Editing Acorn Electron (section)

==Hardware==
Much of the core functionality of the BBC Micro{{snd}} the video and [[memory controller]], cassette input/output, timers and sound generation{{snd}} was replicated using a single customised ULA chip designed by Acorn in conjunction with [[Ferranti]], albeit with only one sound channel instead of three (and one noise channel), and without the character-based [[Teletext]] Mode 7.<ref name="smith20130823" />{{rp|page=1|quote=Out went the BBC’s 6845 video controller, memory controller, serial IO control and timer chips, all replicated to a greater or lesser extent by the ULA’s all-in-one circuitry. “We could pretty much fit all the functionality of the Beeb [BBC Micro] into a ULA with the exception of the microprocessor and the memory,” says Furber.}}

The keyboard includes a form of quick keyword input, similar to that used on the Sinclair [[ZX Spectrum]], through use of the {{keypress|FUNC}} key in combination with other keys labelled with BASIC keywords. However, unlike the Spectrum, this method of rapid keyword entry is optional, and keywords can be entered manually if preferred.<ref name="electronuser198311">{{cite magazine |url=https://archive.org/details/Electron_User_Vol._1_No._2_1983-11_Database_Publications_GB/page/n9/mode/2up |magazine=Electron User |title=Electron v. Spectrum |author=Peters, Nigel |date=November 1983 |access-date=3 September 2020 |pages=10–12}}</ref>

The edge connector on the rear of the Electron exposes all address and data bus lines from the CPU, including the upper eight bits of the address bus, in contrast to the limited selection available via the BBC Micro's expansion ports,<ref name="electronuser198310">{{cite magazine |url=https://archive.org/details/Electron_User_Vol._1_No._1_1983-10_Database_Publications_GB/page/n11/mode/1up |magazine=Electron User |author=Cook, Mike |title=Expanding Electron |date=October 1983 |access-date=3 September 2020 |page=12 |quote=the range of address bus signals available on the Electron edge connector is greater than is available on the BBC Micro with its collection of connectors and sockets.}}</ref> with the One Megahertz Bus as the principal mechanism for general purpose expansion on the BBC Micro only providing the lower eight bits of the address bus.<ref name="BAUG">{{cite book |last1=Bray |first1=Andrew C. |last2=Dickens |first2=Adrian C. |last3= Holmes |first3=Mark A. |date=1983 |title=The Advanced User Guide for the BBC Microcomputer |publisher=Cambridge Microcomputer Centre |page=437 |isbn=0946827001 |quote=A buffered databus and the lower 8 bits of the address bus are connected to this socket together with a series of useful control signals.}}</ref> In addition, various control signals provided by the CPU and ULA are exposed via the Electron's expansion connector.<ref name="EAUG">{{cite book |last1=Dickens |first1=Adrian C. |last2=Holmes |first2=Mark A. |date=September 1984 |title=The Advanced User Guide for the Acorn Electron |publisher=Adder Publishing, Cambridge |page=207 |isbn=0947929037 |quote=In fact, the Electron has more potential for expansion than a BBC Micro. Why? Because all necessary system buses come out on the expansion connector.}}</ref>

The ULA mediates access to 32&nbsp;KB of addressable RAM using four 4164 [[dynamic random-access memory|DRAMs]] (64&nbsp;K×1&nbsp;bit), sharing the RAM between the CPU and the video signal generation (or screen refresh) performed by the ULA itself. Two accesses have to be made to the RAM to get each byte (albeit with a single [[dynamic random-access memory#Page mode DRAM|RAS]]), delivering a maximum transfer rate to or from RAM of one byte per 2&nbsp;MHz cycle.<ref name="SM">{{cite book | url=http://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_ElectronSM.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://chrisacorns.computinghistory.org.uk/docs/Acorn/Manuals/Acorn_ElectronSM.pdf |archive-date=2022-10-09 |url-status=live|title=Electron Service Manual|publisher=Acorn Computers Limited|date=January 1987|issue=2|access-date=24 February 2019|page=7}}</ref> In generating the video signal, the ULA is able to take advantage of this 2&nbsp;MHz bandwidth when producing the picture for the high-bandwidth screen modes.<ref name="acornuser198309" />{{rp|page=26|quote=Now, 640 bits is 80 bytes of information, which means you have to get information out at the rate of 80/40 bytes per microsecond, or 2MHz.}} Due to signalling constraints, the CPU can only access RAM at 1&nbsp;MHz, even when it is not competing with the video system.<ref name="SM" />{{rp|page=7|quote=The processor will run at 1MHz during an access cycle to the RAM.}}

