Editing Acorn Archimedes A420

{{Infobox computer
| name         = Acorn Archimedes A420/1
| image        = [[File:Acorn_Archimedes_A420.jpg|250px]]
| caption      = Acorn Archimedes A420/1
| manufacturer = Acorn Computers Ltd
| type         = Personal computer (three-box desktop)
| release date = May 1989
| discontinued = 1991
| price        = £1,699 ex VAT
| cpu          = ARM2 @ 8 MHz
| memory       = 2 MB RAM, upgradeable to 4 MB
| storage      = 3.5" floppy drive; 20 MB ST506 hard disc
| sound        = 8-channel, 8-bit logarithmic (VIDC1a)
| os           = RISC OS 2.00
| successor    = [[Acorn Archimedes A5000]]
| model        = AKB42
}}
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The '''Acorn Archimedes A420/1''' is the middle machine of Acorn's A4x0/1 range, launched in May 1989 alongside the [[Acorn Archimedes A410|A410/1]] and the A440/1. There was never an original "A420" — the /1 machines replaced a 400 series in which, as far as is known, only the [[Acorn Archimedes A440|A440]] was actually produced.<ref name="chris4001">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A4001.html "Acorn Archimedes A400/1 Series"], Chris's Acorns / The Centre for Computing History.</ref><ref name="chris400">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A400.html "Acorn Archimedes A400 Series"], Chris's Acorns / The Centre for Computing History.</ref>

The A420/1 differs from the A410/1 only in having 2 MB of RAM fitted instead of 1 MB, and a 20 MB ST506 hard disc.<ref name="chris4001" /> Everything else — the Acorn Universal Board (part 0283,000), the ARM2 at 8 MHz, the MEMC1a, the VIDC1a, the four-slot backplane (part 0283,040) and the on-board HD63463P8 ST506/MFM controller — is shared with the rest of the range and is described on the [[Acorn Archimedes A410]] page.

== Range and pricing ==

{| class="wikitable styled-table" style="width:85%; text-align:center;"
! Model !! Product code !! RAM !! Hard disc !! Price ex VAT
|-
| A410/1 || AKB40 || 1 MB || none || £1,199
|-
| '''A420/1''' || '''AKB42''' || '''2 MB''' || '''20 MB ST506''' || '''£1,699'''
|-
| A440/1 || AKB44 || 4 MB || 50 MB ST506 || £2,499
|}

Acorn also sold Learning Curve bundles: AKB45 was an A420/1 Learning Curve and AKB46 an A440/1 Learning Curve.<ref name="chris4001" /><ref name="chrisarc">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/Archimedes.html "Archimedes"], Chris's Acorns / The Centre for Computing History. Acorn product-code list for the Archimedes range.</ref>

Product codes on surviving machines do not always match the badge. Chris Whytehead documents an A420/1 whose serial number carries the AKB46 (A440/1 Learning Curve) code, and another badged A420/1 whose code is AKB44 (A440/1) — a reminder that on this range the label on the front is not evidence of what is inside.<ref name="chris4001" />

== Memory ==

The A420/1 has 2 MB fitted in the back row of RAM positions, with the front row left as empty sockets. Fitting those sockets takes the machine to 4 MB, the limit of a single MEMC1a.<ref name="chris4001" />

== Ports ==

The A420/1's rear panel carries a 75 Ω high-resolution monitor output on two BNC connectors, analogue RGB, serial, headphones and a parallel printer port; the Econet interface is normally not fitted.<ref name="chris4001" />

== Backup batteries ==

Like the rest of the A400/1 range, the A420/1 uses '''two LR06 (AA size) 1.5 V manganese alkaline cells''' in a holder in front of the power supply, wired to PL10 on the main PCB, which Acorn specified for annual replacement.<ref name="sm4001">Acorn Computers Limited, ''Acorn A400/1 &amp; R140 Service Manual'', 1989. Hosted on this wiki: [[Acorn A400-1 and R140 Service Manual]].</ref><ref name="chris4001" /> They are not NiCd, and they leak. Remove them before anything else and inspect the holder and the board beneath — see [[Battery Explosion, Capacitor or Corrosion Damage]].

== Operating system ==

The A420/1 shipped with RISC OS 2.00 in ROM. Earlier Archimedes machines that shipped with Arthur could be taken to RISC OS 2 by replacing the ROM set; A420/1s in the field were commonly taken on to RISC OS 3.10 or 3.11 the same way.<ref name="chris4001" /><ref name="chris300">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A300.html "Acorn Archimedes A300 Series"], Chris's Acorns / The Centre for Computing History. Machine-by-machine specification tables, part numbers and measured benchmark figures.</ref>

== Measured performance ==

From Chris Whytehead's documented A420/1s:<ref name="chris4001" />

{| class="wikitable styled-table" style="width:85%; text-align:center;"
! Machine !! CPU !! RAM !! OS !! ARMSi v4.0 MIPS !! SICK Dhrystone/s !! kWhetstone/s
|-
| AKB46 || Watford Electronics ARM3 25 MHz || 2 MB || RISC OS 3.10 || — || 17,309 || 212
|-
| AKB44 || Aleph One ARM3 30 MHz || 4 MB || RISC OS 3.10 || 14.73 || 20,353 || 258
|}

Both machines are ARM3-upgraded; no measurement of a standard ARM2 A420/1 has been published. A standard ARM2 A410/1 on the same board returns 5,962 Dhrystones per second.<ref name="chris4001" />

== Maintenance and repair ==

* '''Remove the AA alkaline cells''' first.
* '''Acorn's fault diagnosis and functional test procedures''' are in the [[Acorn A400-1 and R140 Service Manual]] hosted here.
* '''The 20 MB ST506 drive''' is the component most likely to be dead; see [[Hard Drive Maintenance and Repair]].
* '''Floppy drive''' — see [[Floppy Drive Repair]].
* '''Tools''' — see [[Recommended Tools]].

== See also ==

* [[Acorn Archimedes A410]] — the entry model, and the never-sold 1987 A410
* [[Acorn Archimedes A440]] — the 1987 A440 and the 1989 A440/1
* [[Acorn Archimedes A540]] — the 1990 flagship
* [[Acorn A400-1 and R140 Service Manual]]

== References ==

<references />

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