Acorn Electron

The Acorn Electron is a budget version of the BBC Micro educational/home computer introduced by Acorn Computers Ltd on 25 August 1983. It has 32 kilobytes of RAM, and its ROM includes BBC BASIC v2 along with its operating system.

Acorn Electron
DeveloperAcorn Computers
Type8-bit microcomputer
Release date25 August 1983 (1983-08-25)
MediaCassette tape, floppy disk (optional), ROM cartridge (optional)
Operating systemAcorn MOS v1.0
CPUSynertek SY6502A clocked at 2MHz when accessing ROM and 1 MHz when accessing RAM
Memory32 KB RAM, 32 KB ROM
DisplayRF modulator, composite video, RGB monitor output
Graphics160×256 (4 or 16 colours), 320×256 (2 or 4 colours), 640×256 (2 colours), 320×200 (2 colours – spaced display with two blank horizontal lines following every 8 pixel lines), 640×200 (2 colours – spaced display)
InputKeyboard

The Electron was able to save and load programs onto audio cassette via a supplied converter cable that connected it to any standard tape recorder that had the correct sockets. It was capable of basic graphics, and could display onto either a television set, a colour (RGB) monitor or a "green screen" monitor.

For a short period, the Electron was reportedly the best selling micro in the United Kingdom,[1] with an estimated 200,000[2] to 250,000 machines[3] sold over its entire commercial lifespan.

History
BASIC prompt after switch-on or hard reset (soft reset was similar but did not contain the Acorn trademark).

After Acorn Computer released the BBC Micro, executives believed that the company needed a less-expensive computer for the mass market. In May 1982, when asked about the recently announced Sinclair ZX Spectrum's potential to hurt sales of the BBC Micro, priced at £125 for the 16K model compared to around twice that price for the 16K BBC Model A, Acorn co-founder Hermann Hauser responded that in the third quarter of that year Acorn would release a new £120-150 computer which "will probably be called the Electron", a form of "miniaturised BBC Micro", having 32 KB of RAM and 32 KB of ROM, with "higher resolution graphics than those offered by the Spectrum".[4]

Acorn's Chris Curry also emphasised the Electron's role as being "designed to compete with the Spectrum... to get the starting price very low, but not preclude expansion in the long term."[5] In order to reduce component costs, and to prevent cloning, the company reduced the number of chips in the Electron from the 102 on the BBC Micro's motherboard to "something like 12 to 14 chips"[6](pp1) with most functionality on a single 2,400-gate Uncommitted Logic Array (ULA).[7] Reports during the second half of 1982 indicated a potential December release,[8] with Curry providing qualified confirmation of such plans, together with an accurate depiction of the machine's form and capabilities, noting that the "massive ULA" would be the "dominant factor" in any pre-Christmas release.[9] However, as the end of the year approached, with the ULA not ready for "main production", the launch of the Electron was to be delayed until the spring.[10]

By June 1983, with the planned March release having passed, the launch of the Electron had been rescheduled for the Acorn User Exhibition in August 1983,[11] and the machine was indeed launched at the event with a new price of £199. The company expected to ship the Electron before Christmas, and sell 100,000 by February 1984.[7] Reviews were generally favorable, noting its excellent graphics compared to the Commodore 64. While its speed was acceptable compared to the Commodore and competing computers the Electron was, however, rather slower than the BBC Micro with one review noting that games designed for the BBC Micro ran "at less than half the speed, with very significant effects on their appeal".[12] The reduced performance can be attributed to the use of a 4-bit wide memory system instead of the 8-bit wide memory system of the BBC Micro to reduce cost. Due to needing two accesses to the memory instead of one to fetch each byte, along with contention with the video hardware also needing access, reading or writing RAM was much slower than on the BBC Micro.[13](pp26)

Production difficulties at Astec in Malaysia delayed the machine's introduction, forcing Acorn to look to other manufacturers such as Wongs in Hong Kong and AB Electronics in Wales. By October 1983, Acorn had received orders for more than 150,000 units, but had production targets of only 25,000 a month before Christmas, meaning that the existing backlog would take more than six months to fulfil. Demand for the Electron was high but only two of WH Smith's London branches had inventory.[14] Ultimately, manufacturing in Malaysia ceased with the anticipated but unspecified number of units having been produced,[15] this having been originally reported as 100,000 units.[16] Acorn's marketing manager, Tom Hohenberg, admitted in early 1984 that "a lot of the trouble stemmed from the ULA" in getting production to the desired levels,[17] but that such difficulties had been resolved, although Acorn faced an order backlog of almost a quarter of a million units.[18]

As the company increased production during 1984, however, the British home computer market greatly weakened. Tom Hohenberg, Acorn's marketing director, later noted that after the 1983 Christmas season, Electron deliveries had increased to meet a demand that was no longer there, with the market having "completely dried up".[19](pp39) Acorn's Christmas 1984 sales were greatly below expectations and by March 1985 the company had reduced the Electron's price to £129.[20] With the company's unsuccessful expansion into the United States abandoned,[21] Acorn's financial situation had deteriorated sufficiently to prompt Olivetti to rescue the company by taking a 49.30% ownership stake.[22] Renewed efforts were made to sell the machine, bundling it with Acorn's own expansions and software, such as one package adding the Plus 1 expansion, joysticks and a ROM cartridge game to the base machine for a total price of £219.[23] Acorn committed to supporting the machine "until the end of 1986", continuing to supply it (as the Merlin M2105) to British Telecom as part of the Healthnet communications system, with small-scale manufacturing continuing while existing stocks were being run down.[24]

