> Production ended in 1983.<ref name="wp" />
 
The minimum machine has 2&nbsp;KB of RAM and an 8&nbsp;KB ROM holding the operating system and Atom BASIC. Further RAM, a floating-point ROM, a utility ROM, a 6522 VIA, a printer port and bus buffers all fit into empty sockets on the same board.<ref name="atomtm">Acorn Computers, ''[[Acorn Atom Technical Manual|Atom Technical Manual]]'', issue 2, October 1980, hosted on this wiki. Parts list pp. 6–7; power supply p. 13; tape interface pp. 14–15; circuit description pp. 16–17; memory map p. 20; extensions pp. 21–23.</ref> Atom BASIC was written by Sophie Wilson, and its interpreter includes a 6502 assembler. The case was designed by Allen Boothroyd of Cambridge Product Design.<ref name="wp" />


== Architecture ==
== Architecture ==


The Atom employed a straightforward single-board design with the 6502 processor running at 1 MHz. The system architecture provided direct CPU access to all peripherals through memory-mapped I/O, eliminating the need for complex decoding logic.
The Atom is built on one double-sided, through-plated PCB, part number 202,000. The ICs, connectors PL/SK1–7 and passive components are on side 2; PL8 and the keyboard are on side 1.<ref name="atomtm" /> The 6502 runs at 1&nbsp;MHz, and every bus cycle is synchronised to that clock. A reset at power-up or from the BREAK key starts the processor in the operating system ROM.<ref name="atomtm" />


=== Memory Organization ===
=== Main board ICs ===


{| class="wikitable styled-table" style="width:80%; text-align:center;"
The base kit fits the following ICs. The designators are Acorn's.<ref name="atomtm" /><ref name="atomcd">Acorn Computers, ''Circuit Diagram for Atom Microcomputer'', drawn 28 February 1980, revised to issue 3 on 14 January 1981. Hosted on this wiki as [[:File:Acorn Atom circuit diagram.png]].</ref>
|+'''Acorn Atom Memory Map'''
 
! Address Range !! Size !! Function
<templatestyles src="Template:StyledTable/styles.css" />
|-
{| class="wikitable styled-table" style="width:100%; text-align:center;"
| $0000 – $09FF || 2.5 KB || Block Zero RAM (lower text space)
|+'''Acorn Atom base-kit ICs'''
|-
! Designator !! Device !! Use
| $0A00 – $0AFF || 256 bytes || Assembler workspace
|-
| $0B00 – $0BFF || 256 bytes || Input buffer/Floating point workspace
|-
| $0C00 – $0FFF || 1 KB || System variables and stack
|-
| $1000 – $1FFF || 4 KB || Extension ROM socket A0
|-
| $2000 – $21FF || 512 bytes || Catalog buffer
|-
| $2200 – $27FF || 1.5 KB || Floating point variables
|-
| $2800 – $28FF || 256 bytes || Graphics workspace
|-
| $2900 – $2FFF || 1.75 KB || User BASIC programs (lower)
|-
| $3000 – $3BFF || 3 KB || Extension RAM (optional)
|-
| $3C00 – $3FFF || 1 KB || User BASIC programs (upper)
|-
| $8000 – $81FF || 512 bytes || Video RAM (graphics mode)
|-
| $8200 – $83FF || 512 bytes || Video RAM (text mode)
|-
| $8400 – $84FF || 256 bytes || I/O space (6522 VIA)
|-
| $A000 – $AFFF || 4 KB || Extension ROM socket A2
|-
| $B000 – $B7FF || 2 KB || Peripheral I/O (optional)
|-
| $B800 – $BBFF || 1 KB || VDU controller (6847)
|-
|-
| $C000 – $CFFF || 4 KB || Atom BASIC interpreter ROM
| IC22 || 6502, 1 MHz || Processor
|-
|-
| $D000 – $DFFF || 4 KB || Extension ROM socket A1
| IC20 || MM52164 || 8 KB operating system and BASIC ROM
|-
|-
| $E000 – $EFFF || 4 KB || Optional floating point ROM
| IC31 || MC6847 || Video display generator
|-
|-
| $F000 – $FFFF || 4 KB || Machine Operating System ROM
| IC25 || INS8255 || Peripheral interface: keyboard, tape, loudspeaker, VDG control
|}
 