When the ULA is consuming all of the RAM bandwidth during the active portion of a display line, the CPU is unable to access the RAM. (The Electron uses the Synertek variant of the 6502 processor as that allowed the clock to be stopped for this 40 microsecond period.)<ref name="SM" />{{rp|page=7|quote=In screen modes 0, 1, 2 and 3 the screen uses all the available memory time slots during the display period. The processor is denied RAM access during 40 microseconds of each 64 microseconds of the 256 lines in 312 which is the display period, and it is made to wait for RAM access until the end of the period.}} In other modes the CPU and video accesses are interleaved with each accessor acquiring bytes at 1&nbsp;MHz.<ref name="SM" />{{rp|page=7|quote=The processor and the video are competing for access to the RAM, and the RAM can only support a bandwidth of 2&nbsp;MHz. In screen modes 4, 5 and 6 this is no problem since the screen only needs 1MHz access to the RAM.}}

In contrast, the [[BBC Micro]] employs one or two sets of eight 16-kilobit devices, with the RAM running at twice the speed (4&nbsp;MHz) of the CPU (2&nbsp;MHz), allowing the video system (screen refresh) and CPU memory accesses to be interleaved, with each accessor able to transfer bytes at 2&nbsp;MHz. The RAM access limitations imposed by the Electron's ULA therefore reduce the effective CPU speed by as much as a factor of four relative to the BBC Micro in the more demanding display modes, and as much as a factor of two otherwise. Byte transfers from ROM occur at 2&nbsp;MHz, however.<ref name="SM" />{{rp|page=7|quote=The processor will run at 2MHz during an access cycle to the ROM.}}

For issue 1–4 motherboards, the ULA had an issue similar to those experienced by other [[CPU socket|socketed]] CPUs. Over time, the thermal heating and cooling could cause the ULA to rise slightly out of its socket just enough to cause the machine to hang or freeze and for other issues to occur preventing the start-up of the machine, in some cases causing the initial beep or tone to sound continuously. This was despite a metal cover and a locking-bar mechanism designed to prevent this from occurring. Pushing down on the metal cover to reseat the ULA was normally sufficient to rectify these issues.{{Citation needed|date=March 2021}}<ref name="computerhistory">{{cite web |url=http://archive.computerhistory.org/resources/physical-object/acorn/102688877.lg.jpg |title=Image: 102688877.lg.jpg, (500 × 359 px) |publisher=archive.computerhistory.org |access-date=17 September 2015}}</ref><ref name="blogspot">{{cite web |url=http://3.bp.blogspot.com/-7zQebyfYlPE/T0dYrkTIIpI/AAAAAAAAAVY/injg8GeCoco/s1600/DSCF3052.JPG|title=Image: DSCF3052.JPG, (1600 × 1200 px) |publisher=3.bp.blogspot.com |access-date=17 September 2015}}</ref><ref name="blogspot2">{{cite web |url=http://1.bp.blogspot.com/-kyRwGNknHnU/T0dZHG8ThnI/AAAAAAAAAVo/JMFmxQLJstY/s1600/DSCF3055.JPG |title=Image: DSCF3055.JPG, (1600 × 1200 px) |publisher=1.bp.blogspot.com|access-date=17 September 2015}}</ref><ref name="bygonebytes">{{cite web |url=http://www.bygonebytes.co.uk/images/electron_issue4.jpg|title=Image: electron_issue4.jpg, (350 × 221 px) |publisher=bygonebytes.co.uk |access-date=17 September 2015}}</ref> Issue 5{{Citation needed|date=March 2021}} and 6 boards utilized a different ULA type, this being known as the Aberdeen ULA (as opposed to the earlier Ferranti ULA) which was mounted on a board that was directly soldered to the main board, with the chip being covered by epoxy resin "insulating material". This arrangement dispensed with the 68-pin socket, and this new type of ULA was expected to be "less prone to failure".<ref name="acorn_information_volume_1">{{ cite tech report | url=http://www.4corn.co.uk/archive/docs/Acorn%20Information%20-%20Volume%201-opt.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.4corn.co.uk/archive/docs/Acorn%20Information%20-%20Volume%201-opt.pdf |archive-date=2022-10-09 |url-status=live | title=Information Volume 1 | publisher=Acorn Computers Limited | date=October 1984 | access-date=21 March 2021 | page=246 }}</ref><ref name="bygonebytes2">{{cite web |url=http://www.bygonebytes.co.uk/images/electron_issue6.jpg|title=Image: electron_issue6.jpg, (350 × 220 px) |publisher=bygonebytes.co.uk |access-date=17 September 2015}}</ref> This type of ULA was also used on the German release of the Electron mainboard which is designated by the marking "GERMAN ELECTRON Issue 1" on the mainboard rather than just "ELECTRON" as for the UK model.<ref name="chris_german_electron">{{ cite web | title=German Acorn Electron | url=http://chrisacorns.computinghistory.org.uk/Computers/ElectronDe.html | date=2 May 2014 | access-date=15 January 2023 | website=Chris's Acorns }}</ref>