By autumn, retailers appeared eager to discount the computer, with prices in stores as low as £100, reportedly less than the distributor prices of the summer months.[25] As the Christmas season approached, Dixons Retail acquired the remaining Electron inventory to sell, bundled with a cassette recorder and software, at a retail price of £99.95,[26] effectively bringing to an end Acorn's interest in the Electron and the home computing market but empowering third-party suppliers whose "inventiveness and initiative" contrasted with Acorn's lack of interest and "false promises".[27] Acorn subsequently released the Master Compact - a model in the Master series of microcomputers with fewer BBC Micro-style ports and a similar expansion connector - with the home audience specifically in mind.[28] Indeed, prior to its release, the Master Compact had been perceived as the successor to the Electron.[29]

With hindsight, the large screen memory required too much processing power to manipulate and left too little space available to programs for the machine to take on the prevailing Spectrum (with less than 7K of screen memory) and Commodore 64 (with hardware sprites) in gaming. Despite this, several features that would later be associated with BBC Master and Archimedes were first features of Electron expansion units, including ROM cartridge slots and the Advanced Disc Filing System, a hierarchical improvement to the BBC's original Disc Filing System.[30]

The availability of the Electron at discounted prices from 1985 onwards led to increased demand for third-party software and expansions for the machine.[31] While it may not have been as popular as the Spectrum, Commodore 64 or Amstrad CPC, it did sell in sufficient numbers to ensure that new software titles from established producers were being produced right up until the early 1990s,[32] with mainstream publications dedicated to the machine having effectively supported it for five years beyond the point at which Acorn's own support had ceased.[33]

Hardware expansions

Since the Electron provided only a selection of video output ports, a cassette port and the expansion connector, a range of additional expansions were produced to offer ports and connections to various peripherals. The first expansions were largely joystick and printer interfaces or sideways ROM boards.[34] For instance, First Byte Computers developed an interface and software which allowed a "switched" joystick to be used with the majority of software titles.[35] This interface became very popular and was sold by W.H. Smiths, Boots, Comet and hundreds of independent computer dealers, selling as many as 23,000 units over a two-year period, helped by a bundling agreement with Dixons.[36]

Acorn's own expansion strategy was led by the Plus 1 which offered a combination of ports and cartridge connectors,[37] followed by the Plus 3 disc drive unit, but by early 1986 the more general range of expansions had broadened to include floppy drive and RS423 interfaces, Teletext adapters, and other fundamental enhancements to the base machine.[38]

Multi-function expansions

Since the Electron's expansion connector was the basis of practically all hardware expansions for the machine, unless an expansion propagated this connector to others, as was done by the Acorn Plus 3, the capabilities of any given expansion would limit the capabilities of the expanded machine. Thus, expansions offering a single function, such as joystick ports or a printer port, would need to be disconnected if other capabilities were needed, and then reconnected later. Consequently, multi-function expansions offering a combination of different capabilities offered a significant degree of convenience as well as avoiding wear on the expansion connector.[39]

Alongside announcements of Acorn's then-unreleased Plus 1,[40] Solidisk previewed a General Purpose Interface for the Electron in early 1984 offering a Centronics printer port, switched joystick port, user port, sideways ROM sockets, and mini-cartridge sockets supported by the 6522 versatile interface adapter (VIA) chip.[41] The Plus 1 itself was released in mid-1984,[42] introducing the influential cartridge format for expansions ultimately used by several other companies.

Acorn Plus 1
An Acorn Electron with Plus 1 expansion unit attached

The Acorn Plus 1 added two ROM cartridge slots, an analogue interface (supporting four channels) and a Centronics parallel port, priced at £59.90.[42] The analogue interface was normally used for joysticks, although trackball[43] and graphics tablet[44] devices were available, and the parallel interface was typically used to connect a printer. Game ROM cartridges would boot automatically. Languages in paged ROM cartridges would take precedence over BASIC.[45] (The slot at the front of the interface took priority if both were populated.)

Access to ROM occurred at 2 MHz until RAM access was required, so theoretically programs released on ROM could run up to twice as fast as those released on tape or disc. Despite this, all of the games released on ROM were packaged as ROM filing system cartridges, from which the micro would load programs into main memory in exactly the same way as if it were loading from tape. This meant that programs did not need to be modified for their new memory location and could be written in BASIC but gave no execution speed benefits.[46] Six ROM cartridge titles were announced for the launch of the Plus 1.[47]

The cartridge slots provided additional control lines (compared to the lines available via the edge connector on the rear of the Electron) to ease implementation of ROM cartridges.[48] Acorn described the hardware extension possibilities in promotional literature, giving an RS423 cartridge as an example of this capability of the Plus 1.[49]

Additional peripheral cartridges by companies such as PRES (via their ARA/ARA2/ASR products) allowed sideways ROM capability, that allowed the standard Acorn ROM space to be programmatically mapped out for alternative EPROMs, either physically via ZIF Sockets, or 'virtually' via ROM images loaded into (battery-backed) RAM in the same ROM address space. This enabled the Electron to achieve the same functionality as that provided by the expansion ROM slots under the keyboard and on the bottom-left of the BBC Micro B keyboard.

The addition of the Plus 1 added a number of new *FX or OSBYTE calls that allowed the OS to read values from the analogue interface and write to the parallel interface.

