Acorn Atom
 | |
| Acorn Atom (photo: Simon Inns, CC BY 2.0) | |
| Specifications | |
|---|---|
| Manufacturer | Acorn Computers Ltd |
| Type | Home computer |
| Released | March 1980[1] |
| Discontinued | 1983[2] |
| Intro price | Kit £120, assembled £170 (1980)[1] |
| CPU | 6502 at 1 MHz (IC22) |
| Memory | 2 KB RAM, expandable on the board to 12 KB; 8 KB system ROM plus sockets for two 4 KB ROMs |
| Storage | Cassette (CUTS, 300 bit/s); Atom Disc Pack optional |
| Display | MC6847 VDG (IC31): 32 × 16 text, graphics up to 256 × 192; UHF and composite video |
| Sound | Loudspeaker driven from the 8255 |
| Dimensions | 381 × 241 × 64 mm[2] |
| OS / Firmware | Atom operating system and BASIC in ROM |
| Predecessor | Acorn System 3 |
| Successor | BBC 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]
| 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]
| 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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]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
[edit | edit source]- 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
[edit | edit source]- 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.
Related pages
[edit | edit source]- Acorn Atom General Maintenance
- Acorn Atom Troubleshooting Guide
- Acorn Atom Capacitor Replacement Guide
- Acorn System 3
- BBC Micro Model A/B
References
[edit | edit source]- ↑ 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.
- ↑ 2.0 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.
- ↑ 3.00 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.
- ↑ 4.0 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.
- ↑ 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.