===Technical specifications===
[[File:Acorn ALF03 Data Recorder (top).jpg|thumb|Acorn ALF03 Data Recorder]]

The hardware specification according to official documentation,<ref name="amp020">{{ cite book | url=https://www.4corn.co.uk/archive/docs/AMPAPP/150/AMP020%20-%20electron-opt.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.4corn.co.uk/archive/docs/AMPAPP/150/AMP020%20-%20electron-opt.pdf |archive-date=2022-10-09 |url-status=live | title=Electron | publisher=Acorn Computers Limited | pages=5–8 }}</ref> combined with more technical documentation and analysis is as follows:

* [[central processing unit|CPU]]: [[Synertek]] [[6502|SY6502A]]<ref name="pastraiser">{{cite web|url=https://www.pastraiser.com/computers/acornelectron/acornelectron.html|title=Acorn Electron Hardware|publisher=pastraiser.com|access-date=17 September 2015}}</ref>
* Clock rate: variable. CPU runs at 2 [[megahertz|MHz]] when accessing ROM and 1&nbsp;MHz when accessing RAM. The CPU is also periodically halted.<ref name="SM" /><ref group=note>The Electron is widely misquoted as operating at 1.79&nbsp;MHz after measurements derived from speed testing against the 2&nbsp;MHz BBC Micro for various pieces of 'common software'.</ref>
* Glue logic: [[Ferranti|Ferranti Semiconductor]] Custom [[gate array|ULA]]
* [[random-access memory|RAM]]: 32 [[kilobyte|KB]]
* [[read-only memory|ROM]]: 32 [[kilobyte|KB]]
* Graphics modes: 160×256 pixels (20×32 characters) in 4 or 16 colours, 320×256 (40×32 characters) in 2 or 4 colours, 640×256 (80×32 characters) in 2 colours
* Text modes: 40×25 characters in 2 colours, 80×25 characters in 2 colours<ref group=note>Text modes are provided using display modes with the appropriate horizontal resolution (320 or 640 pixels), with each character row being 8 pixels high, and with 2 blank display lines being introduced between character rows.</ref>
* Colours: 8 colours (TTL combinations of RGB primaries) + 8 flashing versions of the same colours
* Sound: 1 channel of sound, 7 octaves; built-in speaker. Software emulation of noise channel supported
* Keyboard: 56 key "full travel QWERTY keyboard"<ref name="amp020" />
* Dimensions: 16×34×6.5&nbsp;cm
* I/O ports: Expansion port, tape recorder connector (1200 [[baud]] CUTS variation on the [[Kansas City standard]] for data encoding, via a 7-pin circular [[DIN connector]]), aerial TV connector ([[RF modulator]]), [[composite video]] and [[RGB]] monitor output
* Power supply: External PSU, 19&nbsp;[[volt|V]] [[alternating current|AC]]