After Acorn's change of focus away from the Electron, and with a shortage of Plus 1 units available to purchase, Advanced Computer Products secured the rights to manufacture the unit under licence from Acorn,[50] obtaining the injection moulds and tooling, thus restarting production in 1987 after Acorn's own production of the unit had ceased in November 1985.[51]

The Plus 1 needed memory page &D for its workspace, and the unit added some processing overhead when enabled, both of these things causing issues with the loading and running of software, particularly cassette-based games. To disable the Plus 1, after pressing BREAK, the following commands could be issued:[52]

*FX163,128,1
?&212=&D6
?&213=&F1
?&2AC=0

Slogger Rombox Plus

Following on from Slogger's earlier Rombox product - an expansion similar in profile to the Plus 1 but offering eight ROM sockets and propagating the expansion connector to other units[53] - the Rombox Plus was positioned more directly as a competitor to the Plus 1 in that it offered two cartridge slots and a Centronics print port alongside four ROM sockets. Priced at £49.95, the unit was mostly compatible with cartridges designed for the Plus 1 although one reviewer reported physical issues with some expansion cartridges, suggesting some manufacturing inconsistencies given other users' more positive experiences, but indicated that it was still "worth considering as an alternative to the Plus 1".[54]

Software Bargains Plus 1

In mid-1989, Software Bargains announced an expansion providing different levels of Plus 1 functionality, offered as a bare printed circuit board without casing and in three different variants: the basic model offered one cartridge port and was bundled with View and Viewsheet cartridges for £29.95; an extended model offered one cartridge port and a printer port with the two bundled cartridges for £36.95; the full model offered two cartridge ports, printer port and the bundled cartridges for £39.95. Various board upgrade options were also offered between the variants, with the product being described mainly as a vehicle to expose the bundled software packages to as many as 150,000 owners of the estimated 200,000 Electrons in the UK who "have not yet been able to acquire or use View or Viewsheet".[55]

Communications and networking

To support connectivity, Acorn announced a Plus 2 network interface with availability scheduled for early 1985, together with a RS423 cartridge for the Plus 1.[56] Neither of these products were delivered as announced.

Acorn Plus 2

The Acorn Plus 2 interface was due to provide Econet capability.[57] This interface did not make it to market.[58] However, an Econet interface was produced by Barson Computers in Australia[59] and possibly other individuals and businesses.[58]

Andyk RS423 cartridge

Andyk announced an RS423 cartridge for the Plus 1 providing a serial port, alongside other products, in late 1985. It was priced at £34.99.[60]

Pace Tellstar/Nightingale

Originally reported in mid-1985 as a collaboration between Acorn and Pace Micro Technology,[61] but launched in early 1986, Pace offered a communications product consisting of a RS423 cartridge, bundled with a Nightingale modem and Tellstar communications software,[62] offered at a discounted price of £145.[63]

Jafa Systems RS423 cartridge

Jafa Systems announced an RS423 cartridge in late 1989 to "fill a two year gap in the market", offering a serial connector compatible with the BBC Micro together with an on-board socket for 8 KB or 16 KB EPROM devices or for 32 KB RAM, the latter being configured to present two sideways RAM banks to the system. Write protect functionality was supported to prevent certain ROM software from attempting to overwrite itself if stored in RAM. The cartridge board itself was priced at around £30, with a case costing £5 extra, and an optional 32 KB RAM adding another £20. Support for the E00 ADFS offered by PRES for that company's AP3 disc system was indicated as an application for the sideways RAM.[64]

Disc interfaces

The first disc interface to be announced for the Electron was Pace's Le Box in 1984, offering a single-sided 100 KB floppy drive controlled by the 8271 controller and accessed using the Amcom Disc Filing System, with pricing at £299 plus VAT including the drive or around £199 without. The unit also provided eight sideways ROM sockets and was intended to sit under the Electron itself.[65] The unit was connected via cabling to the expansion edge connector and included its own power supply, and other drives including switchable 40/80 track drives offering up to 400 KB capacity were dealer-supplied options.[66] Although the product was meant to be on sale at the Acorn User Show in August 1984,[65] and had been advertised,[67] it was "discontinued" in early 1985 before getting to market, with a Pace representative indicating that prohibitive pricing of the 8271 chips (each at "over £80 at times") had left the company considering a re-launch of the product should the pricing situation become more favourable.[68]

Following on from Acorn's Plus 3 interface, Cumana, Solidisk, Advanced Computer Products and Slogger all offered disc interfaces for the Electron. Unlike disc systems on the BBC Micro and the Acorn Plus 3, many of the systems released for the Electron did not claim RAM workspace (and raise the PAGE variable affecting applications above the default of &E00), making it easier to use cassette-based software transferred to disc and to run larger programs from disc.[69]

Acorn Plus 3
Acorn Plus 3, showing the connector and disc drive

Launched in late 1984 for a price of £229,[70] the Acorn Plus 3 was a hardware module that connected independently of the Plus 1 and provided a double-density 3½" disc drive connected through a WD1770 drive controller and an ADFS ROM. (The Plus 3 had been rumoured to offer Acorn's DFS and to feature a 8272 double density disk controller before its launch.[56]) There were two versions of the Plus 3 produced: A single-sided and a double-sided drive version. Because the WD1770 is capable of single density mode and uses the same IBM360 derived floppy disc format as the Intel 8271 found in the BBC Micro, it was also possible to run a DFS filing system with an alternate ROM, such as the P.R.E.S AP4 interface.

The Plus 3 reset PAGE to &1D00, reducing the amount of free RAM available to user. The ADFS system could be temporarily disabled (and PAGE reset to &E00) via the *NOADFS command. Alternative WD1770-based DFS and ADFS interfaces such as the P.R.E.S AP4 and 'ADFS E00' products left PAGE at &E00, and did not require the presence of the ZYSYSHELP file (see below)

Disks had to be manually mounted and dismounted using the *MOUNT / *DISMOUNT commands, or using the CTRL-A+BREAK key combination. Disks could also be booted from via the standard ⇧ Shift+BREAK key-combination, if the !BOOT file was present on the disk. This behaviour was the same as on the BBC Micro.

