Acorn Archimedes A420
The Acorn Archimedes A420 was introduced in September 1989 as an updated version of the A410, featuring increased standard memory and the new RISC OS 2.00 operating system. Part of the refreshed Archimedes range alongside the A410/1 and A440/1, the A420 targeted professional users and educational institutions requiring a balance of processing power and storage capacity.
 | |
| Acorn Archimedes A420 | |
| Specifications | |
|---|---|
| Manufacturer | Acorn Computers Ltd |
| Type | Personal Computer |
| Released | September 1989 |
| Discontinued | 1991 |
| Intro price | £1,699 (1989) |
| CPU | ARM2 @ 8 MHz |
| Memory | 2 MB RAM (expandable to 4 MB) |
| Storage | 3.5" floppy drive (800 KB), 20 MB or 40 MB ST506 hard drive |
| Display | 640×512 (16 colours), 640×256 (256 colours), 1152×896 (monochrome) |
| Sound | 8-channel stereo, 8-bit logarithmic DAC |
| Dimensions | 415 mm × 355 mm × 104 mm |
| Weight | 9 kg |
| OS / Firmware | RISC OS 2.00 |
| Predecessor | Acorn Archimedes A410 |
| Successor | Acorn Archimedes A420/1, Acorn Archimedes A540 |
Overview
The A420 addressed criticisms of the original A410's limited memory by including 2 MB RAM as standard, doubling the base specification. This increase reflected both declining memory prices and the greater demands of RISC OS 2.00 compared to the earlier Arthur operating system. The machine shipped in two variants: the A420/20 with a 20 MB hard drive and the A420/40 with a 40 MB drive, though contemporary documentation often referred to both simply as "A420".
Acorn positioned the A420 between the entry-level A410/1 and the high-end A440/1, creating a three-tier product range. The 2 MB standard memory eliminated the immediate need for upgrades that plagued earlier 1 MB systems, while the retained 8 MHz ARM2 processor kept costs below the A440/1's premium pricing.
Architecture
The A420 utilized the mature four-chip Archimedes architecture, benefiting from two years of refinement since the original 1987 launch. The system retained the ARM2 processor, MEMC1a memory controller, VIDC1a video controller, and IOC input/output controller, all operating at 8 MHz synchronized from a single 24 MHz crystal.
Memory Subsystem
The standard 2 MB configuration used four 512K×9-bit SIMMs, filling all available slots on the standard motherboard:
| Configuration | SIMM Type | Slots Used | Total RAM | Expansion Method |
|---|---|---|---|---|
| Standard | 4× 512K×9 | 4 of 4 | 2 MB | Replace all SIMMs |
| Maximum | 4× 1M×9 | 4 of 4 | 4 MB | Direct replacement |
| Alternative | MEMC podule | N/A | 8 MB | Third-party solution |
The MEMC1a controller supported the larger memory capacity without modification, implementing page-mode access for improved performance. Memory timing remained at 125ns access time, though some systems successfully operated with 100ns SIMMs for marginal performance gains.
Hardware Specifications
The A420 motherboard represented the Issue 3 or Issue 4 PCB revision, incorporating lessons learned from earlier production. These boards featured improved power distribution, better ground plane design, and enhanced signal integrity for the high-speed digital circuits.
Component Versions
| Component | Early A420 (1989) | Late A420 (1990-91) | Notes |
|---|---|---|---|
| ARM2 | VL86C010-08 | VL86C010-10QC | Die shrink version |
| MEMC1a | VL86C110 | VL86C110-IC | Improved yield |
| VIDC1a | VL86C310 | VL86C310-IC | Bug fixes |
| IOC | VL86C410 | VL86C410-IC | Timing improvements |
The later component revisions offered improved reliability and slightly reduced power consumption, though performance remained identical. The die-shrink ARM2 ran cooler and proved more resistant to thermal stress.
Storage Controller
The A420 retained the ST506 controller podule design but used an improved revision:
- Controller chip: WD42C22 or HDC9224B
- Data separator: DP8465V-2
- Buffer memory: 16 KB (doubled from A410)
- Maximum drives: 2 (though only one fitted)
- Transfer rate: 625 KB/s raw, 400 KB/s sustained
The doubled buffer memory improved performance for sequential reads and writes, particularly beneficial for database and compiler operations. The controller firmware included enhanced error correction, improving reliability with marginal drives.
Storage Options
The A420 shipped with either 20 MB or 40 MB drives, depending on configuration:
Drive Specifications
| Model | Capacity | Drive Type | Cylinders | Heads | Sectors | Access Time |
|---|---|---|---|---|---|---|
| A420/20 | 20 MB | Conner CP3022 | 636 | 2 | 33 | 29ms |
| A420/40 | 40 MB | Conner CP3044 | 980 | 5 | 17 | 28ms |
| Alternative | 40 MB | Quantum Q540 | 965 | 5 | 17 | 19ms |
The Conner drives represented a significant improvement over the MiniScribe units used in earlier models, offering faster access times and improved reliability. The 3.5" form factor also ran cooler and quieter than the older 5.25" drives.
