Acorn Archimedes A305

Acorn Archimedes A305
	Acorn Archimedes A305
Specifications
Manufacturer	Acorn Computers Ltd
Type	Personal Computer
Released	June 1987
Discontinued	1989
Intro price	£799 (1987)
CPU	ARM2 @ 8 MHz
Memory	512 KB RAM (expandable to 1 MB)
Storage	3.5" floppy drive (800 KB), optional 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	8 kg
OS / Firmware	Arthur 0.30 (later RISC OS 2.00)
Predecessor	BBC Master
Successor	Acorn Archimedes A310, Acorn Archimedes A3000
Codename	Fairway
Model no.	A305, A310, A440

The Acorn Archimedes A305 was the entry-level model in Acorn's revolutionary Archimedes range, released in June 1987. Distinguished from its sibling A310 primarily by shipping with 512 KB of RAM instead of 1 MB, the A305 offered the same groundbreaking ARM RISC processor technology at a lower price point, making it accessible to educational institutions and home users transitioning from 8-bit systems.

Overview

The A305 represented Acorn's strategic decision to offer multiple price points for their new 32-bit architecture. Physically identical to the A310 and sharing the same motherboard design, the A305 simply populated fewer RAM sockets, allowing users to upgrade to full A310 specifications by adding additional memory modules. This approach minimized manufacturing costs while providing a clear upgrade path.

Despite the reduced memory, the A305 delivered exceptional performance compared to contemporary machines. The ARM2 processor's efficiency meant that even with 512 KB, the system outperformed 68000-based computers with twice the memory. Educational buyers particularly appreciated this value proposition, as many BBC Micro applications had been designed to work within similar memory constraints.

Architecture

The A305 utilized the same four-chip VLSI architecture as the entire Archimedes range. The ARM2 processor provided the computational core, while MEMC (Memory Controller) handled memory management and DMA operations. VIDC (Video and Sound Controller) generated display output and processed audio, and IOC (Input/Output Controller) managed peripherals and system timing.

Memory Configuration

Configuration	RAM Chips	Total Memory	Upgrade Path
A305 Standard	2× 256K×9 SIMMs	512 KB	Add 2 more SIMMs
A305 Upgraded	4× 256K×9 SIMMs	1 MB	Equivalent to A310

The 512 KB configuration occupied only two of the four available SIMM slots, using 256K×9-bit modules with parity checking. Users could add two additional SIMMs to reach 1 MB, effectively converting the machine to A310 specifications. The MEMC chip required no reconfiguration, automatically detecting the installed memory capacity at power-on.

Hardware Specifications

The A305 motherboard was identical to the A310, with the same component layout and expansion capabilities. The four-layer PCB design ensured signal integrity for the high-speed digital circuits, while the modular SIMM approach allowed flexible memory configurations.

System Timing

All system timing derived from a single 24 MHz crystal oscillator, divided down to provide:

8 MHz - ARM2 processor clock
8 MHz - MEMC memory controller
24 MHz - VIDC pixel clock (maximum)
2 MHz - IOC timers and slow peripherals
1 MHz - Econet clock (when fitted)

The synchronous design eliminated clock domain crossing issues, improving reliability and simplifying the design compared to asynchronous multi-chip systems.

Power Requirements

Component	Current Draw	Notes
ARM2 CPU	120mA @ 5V	Including cache
MEMC	250mA @ 5V	With 512 KB RAM
VIDC	400mA @ 5V	Active display
IOC	100mA @ 5V	All ports active
System total	3.5A @ 5V	Typical operation

The reduced memory of the A305 resulted in slightly lower power consumption than the A310, improving thermal characteristics and power supply margin. This advantage disappeared when users upgraded to 1 MB, as the additional SIMMs drew approximately 400mA.

Display Capabilities

The A305 supported all standard Archimedes display modes through the VIDC chip. However, the limited memory affected usability of higher resolution modes, as screen memory reduced available application space.