The Plus 3 included an uprated square black power supply unit with mains cord, manufactured by STC, designed and manufactured in England to BS 415 and BS 5850, that was designed to power the Plus 3, in addition to the Electron and the Plus 1 interface as well. This replaced the original cream-coloured "wall wart" style power supply, designed to BS 415 and manufactured in Hong Kong.

  • Original; Part no: unknown; input 220/240 V AC/50 Hz; output 19 V AC/0.737 A/14 W: Usage: Electron, Electron+Plus1
  • Uprated Part no: 865–010; input 240 V AC/50 Hz 50 W; output 21 V AC/1.75 A/36.75 W: Usage: Electron+Plus3, Electron+Plus3+Plus1

The original Electron edge connector was repeated on the back of the Plus3, in addition to a secondary smaller edge connector, that enabled additional drives to be connected (Shugart-compatible connection). These required their own power-supply. The secondary edge connector could not power external drives.

Repair note: If the internal power-supply connector, used to power the existing internal 3.5" drive is damaged, and requires replacement, then the original AMP 800-930 4-pin connector, which was already in short supply during the original production run, may be replaced with a Molex 5264 50-37-5043 "Mini-SPOX" connector as an alternative.[71]

If using the Plus 3 in screen modes 0–3, the pseudo-variable TIME would be thrown off, as the interrupts were disabled during disk access in these modes.

ADFS quirks

The ADFS file format used the ascii bytes Hugo to delimit the directory names on the disc, named after ADFS author Hugo Tyson. Another quirk was the presence of the file ZYSYSHELP which was required by the system, and created during formatting. This was a kludge. Acorn's v1.0 ADFS implementation on the Electron was unreliable when writing to the first few tracks of a floppy disc, so this was a "fix" and simply involved writing a file full of garbage to the suspect part. The ADFS would then skip it. Disc corruption could also occur if attempting to use the *COMPACT command without disabling the blinking cursor with the following command:

VDU23,1,0;0;0;0;

This was due to the fact that the *COMPACT command used screen memory (by default) as working space during the operation, and the software-implemented blinking cursor corrupted that memory space. An alternative would be to give arguments to make it use non-screen memory for workspace, for example *COMPACT 40 20 in MODE 6. Tyson recalls that this bug was found during late testing but not fixed in the initial ROM release in order to avoid late changes, as workarounds exist.[72][73]

Disc formatting was done via the *EFORM command, vs the more familiar *FORM40/*FORM80 DFS commands. Note additionally that the *EFORM command differs from the equivalent *AFORM command for the 1770 ADFS on the BBC Microcomputer. This is possibly as a result of needing to create the ZYSYSHELP file on the Electron. The *EFORM command was only supplied on the Welcome disc that was shipped with the Plus3, and was not included in the ROM.

Advanced Plus 3
The PRES Advanced Plus 3 with a 3½" drive

Produced by Advanced Computer Products (ACP) and sold by PRES, the Advanced Plus 3 (AP3) was a Plus 1 cartridge interface using the WD1770 controller, supplied with Acorn's ADFS and a single-sided 3½" disc drive for £99 plus VAT, offering equivalent functionality to the Acorn Plus 3.[74] Announced in late 1987, the product was made possible by an agreement between ACP and Acorn to license the ADFS software.[75] As with many disc interfaces for the Electron, since the interface provided a connector for the drive, this made it possible to connect a 5¼" floppy disc drive (more common amongst BBC Micro owners) or the more typical 3½" drive.

PRES later released a version of ADFS with support for PAGE at &E00, this being achieved by using RAM provided by the Advanced Battery Backed RAM (ABR) cartridge. This version also fixed two notable bugs in Acorn's ADFS, eliminating unreliability when accessing the first tracks on a disc which had previously necessitated the writing of a file (ZYSYSHELP) as a workaround, and switching off the text cursor during disc compaction which had previously caused disc corruption (since the disc data would be processed using screen memory during this operation, and the cursor would modify that data when blinking). The ROM image was supplied on disc for £17.19, whereas a bundle of the ROM and ABR cartridge was £50.95.[76]

In 1989, the Advanced Plus 3 Mark 2 was launched, offering a double-sided drive in place of the single-sided drive previously offered. This meant that the storage capacity of each disc was increased from the 320 KB of the original Plus 3 to 640 KB (this being supported by ADFS on the Master Compact).[77]

Cumana Floppy Disc System

Early in 1985,[78] Cumana released a cartridge-based interface providing support for double density storage, a real-time clock and calendar for timestamping of files, and a spare ROM socket for user-fitted sideways ROMs.[79] The filing system used was Cumana's own QFS, supporting 89 files per disc,[80] PAGE at &E00,[69] a non-hierarchical catalogue, ten-character filenames, with a format not directly compatible with either of Acorn's DFS or ADFS.[79] The interface itself cost £149.95 when originally announced,[78] but settled at around £115.95 including VAT, also being offered in a promotional bundle with a 5¼" drive for £224.15 including VAT.[81] Later pricing put the interface at £74.95 including VAT.[82]

Solidisk EFS

In mid-1985, Solidisk released a cartridge-based interface with support for single and double density storage and providing Acorn DFS and ADFS compatibility, 16 KB of on-board sideways RAM, and a connector for a Winchester hard drive. The cartridge itself cost £59, with a bundle including a double-sided, double density, 3½" drive costing £200.[83] A 20 MB hard drive was offered at a price of £805.[84]

