Editing Acorn Atom

{{Infobox computer
| name         = Acorn Atom
| image        = [[File:Acorn_Atom.jpg|250px]]
| caption      = Acorn Atom
| manufacturer = Acorn Computers Ltd
| type         = Home Computer
| release date = March 1980
| discontinued = 1983
| price        = Kit: £120, Assembled: £170 (1980)
| cpu          = MOS Technology 6502 @ 1 MHz
| memory       = 2 KB RAM (expandable to 12 KB), 8 KB ROM (expandable to 12 KB)
| storage      = Cassette tape interface (CUTS, 300 baud), optional floppy disk
| display      = 256×192 monochrome graphics, 32×16 text mode (MC6847)
| sound        = 1 channel, internal speaker
| dimensions   = 381 mm × 241 mm × 64 mm
| os           = Acorn MOS with Atom BASIC
| predecessor  = [[Acorn System 3]]
| successor    = [[BBC Micro Model A/B]]
| model        = PCB 202,000
}}

The '''Acorn Atom''' was a home computer made by Acorn Computers Ltd from 1980 to 1983, when it was replaced by the [[Acorn Electron]] and the [[BBC Micro Model A/B|BBC Micro]] as Acorn's low-cost option.<ref name="wp">[https://en.wikipedia.org/wiki/Acorn_Atom "Acorn Atom"], Wikipedia. Source for the 1980&ndash;1983 production span, pricing, the nine MC6847 video modes (64×64 4-colour up to 256×192 monochrome), the 50&nbsp;Hz PAL colour-card requirement, the 300&nbsp;baud CUTS cassette interface, the dual on-board LM7805 regulators, and the memory map.</ref> It was a cut-down [[Acorn System 3]] built into a case with an integral keyboard and a cassette interface, sold in kit or assembled form. The Atom used the MOS Technology 6502 and was designed around an 8&nbsp;KB ROM holding the operating system and Acorn's own Atom BASIC, written by Sophie Wilson.

The minimum machine had 2&nbsp;KB of RAM and 8&nbsp;KB of ROM; the maximum specification had 12&nbsp;KB of each, plus an optional floating-point extension ROM. The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design.<ref name="wp" />

== Architecture ==

The Atom used a single double-sided PCB (part 202,000) with the 6502 running at 1&nbsp;MHz and memory-mapped I/O. The system ROM held the Machine Operating System and Atom BASIC; the BASIC interpreter included an in-line 6502 assembler, which was unusual for the period.<ref name="wp" />

=== Memory map ===

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''Acorn Atom memory map'''
! Address !! Contents
|-
| $0000 || Block Zero RAM (1 KB: zero page, 6502 stack, OS and BASIC variables)
|-
| $0400 || Text / VDG screen RAM (low)
|-
| $0800 || VDG / CRT-controller area
|-
| $0A00 || Optional floppy-disc controller (FDC)
|-
| $1000 || Peripheral / extension space
|-
| $2000 || Catalogue buffer
|-
| $2200 || Sequential file buffers
|-
| $2800 || Floating-point variables (internal RAM, up to 5 KB)
|-
| $2900 || Extension text-space RAM
|-
| $3C00 || Off-board extension RAM
|-
| $8000 || Video and BASIC RAM (mode-dependent, up to 6 KB)
|-
| $A000 || Optional utility ROM
|-
| $B000 || INS8255 PPI I/O device
|-
| $B800 || Optional 6522 VIA (printer interface)
|-
| $C000 || Atom BASIC interpreter ROM (4 KB)
|-
| $D000 || Optional extension ROM
|-
| $E000 || Optional DOS / floating-point ROM
|-
| $F000 || MOS and assembler ROM (4 KB)
|}

Source: Acorn Atom memory map.<ref name="wp" />

== Main board components ==

The IC designators below are from the [[Acorn Atom Technical Manual]] parts list.<ref name="atomtm">[[Acorn Atom Technical Manual]] (Issue 2, October 1980), Acorn Computers &mdash; hosted on this wiki. Source for the IC complement (IC22 6502, IC31 6847, IC25 INS8255, IC26 7445, IC20 MM52164 ROM, IC42/43/51/52 2114 RAM, IC53/IC54 LM340T-5 regulators), the crystals (X1 3.58&nbsp;MHz, X2 4.00&nbsp;MHz), the capacitor list, and the 8&nbsp;V supply with the LK6/LK7 5&nbsp;V conversion.</ref>