== Hardware Specifications ==
 
=== Main Board Components ===
 
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Component !! Specification !! Function
|-
|-
| '''CPU''' || MOS 6502 @ 1 MHz || Central processor
| IC26 || 7445 || Keyboard row driver
|-
|-
| '''VIA''' || MOS 6522 || Versatile Interface Adapter for I/O, timers, and keyboard scanning
| IC42, IC43 || 2114 || 1 KB video and upper text RAM
|-
|-
| '''Video Controller''' || MC6847 || Video Display Generator, 256×192 graphics, 32×16 text
| IC51, IC52 || 2114 || 1 KB block zero RAM
|-
|-
| '''RAM''' || 2114 static RAM (1K×4 bit) || 2 KB minimum, expandable to 12 KB
| IC27, IC28, IC29 || 81LS95, 81LS95, DP8304 || Buffers between the processor and the video RAM
|-
|-
| '''ROM''' || 2532 EPROMs || 8 KB system ROM (4 KB MOS + 4 KB BASIC)
| IC46 || LM358 || Tape input amplifier
|-
|-
| '''Clock Generator''' || 4 MHz crystal oscillator || Divided to 1 MHz CPU clock, 3.58 MHz video clock
| IC44, IC47 || 74LS393, 74LS163 || Clock and tape-tone dividers
|-
|-
| '''Power Regulator''' || 7805 || +5V regulation for logic circuits
| IC6, IC23, IC30 || 74LS138 || Address decoders
|-
|-
| '''Keyboard Decoder''' || 74LS138 || Row/column scanning via 6522 VIA
| IC7, IC8, IC9, IC45, IC48, IC49 || 74LS02, 74LS00, 74LS04, 74LS04, 74LS00, 74LS139 || Glue logic
|-
|-
| '''Address Decoding''' || 74LS138, 74LS00 || Memory and I/O chip select generation
| IC53, IC54 || LM340T-5 || +5 V regulators
|}
|}


=== Power Supply ===
Two crystals are fitted: X1, 3.58&nbsp;MHz, drives the MC6847, and X2, 4.00&nbsp;MHz, feeds the 74LS393 divider IC44, from which the processor clock and the 2.4&nbsp;kHz tape tone are derived.<ref name="atomtm" /><ref name="atomcd" />
 
The Atom required an external power supply providing:
* '''+5V DC''' @ 2A for logic circuits
* '''-5V DC''' @ 100mA for video generation (MC6847)
* '''Unregulated 8V AC''' @ 20VA for cassette motor control
 
Early units shipped with a linear transformer supply, while later models included a more efficient switched-mode unit.


== Video System ==
=== Memory map ===


The MC6847 Video Display Generator provided:
This is Acorn's own map from the Technical Manual. Each 1&nbsp;KB block of RAM is a pair of 2114s.<ref name="atomtm" />


{| class="wikitable styled-table" style="width:70%; text-align:center;"
{| class="wikitable styled-table" style="width:80%; text-align:left;"
! Mode !! Resolution !! Memory Required !! Colours
|+'''Acorn Atom memory map'''
! Address (hex) !! Contents !! Position
|-
|-
| Text || 32×16 characters || 512 bytes || Black on green
| 0000–03FF || Block zero RAM (scratch pad and stack) || IC51, IC52
|-
|-
| Graphics 1 || 64×64 pixels || 1 KB || 4 colours
| 0400–27FF || Reserved for Eurocards (off board) || –
|-
|-
| Graphics 2 || 128×64 pixels || 1 KB || 4 colours
| 2800–3BFF || Lower text space RAM, up to 5 KB || IC10–IC19, added in pairs
|-
|-
| Graphics 3 || 128×96 pixels || 1.5 KB || 4 colours
| 3C00–7FFF || RAM expansion off board || –
|-
|-
| Graphics 4 || 128×96 pixels || 3 KB || 2 colours
| 8000–97FF || Video and upper text RAM, 1 KB to 6 KB || IC42–IC43 first, then IC40–IC41 down to IC32–IC33
|-
|-
| Graphics 5 || 128×192 pixels || 3 KB || 4 colours
| A000 || Utility ROM (MM52132) || IC24
|-
|-
| Graphics 6 || 256×192 pixels || 6 KB || Monochrome
| B000 || 8255 PPI || IC25
|}
 