Advanced Plus 4

Announced in early 1986,[85] the Advanced Plus 4 (AP4) from Advanced Computer Products was a cartridge-based interface employing the WD1770 controller and featuring ACP's 1770 DFS product, providing compatibility with Acorn's DFS from the BBC Micro and thereby supporting seven-character filenames and up to 31 files per disc. However, 8 KB of on-board static RAM was used as workspace for the filing system, keeping PAGE at &E00. An extra ROM socket was provided for a user-fitted sideways ROM, and being a 1770-based interface, it was reported that Acorn's ADFS could be used instead, although since it was not aware of the additional RAM, PAGE would be raised to &1D00 as it would be when using Acorn's Plus 3. The interface was priced at £69.55 plus VAT.[86]

Slogger Electron Disc System and Pegasus 400

Slogger, an established producer of expansions and a reseller of other disc systems, introduced the Electron Disc System in early 1987, priced at £74.95, featuring the Cumana Floppy Disc System interface,[87] which was combined with an Acorn-compatible DFS, SEDFS, having the capability of reading 40-track discs on 80-track drives plus support for Slogger's tape-to-disc conversion products,[88] and reported as offering "virtual 100 per cent 8271 emulation" for compatibility with traditional DFS software.[89] The SEDFS ROM was also available separately for existing Cumana interface owners, priced at £24.95.[87]

The SEDFS was later bundled with Slogger's own cartridge-based interface and a 40/80-track switchable drive offering up to 400 KB storage per disc, with the bundle taking the Pegasus 400 name, introduced as part of a sales tour towards the end of 1987. This package of interface and drive cost £130.[90] The precise DFS variant used by the Pegasus 400 system introduced "typeahead" support permitting keystroke buffering during disc activity on systems with the Turbo-Driver or Master RAM Board fitted and enabled.[91]

Slogger/Elektuur Turbo boards

Announced in early 1986, the Slogger Turbo-Driver was a professionally fitted upgrade priced at £42.[92] The board itself plugged into the CPU and BASIC ROM sockets on the main circuit board of the Electron, which merely involved removing socketed components on very early Electron models, but required desoldering work and therefore benefited from a fitting service for later units. The performance benefit of fitting the board was to make some programs, particularly those running in the high bandwidth modes (0 to 3), run up to three times faster.[93]

The direct origins of the Slogger product appear to be a board designed by Andyk Limited,[94] announced as the Fast Electron Board in late 1985 with a price of £29.99,[60] whereas the Elektuur modification was described in an article in Dutch Electronics magazine Elektuur and intended for users to perform at home.[95]

The Slogger and Elektuur Turbo boards were born out of a hack initially devised at Acorn. By shadowing the lowest 8 KB of RAM with a static RAM chip outside of reach of the ULA, the CPU could always access it at 2 MHz. The tradeoff was that the screen could not be located in that 8 KB. In practice the operating system ROMs always put the screen into the top 20 KB and as a result this probably only broke compatibility with around 2% of software. Speeding up the low portion of memory is particularly useful on 6502 derived machines because that processor has a faster addressing mode for the first 256 bytes and so it is common for software to put any variables involved in time critical sections of program into that region.

The cost of the 64 Kbit SRAM chip would have been more than that of doubling the four 64 KBit DRAM chips to give 8-bit RAM access, fixing both the modest memory and poor performance issues of the Electron.

Slogger Master RAM Board

Introduced at around the start of 1987 and priced at £64.95 fitted or £54.95 as a kit, the Master RAM Board offered the familiar turbo mode from the Slogger Turbo-Driver alongside a shadow mode providing 32 KB of static shadow RAM in addition to the existing 32 KB, thus giving 64 KB in total. So-called "legally written software", this being software using the operating system calls and not writing directly to the screen, could function without significant modification, making substantially more memory available for BASIC, View, Viewsheet, language ROMs and many other applications.[96] By providing extra storage this modification also allowed some games and applications intended for the BBC Micro to function on the Electron despite the lack of a native Mode 7.[97]

Applications could not directly address video memory in shadow mode without modification, so it was incompatible with most games, although there is no inherent reason why a game could not be written to function in shadow mode. A switch mounted through the case switched between normal, turbo and shadow modes.[98]

Towards the end of the Electron's commercial lifetime, the Turbo-Driver and Master RAM Boards were offered already fitted to new Electrons in an attempt to increase sales. For a time, Jafa Systems manufactured their own equivalent of the Master RAM Board in order to support their own product range.[97]

Mode 7 display expansions

Of the capabilities present in the BBC Micro but absent from the Electron, the Teletext-style Mode 7 was particularly conspicuous because of the very low memory usage of that mode (1000 bytes) and the high number of BBC programs that used it to maximise available memory for program code and data while also providing a colourful 40-column textual display with simple low-resolution graphical decorations.[99](pp90)

Such display capabilities, desirable in their own right on low-memory computers, were also desirable for delivering content through low-bandwidth communications channels such as that from Teletext and Viewdata services. However, access to such services can be considered to be a separate capability, and the BBC Micro needed to be upgraded to complement its display capability with the Teletext or Prestel adapters to receive such over-the-air or online content.[99](pp90)

Jafa Systems provided a number of solutions to remedy the absence of a Teletext display capability. Morley Electronics instead chose to offer an expansion combining the display and reception capabilities.

