Josh: New page, cited to Acorn's hosted documents and Chris's Acorns

2026-10-02T01:39:18Z

New page, cited to Acorn's hosted documents and Chris's Acorns

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{{Infobox computer
| name = Acorn Archimedes A500
| image = [[File:Acorn Archimedes A500.jpg|260px|frameless]]
| caption = Chris Whytehead's A500 running RISC OS 2 (photo: Chris Whytehead, CC BY-SA 3.0)
| manufacturer = Acorn Computers
| type = Development workstation (not sold)
| release date = 1986 (internal)
| cpu = ARM1, later ARM2 at 8 MHz; ARM3 on some
| memory = 4 MB (32 x 1 Mbit DRAM)
| storage = 3.5-inch floppy drive; 20 MB ST506 hard disc
| display = VIDC1: up to 1152 x 864 mono, 640 x 480 in 16 colours, 640 x 256 in 256 colours
| sound = 8-channel stereo (VIDC)
| os = Arthur in development; RISC OS 2 (special A500 versions)
| power = BBC Master 128 power supply with an extra 12 V lead
}}
<templatestyles src="Template:StyledTable/styles.css" />
The '''Acorn A500''' is the development machine Acorn built for the Archimedes: a workstation in a heavy metal case with the ARM, MEMC, VIDC and IOC chip set, 4 MB of RAM, a 20 MB hard disc and a four-slot backplane, used inside Acorn to write the operating system and software before the Archimedes went on sale.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A500.html "Acorn A500 (prototype)"], Chris's Acorns, The Centre for Computing History (retrieved 2026-10-02). Paul Fellows's description from the 2003 auction of his A500; the A500 Hardware Guide; photographs and specifications of A500s serial 46 and 104.</ref><ref name="hg">Acorn Computers, ''A500 Hardware Guide'', issue 0.0 (draft, copyright 1986). Hosted on this wiki: [[:File:Acorn A500 Hardware Guide (draft).pdf]]; described on [[Acorn Archimedes A500 Documentation]]. Chapter 1, introduction and features.</ref> It was never sold. Paul Fellows, who used one to develop the operating system at Acorn, wrote that about 100 were built in 1986, some 12 months before the A305 and A310, and that the machine was functionally equivalent to the [[Acorn Archimedes A440|A440]].<ref name="chris" />

== History ==

The A500s first ran with an ARM1 and were upgraded to the ARM2, which added the multiply instruction, while they were in use.<ref name="chris" /><ref name="chrisarc">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/Archimedes.html "Archimedes"], Chris's Acorns (retrieved 2026-10-02). The A500 was the development machine for the whole Archimedes family; it first used the ARM1 and was later upgraded to the ARM2.</ref> Paul Fellows recalled that the machines were at first connected to BBC Micros by an umbilical into the Tube port, with the BBC doing all the input and output. The video, keyboard and filing systems were moved across to the A500 one at a time, until the umbilical was removed and the machine started on its own at the BASIC prompt: "Arthur was born!"<ref name="chris" /> A separate A500 second processor board, which used a BBC Micro in the same way, also survives; Chris Whytehead has one and lists a working example as wanted.<ref name="sp">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/8bit_Upgrades/Acorn_A5002ndProc.html "Acorn A500 second processor"], Chris's Acorns (retrieved 2026-10-02). Board SKB 0000 022A, stripped of most chips; sockets for the ARM, MEMC, IOC and VIDC; Tube interface; circuit diagram.</ref>

Chris Whytehead's second A500, serial number 104, has a board label dated 3 March 1987, so more A500s were built after the first 100. He has also seen an Issue C board in serial number 222.<ref name="chris" />

== Hardware ==

[[File:Acorn Archimedes A500 with top cover removed.jpg|thumb|right|300px|Top cover removed: floppy drive at top left with the ARM2 below it, hard disc at top right, power supply at bottom right and the backplane in the middle (photo: Chris Whytehead, CC BY-SA 3.0)]]

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''A500 features (Acorn hardware guide)'''
! Item !! Specification
|-
| Performance || 8 MIPS peak
|-
| Memory || 4 MB RAM, 256 KB ROM, virtual memory support
|-
| Discs || 1.6 MB floppy, 20 MB Winchester
|-
| Ports || Parallel printer, Econet, RS423 serial, linear RGB, composite video, stereo sound
|-
| Expansion || Four internal I/O expansion slots
|-
| Clock || Battery-backed real-time clock and 256 bytes of RAM
|-
| Display || 1152 x 864 at 1 bit per pixel 60 Hz; 640 x 480 at 4 bits 60 Hz; 640 x 256 at 8 bits 50 Hz
|-
| Keyboard || 95 keys, with speaker and mouse
|}

Source: ''A500 Hardware Guide'', chapter 1.<ref name="hg" />

The main board carries 32 one-megabit DRAMs, which must be 120 ns or faster with an 8 MHz ARM. The floppy controller (IC34) is an FDC9793 or FDC1793 with an FDC9216B data separator, and the real-time clock (IC22) and 256 bytes of CMOS RAM (IC23) sit on the I²C bus. Links set the clock (LK1, 20 or 24 MHz), the DRAM size (LK3 and LK4), the processor (LK5, made for a 3 µm ARM and broken for a 2 µm ARM) and the ROM size (LK6 and LK7).<ref name="hg2">''A500 Hardware Guide'', chapters 9 and 13: detailed hardware description; connectors, links and test points.</ref> Paul Fellows wrote that the A500's RISC OS 2 is a special version, because the serial port controller and hard disc interface differ from the production machines.<ref name="chris" />