Video output options:
* '''UHF modulator''' for television connection (Channel 36)
* '''Composite video''' output (1V peak-to-peak, 75Ω)
* '''RGB output''' (optional modification)
 
== Input/Output Interfaces ==
 
=== Standard Ports ===
 
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Interface !! Connector !! Signals !! Purpose
|-
|-
| '''Cassette''' || 7-pin DIN || Audio in/out, motor control || 300 or 1200 baud data storage
| B400 || Extension on PL8 || PL8
|-
|-
| '''Expansion Bus''' || 64-way edge connector || Full 6502 bus || External peripherals
| B800 || 6522 VIA || IC1
|-
|-
| '''Printer''' || 26-way IDC header || 8-bit parallel + handshake || Centronics printer interface
| C000–CFFF and F000–FFFF || System ROM (MM52164, 8 KB) || IC20
|-
|-
| '''User Port''' || 20-way IDC header || 8-bit bidirectional + control || General purpose I/O
| D000 || Floating-point ROM (MM52132) || IC21
|-
|-
| '''Utility Socket''' || 16-pin DIL || Address/data/control subset || Small expansion cards
| E000 || Reserved for the disk operating system || –
|}
|}


=== 6522 VIA Configuration ===
== Video ==


The 6522 Versatile Interface Adapter managed:
The MC6847 generates the sync and refresh timing from the 3.58&nbsp;MHz crystal and reads the video RAM directly. The processor reaches the video RAM through tri-state buffers. The VDG signals its vertical blanking period to the 8255, and the processor can write to video RAM during that period without disturbing the picture.<ref name="atomtm" /> There are nine display modes, from 64 × 64 in four colours to 256 × 192 in monochrome.<ref name="wp" />
* '''Port A:''' Printer data output (8 bits)
* '''Port B bits 0-3:''' Keyboard row select
* '''Port B bit 4:''' Cassette output
* '''Port B bit 5:''' Loudspeaker output
* '''Port B bit 6:''' Keyboard REPT key
* '''Port B bit 7:''' Keyboard CTRL key
* '''Timer 1:''' Cassette tape timing
* '''Timer 2:''' Sound generation


== Keyboard ==
The standard Atom produces a monochrome picture even on a colour set. Acorn's PAL encoder board fits inside the case to give colour.<ref name="atomtm" /> The UHF modulator (SK1) is set for channel 36, and a 1&nbsp;V into 75&nbsp;Ω composite signal is on pin 9 of PL4, with 0&nbsp;V on pin 10.<ref name="atomtm" />


The Atom featured a 60-key QWERTY keyboard with:
== Keyboard, tape and sound ==
* Full alphanumeric keys
* CTRL, SHIFT, REPT (repeat) modifier keys
* BREAK key (NMI interrupt)
* LOCK key (caps lock)
* COPY key (screen copy to printer)
* Direct keyboard matrix scanning via 6522 VIA


Keyboard matrix organization:
The 8255 drives the 7445 decoder, whose ten outputs scan the rows of the key matrix in turn while the 8255 reads the columns.<ref name="atomtm" />
* 8 rows × 10 columns matrix
* Software debouncing in MOS ROM
* Auto-repeat functionality
* N-key rollover limited to 2 keys


== Expansion Options ==
The tape interface follows the Computer Users' Tape Standard (CUTS): 2.4&nbsp;kHz for a 1, 1.2&nbsp;kHz for a 0, 300 bits per second, crystal controlled.<ref name="atomtm" /> The recorder connects to the 7-pin DIN socket SK2 with a 3-pin lead: pin 1 output, pin 2 common, pin 3 input. Pins 4 to 7 carry 8255 port C outputs, and Acorn warns that they may damage a recorder with a 5- or 7-pin connector. Both levels are set to 300&nbsp;mV RMS. R29 sets the output level, and R34 sets the input gain; Acorn suggests changing R34 from 47&nbsp;kΩ to 220&nbsp;kΩ for hi-fi cassette decks.<ref name="atomtm" /> Port C bit 2 drives the loudspeaker.<ref name="atomtm" />