RISC OS 2.00
The A420 shipped with RISC OS 2.00 in four 512 KB ROM chips, providing the complete desktop environment without disk loading:
Operating System Features
RISC OS 2.00 introduced substantial improvements over Arthur:
- Desktop boot: Direct boot to GUI without command line
- Improved filer: Drag-and-drop file management
- Font manager: Anti-aliased outline fonts
- Printer drivers: PostScript and dot-matrix support
- Task manager: Cooperative multitasking
- Configure utility: GUI-based system configuration
The 2 MB ROM capacity allowed inclusion of several applications:
- Edit: Text editor with BASIC syntax highlighting
- Paint: Bitmap graphics editor
- Draw: Vector graphics application
- PrinterDM: Dot-matrix printer driver
- Modules: Network, serial, and sound support
Memory Management
With 2 MB RAM standard, RISC OS 2.00 memory allocation provided more working space:
| Memory Area | Size | Purpose | User Configurable |
|---|---|---|---|
| Screen memory | 20-320 KB | Display buffer | Via mode selection |
| System heap | 128 KB | OS workspace | No |
| RMA (Module Area) | 256-512 KB | Relocatable modules | Yes |
| Font cache | 64-256 KB | Outline font rendering | Yes |
| System sprites | 64 KB | Desktop icons | Yes |
| RAM disc | 128 KB default | Temporary storage | Yes |
| Application space | 900-1400 KB | User programs | Automatic |
The increased memory allowed comfortable multitasking of two or three applications, compared to the single-application limitation of 1 MB systems.
Performance Characteristics
The A420's performance matched other 8 MHz ARM2 systems, with storage and memory improvements providing practical benefits:
Benchmark Results
| Test | A420 Result | A410 (1 MB) | A440/1 (4 MB) | Notes |
|---|---|---|---|---|
| Dhrystone 2.1 | 4100/sec | 4100/sec | 4100/sec | CPU-bound |
| File copy (HD) | 380 KB/s | 285 KB/s | 385 KB/s | Improved controller |
| Application load | 1.8 sec | 2.4 sec | 1.7 sec | Memory advantage |
| Desktop redraw | 450 ms | 580 ms | 440 ms | Font cache benefit |
The doubled standard memory eliminated paging to disk for most operations, providing perceived performance improvements despite identical processor speeds.
Expansion Capabilities
The A420 provided three free podule slots after hard drive controller installation:
Common Expansion Cards
Popular podule cards for the A420 included:
- SCSI interfaces: Oak, Lingenuity, Morley designs
- Ethernet cards: Acorn AEH54, i-cubed EtherLan 100
- Serial/parallel: Intelligent Interfaces dual serial
- Scanners: Watford Electronics or Pineapple digitizers
- MIDI interfaces: Acorn MIDI, EMR MIDI 4
- PC emulators: Aleph One PC card (80186 processor)
The three available slots permitted reasonable expansion without the limitations of single-slot machines like the A3000. Professional users typically installed SCSI and Ethernet cards, while educational users favored MIDI and digitizer options.
Typical Deployments
The A420 found use in several key markets:
Desktop Publishing
Publishing systems typically configured with:
- A420/40 for document storage
- 4 MB RAM upgrade for large documents
- PostScript printer via parallel port
- Scanner podule for image acquisition
- Impression Publisher or Ovation software
Software Development
Development configurations included:
- A420/20 base system
- Serial podule for debugging
- Acorn Desktop C or GCC compiler
- Source control via Econet
- ARM assembler in ROM
Education Networks
Classroom installations featured:
- A420/40 as file server
- Econet interface for networking
- Level 4 FileStore software
- Shared printers via parallel port
- 10-20 station capacity
Production Variants
Manufacturing changes occurred throughout the A420's production run:
| Period | PCB Issue | ROM Version | Drive Options | Changes |
|---|---|---|---|---|
| Sep-Dec 1989 | Issue 3 | 2.00 | 20/40 MB | Initial release |
| Jan-Jun 1990 | Issue 4 | 2.00 | 20/40 MB | Improved layout |
| Jul-Dec 1990 | Issue 4 | 2.01 | 40 MB only | Bug fixes |
| Jan-Jul 1991 | Issue 4a | 2.01 | 40/60 MB | Cost reduction |
Later units occasionally shipped with larger drives when specified capacities became unavailable, though ADFS limitations restricted usable space to 512 MB per partition.
Known Issues
The A420 exhibited several characteristic problems:
Hard Drive Reliability
While Conner drives proved more reliable than earlier MiniScribe units, age-related failures became common:
- Spindle motor failure: Increasing startup time before complete failure
- Head actuator problems: Seeking errors and data corruption
- Control board failures: Drive not recognized by controller
Modern replacements require SCSI or IDE podule upgrades, as ST506 drives are essentially unobtainable.