The composite video output provides a greyscale image on the standard machine, but an internal modification allows a colour image to be produced, albeit with a degradation in picture quality. Acorn ostensibly intended the composite output to be a high-quality output for monochrome monitors, with the RGB output being the preferred high-quality output for colour images.<ref name="elbug198403_composite">{{ cite magazine | url=https://archive.org/details/elbug-v-01n-10/Elbug%20v01n04/page/16/mode/1up | title=Colour Video Output | magazine=ELBUG | date=March 1984 | access-date=9 August 2022 | volume=1 | issue=4 | pages=16 }}</ref>

===Merlin M2105===
An unusual variant of the Electron was sold by [[British Telecom]] Business Systems as the BT Merlin M2105 Communications Terminal, being previewed by British Telecom at the Communications '84 show.<ref name="acornuser198407_m2105">{{ cite magazine | url=https://archive.org/details/Acorn_User_1984-07_OCR/page/n10/mode/1up | title=Radio cell option for micros | magazine=Acorn User | date=July 1984 | access-date=18 March 2021 | page=9 | quote=British Telecom had the Healthnet terminal on show. This is an Electron with a custom-made expansion box, and it is intended for use where a lot of form-filling is done at many separate sites. }}</ref> This consisted of a rebadged Electron plus a large expansion unit containing 32&nbsp;KB of battery-backed RAM (making up 64&nbsp;KB of RAM in total), up to 64&nbsp;KB of ROM resident in four sockets (making up to 96&nbsp;KB of ROM in total), a [[Centronics printer port]], an RS423 serial port, a [[modem]], and the speech generator previously offered for the BBC Micro.<ref name="acornuser198406">{{cite magazine |url=https://archive.org/details/AcornUser023-Jun84/page/n8/mode/1up |magazine=Acorn User |title=Healthy link for Acorn |date=June 1984 |access-date=8 October 2020 |page=7}}</ref> The ROM firmware provided [[dial-up]] communications facilities, text editing and text messaging functions. The complete product included a monitor and dot-matrix printer.<ref name="electronuser198503" />

Initially trialled in a six-month pilot at 50 florists, with the intention of rolling out to all 2,500 members of the UK network,<ref name="electronuser198503">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume2/Electron-User-02-06/page/n4/mode/1up | title=Say it with Electron flowers! | magazine=Electron User | date=March 1985 | access-date=13 January 2021 | page=5 }}</ref> these were used by the [[Interflora]] florists network in the UK for over a decade.<ref>{{cite web |url=http://www.acornelectron.co.uk/eug/revs/bt/Merlin_M2105_002.html |title=Merlin M2105 | work=Acorn Electron World | publisher=Acornelectron.co.uk |access-date=12 January 2021}}</ref><ref name="chris_bt_m2105">{{cite web|url=http://chrisacorns.computinghistory.org.uk/Computers/BT_MerlinM2105.html |title=Chris's Acorns: BT Merlin M2105 |publisher=chrisacorns.computinghistory.org.uk |date=16 January 2008 |access-date=1 October 2015}}</ref> Used mostly for sending messages, despite providing support for other applications, limited availability of the product led Interflora to look for alternatives after five years, although users appeared to be happy with the product as it was.<ref name="electronuser198905a">{{cite magazine | url=https://archive.org/details/ElectronUserVolume6/Electron-User-06-08/page/n4/mode/1up | title=Flower folk like their Electrons | magazine=Electron User | date=May 1989 | access-date=11 November 2020 | page=5 }}</ref>