=== Internal Expansion ===
== Power supply ==


The Atom motherboard included sockets for:
The Atom needs 8&nbsp;V DC on the power socket SK3. Two LM340T-5 regulators, IC53 and IC54, each feed one half of the board's +5&nbsp;V distribution, and a heatsink sits between the regulators and the PCB.<ref name="atomtm" /> Acorn's high-efficiency mains adaptor delivers up to 1600&nbsp;mA. Acorn's budget for a fully expanded Atom with low-power RAM is 750&nbsp;mA for the minimal system, 750&nbsp;mA for 10&nbsp;KB of expansion RAM and 100&nbsp;mA for the bus buffers and VIA. Atoms supplied before October 1980 used normal-power RAM at 100&nbsp;mA per KB, and fully expanded they draw more than the adaptor can supply.<ref name="atomtm" />
* '''Additional RAM:''' Up to 5 KB lower text RAM (Block Zero)
* '''Video RAM:''' Up to 6 KB for high-resolution graphics
* '''Upper text RAM:''' 3 KB expansion at $3000-$3BFF
* '''Floating Point ROM:''' 4 KB scientific calculator functions
* '''Utility ROMs:''' Three 4 KB ROM sockets for languages or utilities


=== External Expansion ===
The board can instead run from a regulated 5&nbsp;V supply. Link LK6 joins the two halves of the supply and LK7 bypasses the regulators; the 8&nbsp;V adaptor must then never be plugged in, and Acorn asks for a label saying so beside the socket. The manual gives a circuit for a 5&nbsp;V 3&nbsp;A supply (9&nbsp;V 50&nbsp;VA transformer, bridge rectifier, 1500&nbsp;µF 16&nbsp;V reservoir, LM323K regulator) for a fully expanded Atom with an Acorn Eurocard.<ref name="atomtm" />


Via the rear expansion connector:
== Expansion ==
* '''Acorn Disk Pack:''' Dual 5.25" floppy drives with DOS
* '''Econet Interface:''' Network adapter for educational environments
* '''Colour Card:''' PAL colour encoder for MC6847
* '''Atom Speechbox:''' SP0256 speech synthesizer
* '''A/D Converter:''' 8-bit analog input card


== ROM Software ==
The expansions in the Technical Manual are:<ref name="atomtm" />


=== Machine Operating System (4 KB) ===
* Lower text space: up to ten 2114-Ls at IC10–IC19, giving 5&nbsp;KB at #2800–#3BFF. IC6 (74LS138, supplied with the base kit) must be fitted. Each 2114-L draws about 35&nbsp;mA.
* Video graphics RAM: up to six pairs of 2114-Ls, IC32–IC43, for 6&nbsp;KB. IC30 (74LS138) is needed beyond the first pair, and link LK1 must be cut when it is fitted.
* Floating-point ROM at IC21, which adds floating-point arithmetic, scientific functions and the COLOUR statement.
* 6522 VIA at IC1, with its two ports on the 64-way connector PL6 (Vero 17-3704L type). LK2 connects its interrupt.
* Centronics printer port: the VIA plus a 74LS244 at IC50, on the 26-way header PL5.
* Bus buffers IC2–IC5 (two 81LS95, a DP8304 and a 74LS30), which put the Acorn bus on PL6 and on PL7 for one internal Eurocard.
* PL8, a 20-way plug on side 1, for other extension boards.