ROM Bugs
RISC OS 2.00 contained several significant bugs:
- Printer driver crashes: Certain PostScript operations caused system hangs
- Font manager leaks: Memory not released after font operations
- FileCore errors: Occasional directory corruption on power loss
- Network timeouts: Econet operations could freeze system
The 2.01 ROM update addressed most issues, though some users preferred third-party patches.
Power Supply Limitations
The linear power supply design struggled with fully-loaded systems:
- Voltage sag: Under heavy load, 5V rail dropped below specification
- Thermal stress: Continuous operation at maximum load caused component degradation
- Capacitor aging: Filter capacitors lost capacity over time
- Regulation drift: Output voltages wandered with temperature
Regular maintenance and capacitor replacement became essential for reliability.
Upgrade Paths
Common upgrades extended the A420's useful life:
Processor Upgrades
- ARM3 upgrade: 25-33 MHz processor with 4 KB cache
- FPA coprocessor: Hardware floating-point acceleration
- Clocking modifications: 10-12 MHz operation (reduced reliability)
The ARM3 upgrade provided the most significant improvement, offering 4-5× performance increase for compute-intensive tasks.
Storage Upgrades
- SCSI conversion: Replace ST506 with SCSI podule and drive
- IDE adaptation: Third-party IDE podules became available
- Network storage: NFS mounting via Ethernet
- Removable media: SCSI ZIP or SyQuest drives
SCSI conversion became almost mandatory as ST506 drives failed, with the Lingenuity SCSI podule becoming the de facto standard.
Memory Expansion
Upgrading from 2 MB to 4 MB required: 1. Purchase of four 1M×9 SIMMs (approximately £120 in 1990) 2. Removal of existing 512K×9 SIMMs 3. Installation of new SIMMs 4. No configuration required (automatic detection)
The upgrade improved multitasking capability and allowed use of memory-intensive applications like Computer Concepts' Impression Publisher.
Maintenance Considerations
Long-term A420 maintenance focuses on mechanical and analog components:
Critical Maintenance Points
- Hard drive backup: Essential given drive age and irreplaceable nature
- Battery replacement: 3.6V lithium cell requires replacement every 5-7 years
- Thermal management: Clean ventilation paths and check fan operation
- Contact cleaning: Podule slots and connectors benefit from periodic cleaning
- Capacitor inspection: Visual check for bulging or leakage
The ST506 controller podule requires particular attention, as component aging affects data recovery timing. Regular cleaning of edge connectors maintains reliable operation.
General Maintenance
Regular maintenance includes monitoring hard drive health through sound changes and performance degradation. The ST506 controller podule requires periodic edge connector cleaning to maintain reliable communication. Power supply capacitors need inspection for bulging or leakage, with replacement recommended after 30 years. The system benefits from annual internal cleaning to remove dust accumulation, particularly around the power supply and drive bay. CMOS battery replacement every 5-7 years prevents configuration loss.
Troubleshooting
Common fault patterns include hard drive clicking or grinding (mechanical failure imminent), "Dir full" errors with space available (directory corruption), and system freezes during disk access (controller timing drift). Power-on failures often trace to power supply capacitor degradation. Random crashes under load indicate thermal issues or memory failures. The diagnostic process should begin with power supply verification, proceed to cable and connection inspection, then component-level testing.
Capacitor Replacement Guide
Critical capacitors include power supply main filter (6800µF 25V), secondary filters (2200µF 16V ×3, 1000µF 16V ×2), and motherboard electrolytics (100µF 16V ×6, 47µF 16V ×8). The ST506 controller podule contains several 22µF and 47µF capacitors affecting timing accuracy. Replacement requires standard desoldering techniques with attention to polarity and voltage ratings.
Market Position
The A420 competed in a challenging market dominated by PC compatibles and Apple Macintosh systems. While technically superior in many respects, limited software availability restricted market penetration. Educational markets remained receptive due to existing BBC Micro investments and quality educational software. Professional markets proved difficult to penetrate despite competitive pricing and superior performance metrics.
The 1991 discontinuation coincided with Acorn's strategic refocus on the education market and development of the next-generation A5000 series. Total production volumes remained modest, with estimates suggesting 8,000-12,000 units manufactured.
Legacy
The A420 represented the final evolution of the original Archimedes architecture before the transition to ARM3-based systems. Its 2 MB standard memory configuration influenced all subsequent models, establishing a new baseline for RISC OS systems. The machine demonstrated the viability of ARM processors for general-purpose computing, though commercial success remained elusive outside education markets.
Surviving A420 systems face challenges from failing hard drives and aging power supplies. The ST506 interface's obsolescence makes drive replacement difficult, driving many owners to SCSI or IDE conversions. Despite these challenges, the A420's solid construction and modular design enable continued operation with appropriate maintenance and upgrades.