Acorn Electron
 | |
| Acorn Electron | |
| Specifications | |
|---|---|
| Manufacturer | Acorn Computers Ltd |
| Type | Home Computer |
| Released | 25 August 1983 |
| Discontinued | 1985 |
| Intro price | £199 (1983) |
| CPU | 6502A (Synertek SY6502A / Rockwell R6502AP), 2 MHz ROM / 1 MHz RAM |
| Memory | 32 KB RAM, 32 KB ROM |
| Storage | Cassette tape interface (1200 baud), optional floppy disk via Plus 3 |
| Display | 640×256 (2 col), 320×256 (4 col), 160×256 (16 col), 80×25 text |
| Sound | 1 channel + software noise (internal speaker) |
| Dimensions | 340 mm × 160 mm × 65 mm |
| OS / Firmware | Acorn MOS 1.0 with BBC BASIC II |
| Predecessor | BBC Micro Model A/B |
| Successor | BBC Master Compact |
| Codename | Elk |
| Model no. | PCB 205,000 |
The Acorn Electron (nicknamed the "Elk") was a budget home computer released by Acorn Computers Ltd on 25 August 1983 at £199. It was designed as a lower-cost alternative to the BBC Micro and a competitor to the ZX Spectrum, reducing the BBC Micro's roughly 100 chips to around a dozen by moving most of the logic into a single custom Ferranti ULA. It ran BBC BASIC II and Acorn MOS 1.0, giving a degree of software compatibility with the BBC Micro.[1] Around 200,000–250,000 were sold before production wound down in 1985. The case was designed by Allen Boothroyd of Cambridge Product Design.
Architecture
The Electron centred on a custom Ferranti ULA (Uncommitted Logic Array) of about 2,400 gates, handling video generation, RAM control, sound, cassette I/O, keyboard scanning, clock division and reset.[1] Integration came at a performance cost: the memory is 4 bits wide (four 64K×1 DRAMs), so each byte needs two accesses, and the CPU can access RAM only at 1 MHz — and is halted entirely while the ULA reads RAM during the active display.[1][2]
Memory map
| Address Range | Size | Function |
|---|---|---|
| $0000 – $00FF | 256 bytes | Zero page |
| $0100 – $01FF | 256 bytes | 6502 stack |
| $0200 – $03FF | 512 bytes | OS and VDU workspace |
| $0400 – $07FF | 1 KB | Current-language (BASIC) workspace |
| $0800 – $0DFF | 1.5 KB | Buffers and NMI workspace |
| $0E00 – $7FFF | up to ~29 KB | User program and screen memory (mode-dependent) |
| $8000 – $BFFF | 16 KB | Paged (sideways) ROM/RAM — BBC BASIC II |
| $C000 – $FFFF | 16 KB | Operating system ROM (MOS 1.0) |
Main board components
IC designators are from the Service Manual parts list.[2]
| Component | Device | Designator | Notes |
|---|---|---|---|
| CPU | 6502A (Synertek SY6502A / Rockwell R6502AP) | IC3 | Clock stoppable by the ULA |
| ULA | Ferranti 12C021 | IC1 | 68-pin, in a holder (Issue 1–4) |
| RAM | 4× 4164 (64K×1 DRAM) | IC4–IC7 | 32 KB; 4-bit-wide |
| ROM | Single 32 KB mask ROM | IC2 | MOS 1.0 + BBC BASIC II |
| Clock | 16 MHz crystal oscillator | IC8 (74S04) | Divided by the ULA |
| Cassette/audio amp | LM324 | IC13 | Needs +5 V and −5 V |
| Sync logic | 74LS86 | IC14, IC15 | RGB / CSYNC generation |
The ULA
The ULA (IC1) coordinates the whole machine in a 68-pin package. Its pins include the data lines to/from RAM (controlled by RAS/CAS/WE), the multiplexed RAM address lines, the keyboard inputs (KBD0–3), the 16 MHz clock input, the cassette CAS IN/CAS OUT (1.8 V p-p), the sound output, the processor address/data lines, IRQ, NMI (used by the ULA to arbitrate RAM access), R/W, the ROM-select output and RST (asserted at power-up and when BREAK is pressed).[2]
Video system
The Electron produces a 625-line, 50 Hz, fully-encoded PAL signal on UHF channel 36, plus composite and RGB outputs. The composite output is monochrome unless the chrominance link LK4 is made. It supports the BBC Micro's screen modes 0–6 (there is no Teletext Mode 7, which would have needed an SAA5050), with 8 colours plus 8 flashing versions.[1][2]
| Mode | Resolution | Colours | Text |
|---|---|---|---|
| 0 | 640×256 | 2 | 80×32 |
| 1 | 320×256 | 4 | 40×32 |
| 2 | 160×256 | 16 | 20×32 |
| 3 | (text) | 2 | 80×25 |
| 4 | 320×256 | 2 | 40×32 |
| 5 | 160×256 | 4 | 20×32 |
| 6 | (text) | 2 | 40×25 |
Power supply