Sir Computers

In late 1984, Sir Computers announced a Mode 7 adapter unit that plugged into the Electron's expansion connector.[100] Unfortunately, Sir Computers ceased trading before the product was brought to market.[101]

Jafa Systems

Released in 1987 at a price of £89,[102] the Mode 7 Mark 1 Display Unit was a separate unit that connected to the Electron's expansion connector and featured a Motorola 6845 display controller and Mullard SAA5050 character generator to replicate the main elements of the BBC Micro's Teletext display solution. This only used 1 KB of memory for the display, with the expansion listening to display memory write accesses and buffering the data in its own memory.[note 1] A ROM was included to extend the operating system to allow activation of Mode 7 as a genuine screen mode and to provide extra commands and to support keyboard shortcuts used on the BBC Micro to emit Teletext control sequences. To support the output of both the Mode 7 display and the existing video output, a lead connected the Electron's RGB output to the expansion, with the expansion providing only RF (television) output.[103]

Conscious of the relatively high price of the Mark 1 unit,[102](pp19) John Wike of Jafa devised and, at the end of 1988, introduced a software-based Mode 7 Simulator, priced at £25, supplied on a ROM cartridge that rendered the Mode 7 display in a low-resolution, 8-colour graphics mode. Although cheap and effective in enabling use of some software that only used official operating system routines for text output, this solution proved very slow because the Electron had to be placed into the high-bandwidth Mode 2 display to be able to show eight colours at once. In doing so, the CPU spent a lot of time drawing representations of Mode 7 characters and graphics that in a hardware solution would be achieved without any demand on the CPU. It also used up 20 KB of RAM for the graphics display rather than the 1000 bytes of a hardware Mode 7.[104]

A conceptually similar predecessor to the software-based simulator was published by Electron User in early 1987, offering a monochrome Mode 4 simulation of the Teletext display, using the lower 25 character lines of the screen to show the Teletext output, reserving several lines at the top of the screen for a representation of Mode 7 used to prepare the eventual visual output. However, the program did not support direct access to Mode 7 memory locations. The author noted that a Mode 2 version would have been possible but would have required a redesigned character set and "too much memory".[105]

A further refinement of the hardware solution was introduced in 1989 with the Mode 7 Mark 2 Display Unit, which retained the SAA5050 character generator but omitted the 6845 display controller, and was fitted internally in the Electron itself instead of being housed in an external unit, although some kind of ROM expansion unit was needed to hold the driver/utilities ROM. It used software to ensure that the SAA5050 was fed with the correct character data. A software ROM would put the machine into a two-colour, 40-column graphics mode (thus providing one byte per character), and as the ULA read display data from memory in the usual fashion, the SAA5050 would listen to the data it was reading and produce a Mode 7 interpretation of the same information,[106](pp3) this being achieved by fitting a board on top of the ULA connecting to its pins.[106](pp20) When necessary the hardware would switch between the conventional Electron graphics output and the Mode 7 output being produced by the add-on, feeding it to the Electron's built-in video output sockets via the red, green and blue lines on the motherboard.[106](pp22)

The disadvantage to this system is that while the SAA5050 would expect to be repeatedly fed the same 40 bytes of data for every display scanline of each character row, the ULA would read a different set of 40 bytes for every display scanline in order to produce a full graphics display. A software ROM worked around this by duplicating the data intended for a Mode 7 display in memory. Although this produced a Mode 7 that had less of an impact upon CPU performance than a software solution, gave the same visual quality as the BBC Micro, and supported direct access to Mode 7 screen addresses as well as accesses via operating system routines, it still used 10 KB of memory for the display and reduced the amount of readily-usable application memory (as indicated by HIMEM) by another 6 KB.[106](pp3–4)

However, with users increasingly able to rely on expansions such as the Slogger Master RAM board to provide more memory, and with this combination of expansions acknowledged throughout the user manual, the emphasis of the Mode 7 Simulator and Mark 2 Display Unit was arguably to deliver the actual display capabilities for those applications that needed them, instead of using Mode 7 as a way of economising with regard to memory usage, and to do so at a reasonable price. In this latter regard, the Mark 2 model was available as a kit costing £25 or as an assembled product (requiring some soldering) costing £49, with a fitting service available for £10.[107]

The Jafa interfaces did not provide a Teletext or Viewdata reception capability, but the Mark 2 was explicitly stated to work in conjunction with the Morley Electronics Teletext Adapter.[106](pp15) Meanwhile, the manual for the Mark 2 noted that the product would provide the functionality of a Viewdata terminal if combined with Jafa's RS423 cartridge.[106](pp2)

Morley Electronics

Morley Electronics produced a Teletext Adaptor expansion for the BBC Micro and the Electron. Since the BBC Micro has the Mode 7 display capability, the model aimed at the BBC Micro merely provided the content reception capability needed to receive and decode Teletext signals, connecting to the user port and power supply. However, the Electron models provided both display and reception capabilities, doing so by routing either the RGB or UHF signals (depending on the model) through the unit in order to introduce the Mode 7 output produced by the unit, also connecting via a cartridge. The Teletext display capabilities in the Electron models exceeded those of the BBC Micro, with one reviewer noting that the enhanced capabilities permitted "black text on a coloured background, something I've always wanted to do on my Beeb".[99](pp90) The UHF model of the Electron adapter also supported overlaying of Teletext onto video and framing of video.[99](pp91)

Second processor expansions

Acorn did demonstrate a prototype "Tube" interface for the Electron alongside the Plus 3 interface at the Compec exhibition in November 1984,[108] although this was never brought to market directly by Acorn.

Advanced Plus 5

Despite Acorn's withdrawal from the Electron peripheral market, Advanced Computer Products developed the Advanced Plus 5 (AP5) expansion,[109] featuring Tube, 1 MHz bus and user port interfaces, which plugged into a Plus 1 cartridge socket. This provided a sufficient level of compatibility that both the 6502 and Z80 second processor products from Acorn were shown to work, providing a Tube implementation that was "as faithful as you can get", with it also being noted that the Electron being available for as little as £50 at that point in its commercial lifespan was a "very cheap way of getting a CP/M machine".[50] Some differences in the memory map of the Electron meant that BBC Micro software would need modifications to work on the Electron with AP5. The price of the unit in late 1986 was £66.70.[110]

The additional facilities of the AP5 alongside the Tube interface permitted various expansions for the BBC Micro to be made available for the Electron. These included the Hybrid Music 5000[111] and the AMX Mouse.[112]

PMS Electron Second Processor

In 1986, Permanent Memory Systems announced a second processor product for the Electron, the PMS-E2P, as a self-contained cartridge for use with the Plus 1 containing a second 2 MHz 6502A processor plus 64 KB of RAM, priced at £89.[113] This was based on a product originally developed by John Wike of Jafa Systems.[102] It provided an alternative to buying the combination of the P.R.E.S. Advanced Plus 5 and Acorn 6502 2nd Processor.[114]

Sound system expansions

Despite the Electron having only limited sound generation capabilities, few expansions were offered to overcome the machine's limitations.