Memory Impact by Display Mode

Mode	Resolution	Memory Used	Remaining (512 KB)	Remaining (1 MB)
0	640×256×2	20 KB	492 KB	1004 KB
12	640×512×16	160 KB	352 KB	864 KB
13	640×512×256	320 KB	192 KB	704 KB
15	640×512×16	160 KB	352 KB	864 KB
21	640×512×256	320 KB	192 KB	704 KB

Mode 13 and 21 proved particularly challenging on the 512 KB A305, leaving insufficient memory for serious applications. Most A305 users operated in Mode 12 or 15, providing good color depth while preserving adequate application memory. Educational software typically defaulted to these modes, ensuring compatibility across the range.

Operating System Considerations

Arthur, the initial operating system, loaded entirely into RAM on the A305, consuming approximately 160 KB for the core system and desktop. This left roughly 330 KB for applications in Mode 12, comparable to enhanced BBC Master configurations but with vastly superior processing speed.

System Memory Usage

The operating system memory footprint varied significantly between Arthur and RISC OS:

Component	Arthur 0.30	Arthur 1.20	RISC OS 2.00
Kernel	48 KB	52 KB	64 KB
Desktop	64 KB	72 KB	96 KB
Font cache	N/A	N/A	32 KB minimum
System heap	32 KB	40 KB	48 KB
Module area	16 KB	24 KB	32-64 KB
Total	~160 KB	~188 KB	~272-304 KB

RISC OS 2.00's increased memory requirements made the 512 KB configuration marginal for desktop use. The font manager alone could consume 100 KB when using outline fonts, leaving minimal space for applications. This limitation drove many A305 owners to upgrade to 1 MB, particularly after RISC OS became available.

Software Compatibility

The A305's limited memory affected software compatibility, particularly with newer applications designed for 1 MB systems. Early Archimedes software typically supported 512 KB configurations, but this changed as developers targeted the larger installed base of A310 and later machines.

Educational software publishers maintained 512 KB compatibility longer than commercial developers, recognizing the substantial number of A305 machines in schools. Popular educational titles like Podd, Arcventure, and Geordie Racer included special 512 KB modes with reduced features or simplified graphics.

Expansion Options

The A305 provided identical expansion capabilities to other Archimedes models, though memory limitations affected the practical use of some peripherals.

Podule Slots

Four 96-pin expansion slots accepted the same range of podule cards as the A310:

SCSI interfaces - Adding hard drives often required memory upgrade
Network cards - Econet and Ethernet adapters
Video digitizers - Required 1 MB for practical use
MIDI interfaces - Worked acceptably with 512 KB
ROM podules - Ideal for A305 due to no RAM requirement

The backplane operated at full 8 MHz speed with automatic speed detection for slower peripherals. Each slot provided standard power rails, though the linear PSU limited total expansion capacity.

Internal Upgrades

Beyond memory expansion, A305 owners could install:

ARM3 processor upgrade - 25 MHz ARM3 with 4 KB cache
MEMC1a upgrade - Required for 4 MB RAM support
RISC OS carrier board - Upgraded ROM capacity to 2 MB
Econet module - Internal networking interface
Battery backup - For CMOS settings retention

The ARM3 upgrade proved particularly popular with A305 owners, as the 4 KB cache partially compensated for limited main memory by reducing memory bandwidth requirements.

Manufacturing History

Acorn produced the A305 from June 1987 to early 1989, with several PCB revisions addressing various issues:

Revision	Production Period	Changes	Notes
Issue 1	Jun-Oct 1987	Original design	Arthur 0.30 ROM
Issue 2	Nov 1987-Apr 1988	Improved memory timing	Arthur 1.20 ROM
Issue 3	May 1988-Dec 1988	Podule timing fixes	Some with RISC OS upgrade
Issue 4	Jan-Mar 1989	Cost reduction	Final production

Production volumes remained lower than the A310, with educational purchases accounting for approximately 60% of sales. The price differential of £76 between A305 and A310 models narrowed over time as memory costs declined, reducing the A305's market appeal.