Located at $F000-$FFFF, provided:
Acorn also sold the Atom Disc Pack, and an Econet interface could be fitted as a Eurocard.<ref name="chris" />
* Hardware initialization routines
* Keyboard scanning and decoding
* Cassette tape I/O routines
* VDU driver routines
* Interrupt handlers
* Monitor commands (memory examine/modify)


=== Atom BASIC (4 KB) ===
== Board revisions ==


Located at $C000-$CFFF, featured:
The PCB is marked 202,000 with an issue number; a basic Issue 4 board is photographed at Chris's Acorns.<ref name="chris" /> Acorn's circuit diagram went through issues A to D and 1 to 3 between March 1980 and January 1981.<ref name="atomcd" />
* Integer arithmetic only (no floating point in base model)
* Direct memory access via ! and ? operators
* Built-in assembler for 6502 code
* Graphics commands (DRAW, MOVE, PLOT)
* String handling (12 character maximum)
* Array support (integer only)
* Procedures with local variables


=== Optional Floating Point ROM (4 KB) ===
== Known faults ==


Located at $E000-$EFFF when installed:
* Supply current. The adaptor is rated for a fully expanded Atom only with low-power 2114-L RAM. An early machine with normal-power RAM, or one carrying a Eurocard, needs the regulated 5&nbsp;V conversion.<ref name="atomtm" /> Chris Whytehead's Acorn adaptor is labelled 8&nbsp;V 1.8&nbsp;A.<ref name="chris" />
* 5-byte BCD floating point format
* Regulator headroom. An LM340-5 needs at least 7.5&nbsp;V in to hold regulation, and its output may lie between 4.75 and 5.25&nbsp;V across load and temperature.<ref name="lm340">Texas Instruments, ''LM340, LM340A and LM7805 Family Wide V<sub>IN</sub> 1.5-A Fixed Voltage Regulators'', SNOSBT0L, revised September 2016, section 6.6 (LM340/LM7805, V<sub>O</sub> = 5 V): output 4.8–5.2 V at 25 °C, 4.75–5.25 V for 7.5 V ≤ V<sub>IN</sub> ≤ 20 V and 5 mA–1 A; input required to maintain line regulation 7.5 V; dropout 2 V typical at 1 A.</ref> The nominal 8&nbsp;V input leaves little margin, so measure the input at SK3 under load as well as the +5&nbsp;V output. The two regulators get hot.<ref name="wp" />
* 9 significant digits, exponent ±99
* Tape leads. A 5- or 7-pin DIN lead connects the 8255 outputs on SK2 pins 4–7 to the recorder.<ref name="atomtm" />
* Trigonometric functions
* Logarithmic functions
* Scientific notation support


== Technical Innovations ==
Maintenance, fault-finding and capacitor details are on the guide pages below.


The Atom introduced several design features:
== Documentation ==
* '''Single-board construction''' reduced manufacturing costs
* '''Memory-mapped I/O''' simplified hardware design
* '''Built-in assembler''' in BASIC ROM
* '''Direct video memory access''' from BASIC
* '''Modular expansion system''' via edge connector


== Component-Level Details ==
* [[Acorn Atom Technical Manual]]: Acorn's construction and expansion manual, issue 2, October 1980.
* [[:File:Acorn Atom circuit diagram.png]]: Acorn's circuit diagram, issue 3.
* [[:File:Acorn Atom motherboard layout.jpg]]: IC location diagram from ''The Acorn Atom Review''.


=== Clock Generation ===
== Related pages ==


* 4 MHz crystal oscillator (X1)
* [[Acorn Atom General Maintenance]]
* 74LS163 counter divides by 4 for 1 MHz CPU clock
* [[Acorn Atom Troubleshooting Guide]]
* 3.58 MHz NTSC subcarrier for MC6847 video timing
* [[Acorn Atom Capacitor Replacement Guide]]
* Phase relationships maintained for video stability
* [[Acorn System 3]]
 
* [[BBC Micro Model A/B]]
=== Address Decoding Logic ===
 
* IC7 (74LS138) decodes A12-A15 for 4K blocks
* IC8 (74LS138) further decodes I/O space
* IC20 (74LS00) generates chip enables
* Pull-up resistors on unused decoder outputs
 
=== Reset Circuit ===
 
* RC network (R1=4.7kΩ, C1=10µF) provides power-on reset
* Manual reset via BREAK key triggers NMI
* 74LS00 gate buffers and inverts reset signal
* Reset pulse width approximately 47ms
 