This generic product combination of the Electron and accompanying expansion was apparently known as the Chain during development,<ref name="acornuser198406" /> itself having a different board layout,<ref name="chain_m2105">{{ cite web | url=https://stardot.org.uk/forums/viewtopic.php?p=58667#p58667 | title=BT M2105 Version | website=stardot.org.uk | date=23 February 2013 | access-date=6 March 2021 }}</ref> with British Telecom having intended the M2105 to be a product supporting access to an online service known as Healthnet.<ref name="acornuser198406" /><ref name="acornuser198505">{{cite magazine |url=https://archive.org/details/AcornUser034-May85/page/n128/mode/1up |magazine=Acorn User |title=Acorn reviews ABCs |date=May 1985 |access-date=30 August 2020 |page=127}}</ref> This service aimed to improve and speed up communications within hospitals so that patients could be treated and discharged more quickly, and to facilitate transfers of information to doctors and health workers outside hospitals, with communications taking place over conventional telephone lines. The service was to be introduced in the Hammersmith and Fulham district health authority, with installation starting at Charing Cross Hospital. The Electron was said to be particularly suitable for deployment in this application in that it had a "large expansion bus",<ref name="electronuser198406">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume1/Electron_User_01-09/page/n6/mode/1up | title=Electron to speed up NHS communications | magazine=Electron User | date=June 1984 | access-date=14 January 2021 | page=7 }}</ref> ostensibly making the machine amenable to the necessary adaptations required for the role, together with its "price, and the fact it has a real keyboard".<ref name="electronuser198405b">{{ cite magazine | url=https://archive.org/details/ElectronUserVolume1/Electron_User_01-08/page/n7/mode/1up | title=BT sign Electron boards contract | magazine=Electron User | date=May 1984 | access-date=14 January 2021 | page=8 }}</ref> As a Healthnet terminal, the M2105 was intended to support the exchange of forms, letters and memos.<ref name="btjournal198710">{{ cite journal | url=https://archive.org/details/btj-198710/page/n48/mode/1up | title=A prescription for healthy management | journal=British Telecom Journal | date=October 1987 | access-date=18 March 2021 | volume=8 | issue=3 | page=49 | quote=Healthnet is useful in hospitals and the National Health Service as a communications and information system. It transmits forms, letters and memos over the telephone as well as extracting data from the transmitted forms and turning it into clinical management information. Healthnet uses M2105 terminals. }}</ref>

The adoption of an Acorn product in this role was perhaps also unusual in that much of BT's Merlin range of this era had been supplied by ICL, notably the M2226 small business computer and M3300 "communicating word processor".<ref name="btjournal198307">{{ cite journal | url=https://archive.org/details/btj-198307/page/n67/mode/2up | title=Computer wizardry! | journal=British Telecom Journal | volume=4 | issue=2 | date=July 1983 | access-date=16 January 2021 | pages=34–35 }}</ref><ref name="bteng198307">{{ cite journal | url=https://archive.org/details/bte-198307/page/n73/mode/1up | title=First Electronics Office Products from Merlin | journal=British Telecommunications Engineering | date=July 1983 | access-date=26 February 2021 | volume=2 | issue=2 | page=132 | issn=0262-401X }}</ref> Nevertheless, the M2105 offered interoperability with other BT products such as the QWERTYphone which was able to receive messages from the M2105 and the [[One Per Desk|Merlin Tonto]].<ref name="bteng198701a">{{ cite journal | url=https://archive.org/details/bte-198701/page/n19/mode/2up | title=QWERTYphone - A Low-Cost Integrated Voice/Data Terminal | journal=British Telecommunications Engineering | last1=Durkin | first1=G. M. | date=January 1987 | access-date=26 February 2021 | volume=5 | issue=4 | pages=276–280 | issn=0262-401X }}</ref>{{rp|page=280|quote=Receives text messages from other QWERTYphones, Merlin Tonto and M2105 terminals or any terminal with a V.21 modem}}