Common Modifications

A305 owners frequently performed various modifications to enhance their systems:

Memory Upgrade Procedure

Converting an A305 to 1 MB required: 1. Two additional 256K×9 SIMMs (approximately £50 in 1988) 2. Opening the case and locating empty SIMM slots 3. Installing SIMMs in correct orientation 4. No configuration changes required - MEMC auto-detects

The upgrade took minutes and required no technical expertise, making it popular with schools and home users. Many dealers offered installation services for customers uncomfortable opening their computers.

Video Output Modifications

The standard analog RGB output could be modified for different monitor types:

VGA conversion - Sync combiner circuit for PC monitors
Composite video - Added for TV connection
Genlock input - For video production (required 1 MB)

These modifications typically involved adding small daughterboards or replacing the video output socket with a multi-standard connector.

Reliability Issues

The A305 exhibited the same reliability patterns as other early Archimedes models. The linear power supply operated within specifications with 512 KB but approached limits when fully expanded. Thermal management became critical in expanded systems, particularly those with multiple podules installed.

Battery leakage remained the primary long-term reliability concern. The Varta 3.6V NiCd battery typically leaked after 8-10 years, damaging nearby PCB traces. The battery's location near the keyboard connector made damage particularly problematic, as keyboard interface traces were difficult to repair

Performance Analysis

Despite memory limitations, the A305 delivered impressive performance for its era. Integer arithmetic operations completed at speeds comparable to 80386 systems, while the custom chips provided graphics acceleration unavailable on PC platforms.

Benchmark results (512 KB configuration):

Dhrystone 1.1 - 3900 iterations/second
Savage benchmark - 1.2 seconds
BBC BASIC loop test - 0.8 seconds
Screen clear (Mode 12) - 15 milliseconds
Circle drawing - 120 circles/second

These figures remained competitive even against machines costing twice as much, validating Acorn's RISC architecture approach.

Educational Impact

The A305 found particular success in UK schools, where budget constraints made the price difference significant. Many Local Education Authorities negotiated bulk purchases, obtaining additional discounts that made A305 systems attractive alternatives to BBC Master installations.

Teachers appreciated the BBC BASIC compatibility, allowing existing educational software to run with minimal modification. The improved graphics and sound capabilities enabled new categories of educational software, particularly in music and art education.

General Maintenance

Maintenance requirements mirror those of other Archimedes models. Priority tasks include CMOS battery replacement before leakage occurs, cleaning ventilation paths to prevent thermal buildup, and inspection of power supply capacitors for signs of aging. The reduced component count of 512 KB configurations slightly simplifies maintenance, with fewer RAM chips to clean or reseat.

Troubleshooting

Common A305-specific issues include insufficient memory errors when running newer software, system crashes when switching to high-resolution modes, and performance degradation in memory-intensive applications. The limited memory also exacerbates problems with fragmentation, requiring periodic resets to reclaim memory. Standard Archimedes troubleshooting procedures apply to hardware faults.

Capacitor Replacement Guide

Capacitor replacement requirements are identical to the A310, as both models share the same motherboard and power supply designs. Critical capacitors include the main PSU filter (4700µF 25V), auxiliary smoothing capacitors (1000µF 16V ×3), and numerous smaller electrolytics throughout the motherboard. The power supply's linear design makes capacitor replacement straightforward but requires careful attention to polarity and voltage ratings.

Market Position

The A305 occupied an unusual market position, offering revolutionary technology at a price point between 8-bit systems and full 32-bit workstations. While successful in education, the model struggled in the home market where the memory limitations became apparent quickly. Most retail customers opted for the A310, viewing the additional memory as essential for future software compatibility.

By 1989, declining memory prices eliminated the A305's price advantage, leading Acorn to discontinue the model in favor of the unified A3000 design. The A305's brief production run makes it relatively rare today, particularly units still in original 512 KB configuration.