=== Cassette Interface ===
 
* Output: Square wave from 6522 PB4, AC-coupled via C15 (0.1µF)
* Input: Comparator circuit using LM311 or similar
* Motor control: Relay driver using BC108 transistor
* Data encoding: Kansas City Standard (300 baud) or fast mode (1200 baud)
 
== Known Hardware Issues ==
 
Common failure points requiring maintenance:
* '''7805 voltage regulator''' overheating without adequate heatsink
* '''MC6847 video chip''' thermal stress causing intermittent display
* '''Keyboard matrix diodes''' failing causing phantom keypresses
* '''2114 RAM chips''' prone to failure, especially in lower addresses
* '''Edge connector''' oxidation causing expansion problems
* '''Cassette relay''' contacts wearing from repeated use


== Board Revisions ==
== References ==


{| class="wikitable styled-table" style="width:80%; text-align:center;"
<references />
! Revision !! Changes !! Issues Addressed
|-
| Issue 1 || Original design || N/A
|-
| Issue 2 || Improved ground plane || Reduced video interference
|-
| Issue 3 || Modified reset circuit || More reliable power-on reset
|-
| Issue 4 || Updated cassette interface || Better tape reliability
|-
| Issue 5 || Final production version || Cost-reduced component selection
|}


== See Also ==
{{Navbox-AcornComputers}}
* [[Acorn Atom General Maintenance]]
* [[Acorn Atom Troubleshooting]]
* [[Acorn Atom Capacitor Replacement Guide]]


[[Category:Acorn Computers]]
[[Category:Acorn Computers]]

Latest revision as of 01:10, 1 October 2026

Acorn Atom
Acorn Atom (photo: Simon Inns, CC BY 2.0)
Specifications
ManufacturerAcorn Computers Ltd
TypeHome computer
ReleasedMarch 1980[1]
Discontinued1983[2]
Intro priceKit £120, assembled £170 (1980)[1]
CPU6502 at 1 MHz (IC22)
Memory2 KB RAM, expandable on the board to 12 KB; 8 KB system ROM plus sockets for two 4 KB ROMs
StorageCassette (CUTS, 300 bit/s); Atom Disc Pack optional
DisplayMC6847 VDG (IC31): 32 × 16 text, graphics up to 256 × 192; UHF and composite video
SoundLoudspeaker driven from the 8255
Dimensions381 × 241 × 64 mm[2]
OS / FirmwareAtom operating system and BASIC in ROM
PredecessorAcorn System 3
SuccessorBBC Micro Model A/B
Model no.Main PCB 202,000

The Acorn Atom is a 6502-based home computer that Acorn Computers sold from March 1980, either as a kit at £120 or assembled at £170.[1] It is based on the Acorn System 3, in a new case with an integral keyboard, and it accepts the same Acorn Eurocards.[1] Production ended in 1983.[2]

The minimum machine has 2 KB of RAM and an 8 KB ROM holding the operating system and Atom BASIC. Further RAM, a floating-point ROM, a utility ROM, a 6522 VIA, a printer port and bus buffers all fit into empty sockets on the same board.[3] Atom BASIC was written by Sophie Wilson, and its interpreter includes a 6502 assembler. The case was designed by Allen Boothroyd of Cambridge Product Design.[2]

Architecture

[edit | edit source]

The Atom is built on one double-sided, through-plated PCB, part number 202,000. The ICs, connectors PL/SK1–7 and passive components are on side 2; PL8 and the keyboard are on side 1.[3] The 6502 runs at 1 MHz, and every bus cycle is synchronised to that clock. A reset at power-up or from the BREAK key starts the processor in the operating system ROM.[3]

Main board ICs

[edit | edit source]

The base kit fits the following ICs. The designators are Acorn's.[3][4]

