Acorn Electron

Acorn Electron

Acorn Electron
Specifications
Manufacturer	Acorn Computers Ltd
Type	Home Computer
Released	August 25, 1983
Discontinued	1985
Intro price	£199 (1983)
CPU	MOS Technology 6502A @ 2 MHz (1 MHz effective)
Memory	32 KB RAM, 32 KB ROM
Storage	Cassette tape interface, optional floppy disk via Plus 1
Display	640×256 (2 colours), 320×256 (4 colours), 160×256 (16 colours), 80×25 text
Sound	1 channel + white noise (internal speaker)
Dimensions	340 mm × 160 mm × 55 mm
Weight	1.8 kg
OS / Firmware	Acorn MOS 1.0 with BBC BASIC 2
Predecessor	BBC Micro Model A/B
Successor	None
Codename	Egghead, Acorn

The Acorn Electron was a budget home computer released by Acorn Computers Ltd in August 1983. Designed as a low-cost alternative to the BBC Micro, the Electron featured a single custom ULA chip that integrated most of the discrete logic found in its predecessor, reducing manufacturing costs while maintaining software compatibility with BBC BASIC programs.

Architecture

The Electron's architecture centered around a custom Ferranti ULA (Uncommitted Logic Array) containing 2,000 gates, which handled video generation, memory control, sound, and cassette I/O. This integration came at the cost of performance, as the ULA could only access RAM during specific periods, effectively halving the 6502's throughput.

Memory Organization

Acorn Electron Memory Map

Address Range	Size	Function	Access Speed
$0000 – $0040	64 bytes	Zero page workspace	2 MHz
$0041 – $00FF	191 bytes	Zero page user area	2 MHz
$0100 – $01FF	256 bytes	6502 stack	2 MHz
$0200 – $02FF	256 bytes	Operating system workspace	2 MHz
$0300 – $03FF	256 bytes	VDU workspace	2 MHz
$0400 – $07FF	1 KB	Current language workspace	2 MHz
$0800 – $0AFF	768 bytes	Sound/printer buffer	1 MHz
$0B00 – $0CFF	512 bytes	Keyboard buffer/user workspace	1 MHz
$0D00 – $0DFF	256 bytes	NMI workspace	1 MHz
$0E00 – $1FFF	4.5 KB	User BASIC program space (low)	1 MHz
$2000 – $2FFF	4 KB	User BASIC variables	1 MHz
$3000 – $7FFF	20 KB	Screen memory/User space	1 MHz
$8000 – $BFFF	16 KB	Sideways ROM/RAM (paged)	2 MHz
$C000 – $FFFF	16 KB	Operating System ROM	2 MHz

Hardware Specifications

Main Board Components

Component	Part Number	Function	Notes
CPU	MOS 6502A	Central processor	2 MHz clock, 1 MHz effective
ULA	Ferranti 12C021	Video, sound, memory control	68-pin custom chip
RAM	4× HM4864P-2	32 KB total	64K×1 bit DRAMs, 200ns
ROM	2× 23128	32 KB total	OS + BASIC interpreter
Crystal	16 MHz	Master clock	Divided down for all timing
Keyboard Controller	None (ULA)	Key matrix scanning	Via ULA ports
PSU Regulator	7805	+5V regulation	External PSU provides 18V AC

ULA (Uncommitted Logic Array)

The Ferranti-manufactured ULA performed multiple functions:

Function	Implementation	Performance Impact
Video generation	6845 CRTC emulation	Steals RAM cycles during display
DRAM refresh	Automatic during video scan	Hidden refresh, no CPU impact
Memory mapping	Address decode logic	Enforces 1 MHz/2 MHz regions
Sound generation	Simple tone + noise	Single channel only
Cassette interface	1200/300 baud FSK	Integrated modulator/demodulator
Keyboard scanning	14×4 matrix decode	Interrupt-driven scanning
ROM paging	4-bit latch	Supports 16 ROM banks

Video System

The Electron supported seven display modes compatible with the BBC Micro:

Mode	Resolution	Colours	Memory	Text Columns
0	640×256	2	20 KB	80×32
1	320×256	4	20 KB	40×32
2	160×256	16	20 KB	20×32
3	640×256	2 (text only)	16 KB	80×25
4	320×256	2	10 KB	40×32
5	160×256	4	10 KB	20×32
6	320×256	2 (text only)	8 KB	40×25

Video timing specifications:

• Horizontal frequency: 15.625 kHz
• Vertical frequency: 50 Hz
• Pixel clock: 8 MHz (modes 0,3) or 4 MHz (modes 1,4,6) or 2 MHz (modes 2,5)
• Active display: 256 lines × 640/320/160 pixels
• Blanking: 56 lines vertical, 160 pixels horizontal

Input/Output

Expansion Port

The rear expansion connector provided a 50-way edge connector with:

Pin Group	Signals	Purpose
1-8	D0-D7	Data bus
9-24	A0-A15	Address bus
25-26	R/W, Φ2	Control signals
27-30	nNMI, nIRQ, nRST, RDY	Interrupt/control
31-35	16MHz, nPAGE, nROM	Timing/selection
36-40	Sound, Cassette	Audio I/O
41-50	Power, Ground	+5V, -5V, 0V

Plus 1 Expansion Unit

The Plus 1 add-on provided:

• Printer port: Centronics parallel interface
• Joystick port: Analogue joysticks (BBC-compatible)
• Cartridge slots: Two ROM cartridge sockets
• Additional logic: 8255 PPI for I/O expansion

Plus 3 Disk Interface

The Plus 3 added:

• WD1770 FDC: Floppy disk controller
• 3.5" drive: Single 640 KB drive
• ADFS ROM: Advanced Disk Filing System
• Additional RAM: 3 KB workspace

Power Supply

The Electron used an external transformer unit:

• Input: 230V AC (UK model)
• Output: 18V AC @ 1.4A
• Internal regulation: 7805 provides +5V DC
• Consumption: 6W typical

Power distribution:

• +5V: Logic circuits, ULA, CPU, RAM
• -5V: Generated on-board for cassette interface
• Current draw: 1.2A maximum on +5V rail

Keyboard

The Electron featured a 56-key keyboard:

• Layout: QWERTY with BREAK, COPY, CAPS LOCK/FUNC
• Matrix: 14 columns × 4 rows scanned by ULA
• Debouncing: Software-based in OS ROM
• Key switches: Membrane or mechanical (varied by production run)
• Auto-repeat: OS-controlled with adjustable rate

Sound System

Limited compared to the BBC Micro:

• Channels: 1 tone generator + white noise
• Frequency range: 122 Hz to 3.9 kHz
• Volume: 16 levels
• Output: Internal speaker only (no line output)
• SOUND command: BBC BASIC compatible subset

ROM Software

Operating System ROM (16 KB)

Located at $C000-$FFFF:

• MOS 1.0: Acorn Machine Operating System
• VDU drivers: Text and graphics routines
• File system: Cassette filing system
• I/O routines: Keyboard, sound, printer support
• Interrupt handlers: IRQ and NMI processing

BASIC ROM (16 KB)

Paged in at $8000-$BFFF:

• BBC BASIC 2: Full implementation
• Assembler: Built-in 6502 assembler
• Graphics commands: PLOT, DRAW, MOVE, etc.
• Sound commands: SOUND, ENVELOPE (limited)
• Error handling: ON ERROR support

Technical Limitations

The ULA design imposed several constraints:

Limitation	Cause	Impact
1 MHz RAM access	ULA video priority	50% speed reduction vs BBC Micro
Single sound channel	Simplified ULA design	No music capabilities
No hardware multiply	Cost reduction	Slower arithmetic
No Teletext mode	Missing SAA5050 chip	No Mode 7 support
Limited I/O	No 6522 VIA chips	Requires expansion units
No RS423 serial	Cost reduction	No built-in comms

Manufacturing Variants

Issue	ULA Version	Changes	Production Period
1	12C021	Initial release	Aug 1983 - Dec 1983
2	12C021A	Improved yield	Jan 1984 - Jun 1984
3	12C021B	Video timing fix	Jul 1984 - Dec 1984
4	12C021C	Final revision	Jan 1985 - Aug 1985

Memory Access Timing

The ULA controlled all RAM access with strict timing:

• Video fetch: Every 2 MHz cycle during active display
• CPU access: Alternate cycles in screen area
• Refresh: During horizontal blanking
• Contention: CPU stalled when accessing video RAM

Timing diagram (per 1 µs):

1. 0-250ns: ULA video fetch
2. 250-500ns: CPU access window
3. 500-750ns: ULA video fetch
4. 750-1000ns: CPU access window

Known Hardware Issues

Power Supply

• 7805 regulator overheating: Inadequate heatsinking causes thermal shutdown
• Transformer hum: Loose laminations in external PSU
• DC offset: Electrolytic capacitor degradation

ULA Problems

• Heat sensitivity: ULA failures increase above 40°C
• Video corruption: Internal gate degradation causes display artifacts
• Total failure: No modern replacement available

Keyboard Issues

• Membrane fatigue: Early units suffer key bounce
• Matrix diodes: 1N4148 diodes fail causing phantom keys
• Connector oxidation: Poor contact at ribbon cable

General Maintenance

Main article: Acorn Electron General Maintenance

Regular maintenance includes cleaning the case with mild detergent, ensuring adequate ventilation around the ULA heatsink, and checking the keyboard membrane for wear. The external power supply transformer should be inspected for overheating. Internal dust accumulation should be removed with compressed air, particularly around the ULA and voltage regulator heatsinks.

Troubleshooting

Main article: Acorn Electron Troubleshooting

Common faults include no display output (check ULA and crystal oscillator), corrupted graphics (failing ULA or RAM), and keyboard non-response (check ribbon cable and matrix diodes). The external PSU frequently fails with dried capacitors causing ripple. Memory faults typically manifest as system crashes or corrupted BASIC programs.

Capacitor Replacement Guide

Main article: Acorn Electron Capacitor Replacement Guide

Critical capacitors requiring replacement after 40 years include:

• C1, C2: 2200µF 25V (main PSU filtering)
• C3: 100µF 16V (ULA supply decoupling)
• C4-C7: 47µF 16V (RAM refresh timing)
• C8: 10µF 25V (reset circuit)
• C9-C12: 0.1µF ceramic (IC decoupling)

Easter Eggs

The Electron ROM contained hidden features:

• Development team credits: Located at ROM address $FFB7
• Test routines: Accessible via *FX calls
• Hidden error messages: "Roger" appears for certain crashes

See Also

• BBC Micro Model A/B
• Acorn Atom
• Acorn Communicator