Millsgrade Voxbox

Advertised in late 1985, the Voxbox by Millsgrade Limited was an expansion connecting to the Electron's expansion connector that provided allophone-based speech synthesis, with driver software provided on cassette.[115] The supplied software supported the definition of spoken words built up from the allophones - these allophones or sounds being stored in the expansion's own ROM - and for catalogues of words to be created and saved. A program was supplied that extended BASIC to allow the use of the synthesiser in user programs.[116]

Sound Expansion cartridge

Originally announced in 1987 by Project Expansions to be priced at around £40,[117] the Sound Expansion cartridge could be fitted in a Plus 1 (or compatible) slot and provide sound output equivalent to that of the BBC Micro, with Superior Software's Speech bundled as a "limited offer".[118] A product of the same name and with similar functionality was subsequently sold by Complex Software for around £55, employing its own adjustable speaker in the cartridge unit.[119]

Hybrid Music 5000

Hybrid Technology's Music 5000 was adapted and released by PRES for use with the 1 MHz bus of the Advanced Plus 5 expansion, with the Music 5000 itself priced at £113.85. The only functional differences between the Electron adaptation and the original BBC Micro unit involved the use of Mode 6 for the display and the reduced performance of the Electron imposing some limitations on processing in programs written for the system.[111]

Merlin M2105

An unusual variant of the Electron was sold by British Telecom Business Systems as the BT Merlin M2105 Communications Terminal. This consisted of a de-badged Electron plus a large expansion unit containing 32 KB of battery-backed RAM (making up 64 KB of RAM in total), 48 KB of ROM, a Centronics printer port, a modem, and the speech generator previously offered for the BBC Micro.[120] The ROM firmware provided dial-up communications facilities. 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,[121] these were used by the Interflora florists network in the UK for over a decade.[122][123] 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.[124]

This generic product combination of the Electron and accompanying expansion was apparently known as the Chain during development,[120] with British Telecom having intended the M2105 to be a product supporting access to an online service known as Healthnet.[120][125] 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",[126] 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".[127]

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".[128]

Technical information

Much of the core functionality of the BBC Micro - the video and memory controller, cassette input/output, timers and sound generation - was replicated using a single customized 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.[6](pp1)

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,[129] 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.[130] In addition, various control signals provided by the CPU and ULA are exposed via the Electron's expansion connector.[131]

For Issue 1-4 motherboards, the ULA had an issue similar to those experienced by other 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 start exhibiting 'hanging' or other startup-failure issues, such as a continuous 'startup beep'. This was despite a metal cover, and 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.[132][133][134][135] Issue 5 and 6 boards utilized a different epoxy resin covering directly over the ULA, which resolved this issue.[136]

The keyboard includes a form of quick keyword input, similar to that used on the Sinclair Spectrum, through use of the '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.[137]

The ULA mediates access to 32 KB of addressable RAM using 4 64-kilobit RAM chips (4164), 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 RAS), delivering a maximum transfer rate to or from RAM of one byte per 2 MHz cycle.[138] In generating the video signal, the ULA is able to take advantage of this 2 MHz bandwidth when producing the picture for the high-bandwidth screen modes.[13](pp26) Due to signalling constraints, the CPU can only access RAM at 1 MHz, even when it is not competing with the video system.[138](pp7)

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.)[138](pp7) In other modes the CPU and video accesses are interleaved with each accessor acquiring bytes at 1 MHz.[138](pp7)

In contrast, the BBC Micro employs one or two sets of 8 16-kilobit devices, with the RAM running at twice the speed (4 MHz) of the CPU (2 MHz), allowing the video system (screen refresh) and CPU memory accesses to be interleaved, with each accessor able to transfer bytes at 2 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 MHz, however.[138](pp7)

Hardware
Acorn ALF03 Data Recorder
  • CPU: Synertek SY6502A[139]
  • Clock rate: variable. CPU runs at 2 MHz when accessing ROM and 1 MHz when accessing RAM. The CPU is also periodically halted.[138] The Electron is widely misquoted as operating at 1.79 MHz after measurements derived from speed testing against the 2 MHz BBC Micro for various pieces of 'common software'.
  • Glue logic: Ferranti Semiconductor Custom ULA
  • RAM: 32 KB
  • ROM: 32 KB
  • Text modes: 20×32, 40×25, 40×32, 80×25, 80×32 (all text output produced by software in graphics modes)
  • Graphics modes: 160×256 (4 or 16 colours), 320×256 (2 or 4 colours), 640×256 (2 colours), 320×200 (2 colours – spaced display with two blank horizontal lines following every 8 pixel lines), 640×200 (2 colours – spaced display)
  • 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
  • Dimensions: 16×34×6.5 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, 19V AC

Quirks
Exile is an example of a game where the developers left non-graphical data visible in the display buffer to gain additional memory space.

Like the BBC Micro, the Electron was constrained by limited memory resources. Of the 32 KB RAM, 3½ KB was allocated to the OS at startup and at least 10 KB was taken up by the display buffer in contiguous display modes.