The Electron uses an external mains adaptor that delivers 19 V AC, 50 Hz, 14 W (some sources quote 18 V AC). The AC is rectified, smoothed and regulated on an internal PSU PCB (part 332,002), which produces +5 V via a 7805 and also generates −5 V; the +5 V rail should sit between 4.75 V and 5.25 V with no more than 50 mV of noise. There is no mains voltage or CRT inside the system unit.[2] Because the internal bridge rectifier accepts either polarity, the machine can also be run from a DC supply (about 18 V DC ideal, 9 V DC for a bare machine).[3]
Keyboard, sound and cassette
The keyboard is a 56-key full-travel QWERTY assembly with two-key rollover, auto-repeat, and ten function keys; it is scanned directly by the ULA, and the machine will not run with the keyboard disconnected.[2] Sound is a single channel with envelope control over seven octaves through the internal speaker, with software emulation of a noise channel. The cassette interface runs at 1200 baud (CUTS) through a 7-pin DIN with automatic recorder motor control.[1][2]
Expansion
Acorn's Plus 1 added a Centronics printer port, an analogue-joystick port and two ROM-cartridge slots. The Plus 3 added a WD1770 floppy-disc controller, ADFS, and a 3.5-inch drive.[1] The rear edge connector exposes the full address and data bus plus control signals and the AC feed for expansions.[2]
Board and ULA variants
The main PCB is part 205,000 with an issue number. On Issue 1–4 boards the Ferranti ULA sits in a 68-pin holder under a metal cover; thermal cycling can lift it out of the holder, which is the Electron's best-known fault (hang, freeze, or a continuous power-on tone, cured by pressing the cover to reseat the chip). Issue 5 and 6 boards (and the German Electron) use a later "Aberdeen" ULA soldered to the board and covered in epoxy, dispensing with the socket and far less prone to the fault.[1]
Known hardware issues
- ULA reseating (Issue 1–4): hang/freeze/continuous tone from the socketed ULA working loose; press the metal cover to reseat.[1]
- Cracked DC-jack joints: a very common cause of dead or intermittent power; reflow or replace the jack.[3]
- 74LS86 sync failure (IC14/IC15): RGB output blank while composite still works.[3]
- 4164 RAM faults: corrupted display and crashes.
- PSU electrolytics: ripple on the +5 V rail and random resets as they age.
For diagnosis and repair, see the maintenance, troubleshooting and capacitor pages below.
Capacitors
The main board carries only a few electrolytics and tantalums — e.g. C1 47 µF/16 V, C2 10 µF/16 V, C7 and C11 10 µF/35 V, C25 33 µF/16 V, plus tantalums C8/C9 1 µF/35 V and C23 47 µF/16 V — while the larger smoothing electrolytics are on the internal PSU PCB. Full list and procedure: Acorn Electron Capacitor Replacement Guide.[2]
ROM credits
The operating-system ROM contains the names of the Electron's design team in the locations $FC00–$FFFF, and the last bytes of both the BASIC ROM and the Plus 3 ADFS ROM contain the word "Roger", a reference to Roger (later Sophie) Wilson.[1]
See also
- Acorn Electron Maintenance Guide
- Acorn Electron Troubleshooting Guide
- Acorn Electron Capacitor Replacement Guide
- Acorn Atom
- BBC Micro Model A/B
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 "Acorn Electron", Wikipedia. Source for the 25 August 1983 launch and £199 price, the ~2,400-gate Ferranti ULA, the chip-count reduction, the Synertek SY6502A clock-stopping requirement and 2 MHz/1 MHz/halted clocking, the four 4164 DRAMs and 4-bit-wide memory, the 32 KB ROM, the screen modes, the 19 V AC PSU, the socketed-ULA fault and the design-team ROM credits.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Acorn Electron Service Manual (Part no. 0405001, Issue 2, January 1987), Acorn Computers — hosted on this wiki. Source for the IC complement (IC1 ULA 12C021 in a 68-pin holder, IC2 single 32 KB OS/BASIC ROM, IC3 6502A, IC4–7 4164 DRAM, IC8 74S04 16 MHz oscillator, IC13 LM324), the 16 MHz clocking, the 19 V AC 14 W supply and +5 V/−5 V rails, the capacitor list, and that the machine will not run without the keyboard connected.
- ↑ 3.0 3.1 3.2 Wilson, Adam. "Acorn Electron Repair & Restoration", Adam's Vintage Computer Restorations (2021). Source for the internal bridge/7805 PSU, the DC-power substitution, the cracked DC-jack fault, and a 74LS86 RGB-sync failure.