Acorn Atom base-kit ICs
Designator Device Use
IC22 6502, 1 MHz Processor
IC20 MM52164 8 KB operating system and BASIC ROM
IC31 MC6847 Video display generator
IC25 INS8255 Peripheral interface: keyboard, tape, loudspeaker, VDG control
IC26 7445 Keyboard row driver
IC42, IC43 2114 1 KB video and upper text RAM
IC51, IC52 2114 1 KB block zero RAM
IC27, IC28, IC29 81LS95, 81LS95, DP8304 Buffers between the processor and the video RAM
IC46 LM358 Tape input amplifier
IC44, IC47 74LS393, 74LS163 Clock and tape-tone dividers
IC6, IC23, IC30 74LS138 Address decoders
IC7, IC8, IC9, IC45, IC48, IC49 74LS02, 74LS00, 74LS04, 74LS04, 74LS00, 74LS139 Glue logic
IC53, IC54 LM340T-5 +5 V regulators

Two crystals are fitted: X1, 3.58 MHz, drives the MC6847, and X2, 4.00 MHz, feeds the 74LS393 divider IC44, from which the processor clock and the 2.4 kHz tape tone are derived.[3][4]

Memory map

[edit | edit source]

This is Acorn's own map from the Technical Manual. Each 1 KB block of RAM is a pair of 2114s.[3]

Acorn Atom memory map
Address (hex) Contents Position
0000–03FF Block zero RAM (scratch pad and stack) IC51, IC52
0400–27FF Reserved for Eurocards (off board) –
2800–3BFF Lower text space RAM, up to 5 KB IC10–IC19, added in pairs
3C00–7FFF RAM expansion off board –
8000–97FF Video and upper text RAM, 1 KB to 6 KB IC42–IC43 first, then IC40–IC41 down to IC32–IC33
A000 Utility ROM (MM52132) IC24
B000 8255 PPI IC25
B400 Extension on PL8 PL8
B800 6522 VIA IC1
C000–CFFF and F000–FFFF System ROM (MM52164, 8 KB) IC20
D000 Floating-point ROM (MM52132) IC21
E000 Reserved for the disk operating system –

Video

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The MC6847 generates the sync and refresh timing from the 3.58 MHz crystal and reads the video RAM directly. The processor reaches the video RAM through tri-state buffers. The VDG signals its vertical blanking period to the 8255, and the processor can write to video RAM during that period without disturbing the picture.[3] There are nine display modes, from 64 × 64 in four colours to 256 × 192 in monochrome.[2]

The standard Atom produces a monochrome picture even on a colour set. Acorn's PAL encoder board fits inside the case to give colour.[3] The UHF modulator (SK1) is set for channel 36, and a 1 V into 75 Ω composite signal is on pin 9 of PL4, with 0 V on pin 10.[3]

Keyboard, tape and sound

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The 8255 drives the 7445 decoder, whose ten outputs scan the rows of the key matrix in turn while the 8255 reads the columns.[3]

The tape interface follows the Computer Users' Tape Standard (CUTS): 2.4 kHz for a 1, 1.2 kHz for a 0, 300 bits per second, crystal controlled.[3] The recorder connects to the 7-pin DIN socket SK2 with a 3-pin lead: pin 1 output, pin 2 common, pin 3 input. Pins 4 to 7 carry 8255 port C outputs, and Acorn warns that they may damage a recorder with a 5- or 7-pin connector. Both levels are set to 300 mV RMS. R29 sets the output level, and R34 sets the input gain; Acorn suggests changing R34 from 47 kΩ to 220 kΩ for hi-fi cassette decks.[3] Port C bit 2 drives the loudspeaker.[3]

Power supply

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The Atom needs 8 V DC on the power socket SK3. Two LM340T-5 regulators, IC53 and IC54, each feed one half of the board's +5 V distribution, and a heatsink sits between the regulators and the PCB.[3] Acorn's high-efficiency mains adaptor delivers up to 1600 mA. Acorn's budget for a fully expanded Atom with low-power RAM is 750 mA for the minimal system, 750 mA for 10 KB of expansion RAM and 100 mA for the bus buffers and VIA. Atoms supplied before October 1980 used normal-power RAM at 100 mA per KB, and fully expanded they draw more than the adaptor can supply.[3]