{| class="wikitable styled-table" style="width:95%; text-align:center;"
! Component !! Device !! Designator !! Function
|-
| '''CPU''' || MOS 6502 @ 1 MHz || IC22 || Central processor
|-
| '''Video''' || Motorola MC6847 VDG || IC31 || 256×192 graphics / 32×16 text
|-
| '''Peripheral interface''' || INS8255 PPI || IC25 || Keyboard, cassette and loudspeaker I/O
|-
| '''Keyboard row driver''' || 7445 BCD-to-decimal decoder || IC26 || Drives the keyboard matrix rows
|-
| '''VIA (optional)''' || MOS 6522 || (B800) || Printer (Centronics) interface &mdash; fitted only on expanded machines
|-
| '''RAM''' || 2114 static RAM (1K×4) || IC42, IC43, IC51, IC52 (+ expansion) || 2 KB base, expandable to 12 KB
|-
| '''ROM''' || MM52164 mask ROM (base) || IC20 || 8 KB system ROM (MOS + BASIC)
|-
| '''Clocks''' || 4.00 MHz + 3.58 MHz crystals || X2, X1 || CPU clock (÷ to 1 MHz) and MC6847 video timing
|-
| '''Cassette amp''' || LM358 op-amp || IC46 || CUTS read/write circuit
|-
| '''Regulators''' || 2× LM340T-5 (7805) || IC53, IC54 || +5 V regulation, one per board half
|}

== Video system ==

The MC6847 Video Display Generator provided text (32×16) and graphics up to 256×192 monochrome, across nine modes from 64×64 in four colours to 256×192 monochrome. The MC6847 is a 60&nbsp;Hz NTSC device, so on European 50&nbsp;Hz sets the standard machine produced a monochrome picture; an Acorn 50&nbsp;Hz PAL colour card (Acorn drawing 102,006-C) was available to add colour.<ref name="wp" /> Output was via a UHF modulator (channel 36); a composite-video output was available by modification.

== Keyboard, cassette and sound ==

The keyboard, cassette interface and loudspeaker are handled by the '''INS8255 PPI (IC25)''', with the '''7445 (IC26)''' driving the keyboard-matrix rows; the optional 6522 VIA is used only for the printer interface on expanded machines.<ref name="atomtm" /> The full-travel QWERTY keyboard includes CTRL, SHIFT, REPT (repeat) and BREAK keys, with software debouncing in the MOS ROM.

The cassette interface uses the CUTS variant of the Kansas City standard at '''300&nbsp;baud only''', built around the LM358 op-amp (IC46) and transistors Q1/Q2 (BC107). There is no internal cassette deck or motor relay. Commercial BBC Micro tapes (1200&nbsp;baud) cannot be loaded on the Atom.<ref name="wp" />

== Power supply ==

The Atom takes '''8&nbsp;V DC (unregulated)''' on a 2.1&nbsp;mm jack (SK3) and regulates it on the board to '''+5&nbsp;V''' using two LM340T-5 (7805-type) regulators, IC53 and IC54, each feeding one half of the logic, with a heatsink fitted between them. There is no &minus;5&nbsp;V rail and no AC inside the machine.<ref name="atomtm" /> The Acorn adaptor is rated 8&nbsp;V at about 1.5&ndash;1.8&nbsp;A, which is marginal: a fully expanded Atom can draw up to 3&nbsp;A, and the two regulators run hot.<ref name="wp" /> The board can instead be powered from a 5&nbsp;V regulated supply by fitting links LK6 and LK7, which join the two power sections and bypass the regulators.<ref name="atomtm" />

== Input/output and expansion ==

Standard ports: a 7-pin DIN cassette socket (SK2), the UHF output (SK1), and the 8&nbsp;V DC jack (SK3). The rear 64-way edge connector carries the full 6502 bus for Acorn Eurocard expansion. Expansion options fitted into the on-board sockets and via the bus included additional lower-text RAM (in 1&nbsp;KB steps to 5&nbsp;KB), video graphics RAM (to 6&nbsp;KB), the 4&nbsp;KB floating-point ROM, a 4&nbsp;KB utility ROM, the 6522 VIA and Centronics printer port, the Acorn Disc Pack, an Econet interface, the PAL colour card and a speech board.<ref name="atomtm" />

== Board revisions ==

The Atom main PCB carries the part number '''202,000''' on the silkscreen with an issue number. The Technical Manual documents Issue&nbsp;2 (October 1980); production boards are commonly seen up to Issue&nbsp;4.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/Atom.html "Acorn Atom"], Chris's Acorns / The Centre for Computing History. Documents the 202,000 Issue&nbsp;4 board, the base IC complement, and the recommendation to use a regulated 5&nbsp;V supply for a fully expanded machine.</ref>

== Known hardware issues ==

* The two LM7805 regulators (IC53, IC54) run hot; on a fully expanded machine the supply is marginal and thermal shutdown causes intermittent resets.<ref name="wp" />
* 2114 static RAM is a common failure part, producing corrupted text/graphics or crashes.
* The rear edge-connector fingers are exposed and oxidise, causing expansion-bus faults.
* Socketed ICs (ROM, RAM, MC6847, 8255) develop intermittent contacts and benefit from reseating.

For diagnosis and repair, see the maintenance, troubleshooting and capacitor pages below.

== See also ==
* [[Acorn Atom General Maintenance]]
* [[Acorn Atom Troubleshooting Guide]]
* [[Acorn Atom Capacitor Replacement Guide]]
* [[Acorn Electron]]
* [[BBC Micro Model A/B]]

== References ==

<references />

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[[Category:Acorn Computers]]