Due to the timing of interrupts it was possible to disable either the top 100 or bottom 156 lines of the display with palette changes. Many games took advantage of this, gaining storage by leaving non-graphical data in the disabled area.

Other games would load non-graphical data into the display, leaving it visible as regions of apparently randomly coloured pixels.

Although page flipping was a hardware possibility, the limited memory forced most applications to do all their drawing directly to the visible screen, often resulting in graphical flicker or visible redraw. A notable exception is Players' Joe Blade series.

FireTrack: smooth vertical scrolling

Although programs can alter the position of the screen in memory, the non-linear format of the display means that vertical scrolling can only be done in blocks of 8 pixels without further work.

FireTrack exploits a division in the way the Electron handles its display – of the seven available graphics modes, two are configured so that the final two of every ten scanlines are blank and are not based on the contents of RAM. If 16 scanlines of continuous graphical data are written to a character-block-aligned portion of the screen then they will appear as a continuous block in most modes but in the two non-continuous modes they will be displayed as two blocks of eight scanlines, separated in the middle by two blank scanlines.

In order to keep track of its position within the display, the Electron maintains an internal display address counter. The same counter is used in both the continuous and non-continuous graphics modes and switching modes mid-frame does not cause any adjustment to the counter.

FireTrack switches from a non-continuous to a continuous graphics mode part way down the display. By using the palette to mask the top area of the display and taking care about when it changes mode it can shift the continuous graphics at the bottom of the display down in two pixel increments because the internal display counter is not incremented on blank scanlines during non-continuous graphics modes.

Exile: sampled speech

Exile turns the Electron's one channel output into a digital speaker for PCM output.

The speaker can be programmatically switched on or off at any time but is permanently attached to a hardware counter so is normally only able to output a square wave. But if set to a frequency outside the human audible range then the ear can't perceive the square wave, only the difference between the speaker being switched on and off. This gives the effect of a simple toggle speaker similar to that seen in the 48 KB Sinclair ZX Spectrum. Exile uses this to output 1-bit audio samples.

Frak! and Zalaga: Polyphonic music

As part of their copy protection, illegal copies of Aardvark Software's "Frak!" and "Zalaga" would cause a pseudo-polyphonic rendition of Trumpet Hornpipe, the Captain Pugwash theme tune, to play endlessly rather than loading the game properly (Pugwash being a pirate). On the Electron version of Frak!, the tune was the main theme from "Benny Hill" (Boots Randolph's "Yakety Sax"). The polyphony was achieved via fast note-switching to achieve the necessary chords.

See also: List of Acorn Electron games

Although not as well supported by the biggest software publishers as rivals like the Commodore 64 and Sinclair ZX Spectrum, a good range of games were available for the Electron. The traditional BBC Micro publishers such as Acornsoft, Superior Software and Micro Power offered the widest support. Notable popular games particularly associated with the Electron include:

Crystal Castles is an example of a popular arcade game officially ported to the Electron (US Gold).

There were also many popular games officially converted to the Electron from arcade machines (including Crystal Castles, Tempest, Commando, Paperboy and Yie Ar Kung-Fu) or other home computer systems (including Impossible Mission, Jet Set Willy, The Way of the Exploding Fist, Tetris, The Last Ninja, Barbarian and SimCity).

Despite Acorn themselves effectively shelving the Electron in 1985, games continued to be developed and released by professional software houses until 1991. There were around 1,400 games released for the Acorn Electron, several thousand extra public domain titles were released on disc through Public Domain libraries. Notable enterprises which produced discs of such software are BBC PD, Electron User Group and HeadFirst PD.

Emulation

Several emulators of the machine exist: ElectrEm[140] for Windows/Linux/macOS, Elkulator[141] for Windows/Linux/DOS, ElkJS[142] is a browser-based (JavaScript/HTML5) emulator, and the multi-system emulators MESS and Clock Signal[143] feature support for the Electron. Electron software is predominantly archived in the UEF file format.

There are also two known FPGA based recreations of the Acorn Electron hardware. ElectronFPGA[144] for the Papilio Duo hardware and the Acorn-Electron[145] core for the FPGA Arcade "Replay" board.

Design team

The operating system ROM locations 0xFC00-0xFFFF contain the following text, which is different from the 'thanks' list in the original BBC Model B (Code here):

(C) 1983 Acorn Computers Ltd. Thanks are due to the following contributors to the development of the Electron (among others too numerous to mention):- Bob Austin, Astec, Harry Barman, Paul Bond, Allen Boothroyd, Ben Bridgewater, Cambridge, John Cox, Chris Curry, 6502 designers, Jeremy Dion, Tim Dobson, Joe Dunn, Ferranti, Steve Furber, David Gale, Andrew Gordon, Martyn Gilbert, Lawrence Hardwick, Hermann Hauser, John Herbert, Hitachi, Andy Hopper, Paul Jephcot, Brian Jones, Chris Jordan, Computer Laboratory, Tony Mann, Peter Miller, Trevor Morris, Steve Parsons, Robin Pain, Glyn Phillips, Brian Robertson, Peter Robinson, David Seal, Kim Spence-Jones, Graham Tebby, Jon Thackray, Topexpress, Chris Turner, Hugo Tyson, John Umney, Alex van Someren, Geoff Vincent, Adrian Warner, Robin Williamson, Roger Wilson.

Additionally, the last bytes of both the BASIC ROM and 'Plus 3 Interface' ADFS v1.0 ROM include the word "Roger", thought to be a reference to Roger Wilson.[146]

The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.

See also

References
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Notes
  1. Here, the MAME source code emulating the Mark 1 adapter is informative.
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