The hardware specifications of the M2105, observed from manufactured units, include the 6502 CPU (SY6502<ref name="beebmaster_m2105">{{ cite web | url=http://www.beebmaster.co.uk/Elk/BTMerlin9.html | title=BT Merlin | website=BeebMaster | access-date=25 February 2021 }}</ref> or R6502), ULA and 32&nbsp;KB of dynamic RAM fitted in the Electron main unit, plus 32&nbsp;KB of static RAM, two 6522 VIA devices for interfacing, AM2910PC modem, SCN2681A [[UART]], and [[Texas Instruments LPC Speech Chips|TMS5220]] plus [[TMS6100]] for speech synthesis.<ref name="chris_bt_m2105" /> The speech synthesis was used for the "voice response" function which answered incoming voice calls by playing a synthesised message to the caller.<ref name="m2105_user_guide">{{ cite book | url=http://www.computinghistory.org.uk/det/10302/BT-Merlin-User-Guide-M2105-Messaging-Terminal/ | title=BT Merlin User Guide for M2105 Messaging Terminal | publisher=British Telecom Business Systems | date=June 1985 | volume=2 | pages=54, 78 }}</ref> The components chosen and the capabilities provided (excluding speech synthesis) are broadly similar to those featured by the [[Acorn Communicator]] which was another product of Acorn's custom systems division.

The product documentation indicates a specification with 48&nbsp;KB of RAM plus 16&nbsp;KB of "non volatile [[CMOS]] RAM" and 96&nbsp;KB of ROM,<ref name="m2105_user_guide" />{{rp|page=87}} although this particular composition of RAM is apparently contradicted by the RAM devices present on surviving M2105 machines.<ref name="chris_m2105_expansion">{{ cite web | url=http://chrisacorns.computinghistory.org.uk/Pics/BT_MerlinM2105LL.html | title=BT Merlin M2105 Expansion box circuitboard | website=Chris's Acorns | access-date=25 February 2021 }}</ref> However, the earlier Chain variant of the board does appear to provide only 16&nbsp;KB of static RAM using two HM6264LP-15 chips, also providing an extra 16&nbsp;KB of dynamic RAM using eight MK4516-15 chips, suggesting that the product evolved during development.<ref name="chain_m2105" />

===Recreations and emulators===
Several emulators of the machine exist:

# ElectrEm<ref>{{cite web |url=http://electrem.emuunlim.com |title=ElectrEm |publisher=Electrem.emuunlim.com |access-date=28 May 2013}}</ref> (Windows, Linux, Mac)
#Elkulator<ref>{{cite web|url=http://elkulator.acornelectron.co.uk |title=Elkulator - The Acorn Electron emulator |publisher=Elkulator.acornelectron.co.uk |access-date=28 May 2013}}</ref> (Windows, Linux)
#ElkJS<ref>{{cite web|url=http://elkjs.azurewebsites.net |title=ElkJS}} - JavaScript browser based Acorn Electron emulator</ref> is a browser-based emulator
#Multi-system emulators [[MESS]] and Clock Signal<ref>{{cite web |last1=Harte |first1=Thomas |title=Clock Signal |url=https://github.com/TomHarte/CLK |publisher=GitHub |access-date=31 March 2019}}</ref> support the Electron.

Electron software is predominantly archived in the [[UEF (file format)|UEF]] [[file format]].

There are also two publicly documented [[Field-programmable gate array|FPGA]] based recreations of the Acorn Electron hardware: ElectronFPGA<ref>{{cite web|url=https://github.com/hoglet67/ElectronFpga|title=ElectronFPGA|website=[[GitHub]] }}{{snd}} Acorn Electron core for the Papilio Duo</ref> for the Papilio Duo hardware and the Acorn-Electron<ref>{{cite web|url=http://www.fpgaarcade.com/kb/acorn-electron/|title=Acorn-Electron}}{{snd}} Acorn Electron core for the FPGA Arcade "Replay" board</ref> core for the FPGA Arcade "Replay" board. In addition, an implementation of the ULA for the [[ICE (FPGA)#iCE40 (40 nm)|Lattice ICE40]] series has been made available.<ref name="moogway82_ula">{{ cite web | url=https://github.com/moogway82/JamSoftElectronULA | title=JamSoftElectronULA | website=GitHub }}</ref>