[[File:Acorn Archimedes A500 main board.jpg|thumb|right|220px|Main board of A500 serial 46, front edge at the bottom: 4 MB of DRAM, ARM2 and MEMC1, the blue ST506 sockets and the port connectors along the top (photo: Chris Whytehead, CC BY-SA 3.0)]]

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Chris Whytehead's A500s (Chris's Acorns)'''
! Item !! Serial 46 !! Serial 104
|-
| Main board || 0000,025 issue B || 0000,025 issue B
|-
| Processor || VLSI VL2333-QC ARM2, 8 MHz || Prototype VL86C020 ARM3 on daughterboard 0283,600 issue A
|-
| Memory controller || MEMC1 || MEMC1a
|-
| Memory and video || 4 MB, VIDC1 || 4 MB, VIDC1
|-
| Floppy drive || Sony MP-F73W-00D || Sony MP-F73W-00D
|-
| Hard disc || 20 MB Miniscribe 8425 on the on-board HD63463 MFM controller || 50 MB Rodime RO3065
|-
| Backplane || Acorn four-slot, SKEA,007 issue A || Acorn four-slot, SKEA,007 issue A
|-
| Operating system || RISC OS 2.00 for the A500 (15 September 1988) || RISC OS 2.06 development version
|}

Chris Whytehead's specification table records the hard disc controller as "HD64363"; his description of the second machine names it as the HD63463, the Hitachi controller also used on the 400 series. Neither of the usual benchmark programs, !SICK or !ARMSI, runs on the A500. His serial 46 boots RISC OS 2 with most modules loaded from disc, and he has also run Arthur 1.20 ROMs made for the A500.<ref name="chris" /><ref name="sm440">Acorn Computers, ''High Performance Computer Systems Model 440 Service Manual'', part number 0476,155, issue 1, November 1988, p. 28: Hitachi HD63463 hard disc controller. Hosted on this wiki: [[Archimedes 440 Service Manual]].</ref>

=== Keyboard and mouse ===

[[File:Acorn Archimedes A500 keyboard and mouse.jpg|thumb|right|300px|The metal-cased A500 keyboard and three-button mouse (photo: Chris Whytehead, CC BY-SA 3.0)]]

The keyboard is a metal-cased unit that plugs into the front of the computer with a telephone-style plug. It has extra keys such as HELP and MENU, which Paul Fellows says were specified for the ARX operating system that was never released. The three-button mouse, made by Penny & Giles (model M3/100), plugs into the keyboard through a 9-way D connector.<ref name="chris" /> The keyboard has its own 6500/11 microcontroller, an LM7805 regulator taking +5 V from the +12 V feed, and a reset button on the rear that resets both the keyboard and the computer. It has its own power-on reset, so it can be unplugged and replugged without resetting the A500.<ref name="hg3">''A500 Hardware Guide'', chapter 11 (reset and power-on) and appendix B (the A500 keyboard: hardware description, connectors and parts list).</ref>

== Care and repair ==

No service manual for the A500 is known, and the hardware guide's parts list and mechanical chapter, including "Changing the batteries", are empty headings. The following comes from the hardware guide and Chris Whytehead's photographs.<ref name="hg4">''A500 Hardware Guide'', sections 9.7, 9.10.4, 9.12, 12, 14.2 and 15.</ref>

* Batteries: the clock and CMOS RAM run from two dry cells, which the guide says can be changed with the power on without losing the settings. The cells sit at the front of the case behind the moulded front panel.<ref name="hg4" /><ref name="chris" /> Old cells leak; see [[Battery Explosion, Capacitor or Corrosion Damage]] and [[Battery Refurbishment]].
* Power supply: a BBC Master 128 unit with an extra 12 V (yellow) lead. Acorn's guide calls it "a compromise and probably inadequate for a production machine", able to run one low-power 3.5-inch Winchester.<ref name="hg4" /><ref name="chris" /> It runs from the mains; discharge and measure its capacitors before working on it.
* Keyboard: the keyboard's polarised capacitors are C1 and C8, 10 µF 16 V, and C12, 470 µF 6.3 V.<ref name="hg3" />
* Modifications: Acorn listed eight changes for Issue B main boards, among them smoothing the +12 V and +5 V feeds to the floppy drive. Most of the wire changes on Chris Whytehead's serial 104 appear to come from the "Fast A500" upgrade, which runs the ARM and memory faster than the 8 MHz I/O system. Acorn's note on that upgrade says the EPROMs are the first parts to fail when a machine is pushed too fast.<ref name="hg4" /><ref name="fast">Muller, Mike. ''Application Note: Fast A500's'', issue 1.4, Acorn Computers, 12 December 1988 (draft). Hosted on this wiki: [[:File:Acorn Fast A500 Application Note.pdf]].</ref><ref name="chris" />
* Hard and floppy drives: see [[Hard Drive Maintenance and Repair]] and [[Floppy Drive Repair]].

== Documentation ==

* [[Acorn Archimedes A500 Documentation]]: the draft ''A500 Hardware Guide'' and the ''Fast A500's'' application note.
* [[Archimedes 440 Service Manual]] for the production board most like the A500's.

== Related pages ==

* [[Acorn ARM Evaluation System]]
* [[Acorn Archimedes A305]] and [[Acorn Archimedes A310]]
* [[Acorn Archimedes A440]]

== References ==

<references />

{{Navbox-AcornComputers|state=collapsed}}

[[Category:Acorn Computers]]

Acorn Archimedes A500 - Revision history

Josh: New page, cited to Acorn's hosted documents and Chris's Acorns