Editing Acorn Archimedes A540

{{Infobox computer
| name         = Acorn Archimedes A540
| image        = [[File:Acorn_Archimedes_A540.jpg|250px]]
| caption      = Acorn Archimedes A540
| manufacturer = Acorn Computers Ltd
| type         = Personal computer (three-box desktop)
| release date = June 1990
| discontinued = 1992
| price        = £2,495 ex VAT, no monitor
| cpu          = ARM3 on a replaceable CPU card, 25/26 MHz as shipped; 33 MHz cards exist
| memory       = 4 MB RAM, expandable to 16 MB with up to three 4 MB RAM cards
| storage      = 3.5" floppy drive (800 KB); 100 MB SCSI hard disc on an Acorn SCSI podule
| sound        = 8-channel, 8-bit logarithmic (VIDC1a)
| os           = RISC OS 2.01
| predecessor  = [[Acorn Archimedes A440]]
| successor    = [[Acorn Archimedes A5000]]
| model        = AKB50
}}
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The '''Acorn Archimedes A540''' was launched in June 1990 as the flagship of the Archimedes range. It was the first Acorn machine with an ARM3 as standard, the only one that could be taken to 16 MB of memory, and — with an ARM3 and an FPA10 fitted — more powerful than the later [[Acorn Archimedes A5000|A5000]].<ref name="chris540">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A540.html "Acorn Archimedes A540"], Chris's Acorns / The Centre for Computing History.</ref>

It costs £2,495 plus VAT without a monitor, and shares its hardware with Acorn's R260 RISC iX workstation: the A540 is an R260 without RISC iX installed, which could be bought separately and installed later.<ref name="chris540" /> Acorn documented the two together in the [[Acorn Archimedes 500 Series and R200 Series Service Manual]].<ref name="sm500">Acorn Computers Limited, ''Acorn Archimedes 500 series / Acorn R200 series Service Manual'', Issue 2, June 1991. Part 5 component-level fault diagnosis and Part 6 parts lists. Hosted on this wiki: [[Acorn Archimedes 500 Series and R200 Series Service Manual]].</ref>

== Memory system ==

This is what makes the A540 unusual. A single MEMC1a addresses a maximum of 4 MB, so Acorn put a MEMC1a on the motherboard and gave the machine three further sockets, SK5, SK6 and SK7, for '''RAM cards that each carry their own MEMC1a and their own 4 MB'''. Configurations are therefore 4, 8, 12 or 16 MB.<ref name="chris540" /><ref name="sm500" />

The motherboard's DRAM is 1M × 1 80 ns in 20-pin ZIP packages; the 4 MB cards use 1M × 4 80 ns SOJ devices with a MEMC1a at IC10. Acorn's parts list carries an explicit warning: '''MEMCs must be Acorn part number 2201,393''' to guarantee correct timing.<ref name="sm500" /> The memory clock runs at '''12 MHz''', against 8 MHz on the A300, A400 and A400/1 machines.<ref name="chris540" /><ref name="sm500" /> Acorn and Simtec both made RAM cards.<ref name="chris540" />

The memory cards are the machine's known weak point. Chris Whytehead reports that an A540 left unused for a while becomes temperamental and that the RAM cards in particular are liable to work loose; reseating them is the first thing to try on an unstable machine.<ref name="chris540" />

== Processor ==

The ARM3 sits on a replaceable CPU card. Machines shipped with 25 or 26 MHz parts; 33 MHz cards exist and are found on upgraded machines. Chris Whytehead's two documented A540s carry a VL86C020 at 26 MHz (the original card, with no FPA socket) and a VY86C020-35 at 33 MHz with an FPA10 fitted.<ref name="chris540" />

Measured on those machines with ARMSi v4.0: 14.14 MIPS for the 26 MHz card and 16.8 MIPS for the 33 MHz card with FPA10.<ref name="chris540" />

=== FPA10 floating point accelerator ===

Acorn released the FPA10 in July 1993 for the A540, the A5000 and the R260, at '''£99 ex VAT'''. It fits a socket designed for it on the A540's processor card. Acorn's figures: 250 mW dissipation, peak throughput up to 5 MFLOPS at 26 MHz and an average above 3 MFLOPS, with floating point performance under the RISC OS floating point emulator increasing by as much as a factor of 50.<ref name="prfpa">Acorn Computers Limited press release, "Acorn Releases Floating Point Accelerator", 5 July 1993. [https://chrisacorns.computinghistory.org.uk/docs/Acorn/PR/FPA_release.txt Text] via Chris's Acorns.</ref>

Acorn's own release note adds a repair-relevant point: '''owners of early A540s and R260s may need a free-of-charge modification to their processor cards''' before an FPA10 will work, and RISC iX users need an additional software driver.<ref name="prfpa" />

The FPA10 is the only floating point accelerator for this machine. The FPA11 is a different part and is not an A540 upgrade.