The board can instead run from a regulated 5 V supply. Link LK6 joins the two halves of the supply and LK7 bypasses the regulators; the 8 V adaptor must then never be plugged in, and Acorn asks for a label saying so beside the socket. The manual gives a circuit for a 5 V 3 A supply (9 V 50 VA transformer, bridge rectifier, 1500 µF 16 V reservoir, LM323K regulator) for a fully expanded Atom with an Acorn Eurocard.[3]

Expansion

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The expansions in the Technical Manual are:[3]

  • Lower text space: up to ten 2114-Ls at IC10–IC19, giving 5 KB at #2800–#3BFF. IC6 (74LS138, supplied with the base kit) must be fitted. Each 2114-L draws about 35 mA.
  • Video graphics RAM: up to six pairs of 2114-Ls, IC32–IC43, for 6 KB. IC30 (74LS138) is needed beyond the first pair, and link LK1 must be cut when it is fitted.
  • Floating-point ROM at IC21, which adds floating-point arithmetic, scientific functions and the COLOUR statement.
  • 6522 VIA at IC1, with its two ports on the 64-way connector PL6 (Vero 17-3704L type). LK2 connects its interrupt.
  • Centronics printer port: the VIA plus a 74LS244 at IC50, on the 26-way header PL5.
  • Bus buffers IC2–IC5 (two 81LS95, a DP8304 and a 74LS30), which put the Acorn bus on PL6 and on PL7 for one internal Eurocard.
  • PL8, a 20-way plug on side 1, for other extension boards.

Acorn also sold the Atom Disc Pack, and an Econet interface could be fitted as a Eurocard.[1]

Board revisions

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The PCB is marked 202,000 with an issue number; a basic Issue 4 board is photographed at Chris's Acorns.[1] Acorn's circuit diagram went through issues A to D and 1 to 3 between March 1980 and January 1981.[4]

Known faults

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  • Supply current. The adaptor is rated for a fully expanded Atom only with low-power 2114-L RAM. An early machine with normal-power RAM, or one carrying a Eurocard, needs the regulated 5 V conversion.[3] Chris Whytehead's Acorn adaptor is labelled 8 V 1.8 A.[1]
  • Regulator headroom. An LM340-5 needs at least 7.5 V in to hold regulation, and its output may lie between 4.75 and 5.25 V across load and temperature.[5] The nominal 8 V input leaves little margin, so measure the input at SK3 under load as well as the +5 V output. The two regulators get hot.[2]
  • Tape leads. A 5- or 7-pin DIN lead connects the 8255 outputs on SK2 pins 4–7 to the recorder.[3]

Maintenance, fault-finding and capacitor details are on the guide pages below.

Documentation

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Whytehead, Chris. "Acorn Atom Computers" and "Acorn Atom", Chris's Acorns, archived by The Centre for Computing History. Release in March 1980; kit £120, assembled £170, fully expanded over £200; based on the System 3; a basic machine with motherboard 202,000 Issue 4; Acorn mains unit labelled 8 V 1.8 A DC; Atom Disc Pack and Econet via Eurocards.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 "Acorn Atom", Wikipedia (retrieved 2026-10-01). Source for the 1983 end of production, the case designer, the author of Atom BASIC, the nine graphics modes, the case dimensions and the regulators running hot.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 Acorn Computers, Atom Technical Manual, issue 2, October 1980, hosted on this wiki. Parts list pp. 6–7; power supply p. 13; tape interface pp. 14–15; circuit description pp. 16–17; memory map p. 20; extensions pp. 21–23.
  4. ↑ 4.0 4.1 4.2 Acorn Computers, Circuit Diagram for Atom Microcomputer, drawn 28 February 1980, revised to issue 3 on 14 January 1981. Hosted on this wiki as File:Acorn Atom circuit diagram.png.
  5. ↑ Texas Instruments, LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators, SNOSBT0L, revised September 2016, section 6.6 (LM340/LM7805, VO = 5 V): output 4.8–5.2 V at 25 °C, 4.75–5.25 V for 7.5 V ≤ VIN ≤ 20 V and 5 mA–1 A; input required to maintain line regulation 7.5 V; dropout 2 V typical at 1 A.


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