== SCSI ==

The A540's SCSI is '''not''' on the motherboard. Unlike the rest of the range it shipped with a SCSI podule as standard, together with a 100 MB SCSI hard disc — so one of the four podule slots is occupied from new.<ref name="chris540" /> Documented machines carry the Acorn AKA31 (part 0173,010) or the AKA32 Mk3 (part 0173,080).<ref name="chris540" />

Acorn's service manual documents the SCSI expansion card down to component level. It is built around a '''Western Digital WD33C93A''' SCSI bus interface controller and an NEC 71071 DMA controller, with two 32K × 8 static RAMs, a driver EPROM and five PALs.<ref name="sm500" />

== Video ==

The A540 uses the VIDC1a, as the rest of the Archimedes range does.<ref name="chris540" /> It and the R260 carry '''three BNC connectors''' on the rear panel that appear on no other Acorn machine.<ref name="chris540" />

== Expansion and construction ==

The backplane is Acorn part 0186,001, four slots; the motherboard is part 0186,000 Issue 1.<ref name="chris540" /> The power supply is physically larger than a standard Archimedes unit and contains '''two fans'''.<ref name="chris540" />

Cards documented on surviving A540s include the Atomwide Ethernet IV, the Wild Vision / Computer Concepts Colour Card and Colour Card Gold, the Computer Concepts ScanLight scanner interface, and the Aleph One 386 PC card.<ref name="chris540" />

== Backup battery ==

The A540's backup battery is '''BT1, a NiCd 1.2 V 280 mAh cell mounted on the PCB''', with a foam pad beneath it. It is a soldered rechargeable cell, not a holder of AA cells as in the A300, A400 and A400/1 machines, and not a 3.6 V pack.<ref name="sm500" />

Acorn's component-level check, from the service manual: if the non-volatile memory loses data and the real-time clock fails, then '''with the machine switched off, look for about 2.8 V on IC22 pin 8'''. If that voltage is absent, check the charge state of BT1. If the NVM at IC22 consistently fails on the same data bits, change the device. If the clock runs inaccurately or not at all, check and if necessary replace crystal X3; LK8 gives access to the clock signal.<ref name="sm500" />

Thirty-year-old NiCd cells on Acorn boards leak and corrode the tracks around them. Remove BT1 before powering a stored machine — see [[Battery Explosion, Capacitor or Corrosion Damage]] and [[Battery Refurbishment]].

== Operating system ==

The A540 launched with RISC OS 2.01, dated 5 July 1990 in the ROM.<ref name="chris540" /> Many machines were later taken to RISC OS 3.10 or 3.11 by ROM swap; Chris Whytehead's first A540 runs 3.11 with the Acorn Universal Boot installed.<ref name="chris540" /> His second, bought from an ex-Acorn employee, still runs RISC OS 2.01 with SJ Research Nexus Level 4 fileserver software for Econet — a reminder that an unmodified A540 is worth leaving unmodified.<ref name="chris540" />

== Serial port ==

The A500/R200 serial port is based on the '''RS232''' standard, unlike the RS423-based port on the earlier Archimedes machines, and the RS423 Drive software patch (version 1.24) needed on those machines is not required here.<ref name="sm500" />

== Maintenance and repair ==

* '''Remove BT1''' and inspect the board around it before applying power to a stored machine.
* '''Reseat the RAM cards''' in SK5–SK7 on any machine that is unstable or reports the wrong memory size.
* '''Check the FPA card modification''' if an FPA10 does not work in an early machine.<ref name="prfpa" />
* '''Two fans''' — check both turn. The A540 runs hot and the memory cards are heat-sensitive.
* '''SCSI drive''' — see [[Hard Drive Maintenance and Repair]].
* '''Floppy drive''' — documented machines use the Citizen OSDC-65C. See [[Floppy Drive Repair]].
* '''Tools''' — see [[Recommended Tools]].

Component-level fault diagnosis, the functional test suite and full parts lists for the main PCB, RAM card, backplane, ARM3 card, keyboard, Ethernet and SCSI cards — including every capacitor designator and value — are in the [[Acorn Archimedes 500 Series and R200 Series Service Manual]] hosted here.<ref name="sm500" />

== See also ==

* [[Acorn Archimedes A440]] — the flagship it replaced
* [[Acorn Archimedes A5000]] — the 1991 ARM3 machine that undercut it
* [[Acorn Archimedes A410]] and [[Acorn Archimedes A420]] — the A400/1 range
* [[Acorn Archimedes 500 Series and R200 Series Service Manual]]

== References ==

<references />

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