Acorn System Computer Build Guidance

I have created this page after being asked what boards are required to create a System Computer so here goes:

System 1

A few years ago Trevor Hamblett was selling replica System 1 boards and in fact I have several of these in my Systems, unfortunately I think he has moved on to other projects and no longer sells these boards.  I have created my System 1 Trainer which is a single board version of the original System 1 design.  It incorporates parts that are still available and is 100% compatible with the original hardware.

System 2

A minimum System 2 Computer requires the following:

The System 1 Trainer board is designed to be cut down to Eurocard size creating a 6502 CPU and Cassette Interface board for a System 2.

The original System 2 used an 8K RAM & 8K ROM Board which provides 8K of RAM (using 2114's so keeps you warm in the winter) and sockets for two 4K EPROM's, typically BASIC or ADE and a Floating Point ROM.  A second option is the Control Universal CUMEM Memory Carrier Board, obviously not an Acorn product but it offers more flexibility though is a little more complicated to configure.

The System 2 was intended to be used with the 40x25 Teletext VDU Board which gives you colour and (very) chunky graphics.  An alternative is the 80x25 VDU Board though this is only black and white.  I have created versions of COS, the Cassette Operating System for either board.  There are two options for the 80x25 VDU, either a replica of the original Acorn VDU Board or my enhanced version, this corrects the rather crude RC delays used in the Acorn design with proper timed delays giving a better and more repeatable picture.

The System 2 (and 3/4) all use the (1MHz) 6502 Card which does not have address buffering (unlike the 6502A board which does) so you need to use one of the Buffered Backplanes either the 8-Slot or 12-Slot, the number of slots is actually 1 more as the CPU board slot is not counted.

Of course you can if you wish build a System 2 with the 6502A CPU Board and run it at 1MHz (some of the boards may not work reliably at 2MHz) then you can use the unbuffered Backplane

There are two keyboard options, either a replica of the original Acorn ASCII Keyboard or my Electron Keyboard, both require parts from donor computers.

The ASCII Keyboard is designed to fit in an Atom Case as this is what Acorn originally used, you may not want (or be able to afford !) to destroy an Atom but the keyboard can be used as a bare PCB.  The keyboard itself comes from a BBC Computer.

The Electron Keyboard is design to fit in an Electron Computer Case, I have less of an issue with destroying one of those (!) and they are cheaper and more widely available, in fact I have sold on the unused parts and recovered most of the original cost of the computer.  It uses the keyboard from the Electron and can be fitted with the Electron keycaps though I think it looks smarter with BBC Keycaps ?

I have built two of each but only one of the Atom keyboards is cased, the Electron Keyboards are the ones I use on my most used machines as they are more compact taking up less desk space.

Once you have finished your System 2 then you will want to upgrade it of course, there are a few options:

or you may want to go the whole hog and upgrade to a System 3 . . .  

System 3/4

The System 3/4 is just a System 2 with Floppy Disks for which you need a Floppy Disk Controller (FDC) Board.  The original System 3 was intended to be used with 5.25" disk drives and in fact because it was based on the 8271 controller (as was the BBC) its not reliable with 3.5" drives.  My replica board incorporates a mod which allows it to work with 3.5" drives.  You can also use GOTEKs which is actually what I use day-to-day as it makes transfer to/from the PC world much easier.

The System 3 was usually used with the 40x25 Teletext VDU Board however it is possible to use it with the 80x25 VDU Board, I have created versions of DOS that work with either VDU.

The System 4 is just a larger version providing more room for expansion.

System 5

The System 5 moves up a gear to a 2MHz 6502A CPU (a precursor of things to come - the BBC Computer) so this uses a completely different and more complex 6502A CPU Board which has the address bus buffered so a 10-Slot Unbuffered Backplane is required.

The System 5 was intended to be used without BASIC etc. in EPROM but loaded off disk as required, to achieve this more memory is required.  The 32K Dynamic RAM Board provides that but does not have slots for EPROMS like the 8K RAM board, if you want EPROM sockets then I suggest fitting a CUMEM.

The System 5 was usually used with the 80x25 VDU Board.

Enclosures

There was a time when 19" Eurocard racks appeared on eBay but I haven't seen any for some time, VeroTec, who are latest incarnation of the original Vero company, still sell KM6 racks and all the accessories required, the last time I ordered from them there was a minimum £25 order value and £10 postage so make you sure you order all that you need in one go !

They will send you a catalogue which is worth having as it has some useful drawings at the beginning.

They don't seem to publish prices so I have included a table of things I have purchased and what I paid at the time which may help.

957-235357 14HP Panel each £9.90
950-232664 160mm Module Guide pack of 10 £3.70
957-236390 24HP 3U 160mm Module each £33.87
950-232054 3U Rack each £54.70
957-236071 42HP 6U Panel (no handle) each £14.28
174-39399 42HP Handle pack of 10 £30.68
957-236391 42HP Module each £40.72
957-235360 42HP Panel each £17.50
957-235348 4HP 3U Front Panel each £6.55
957-235349 5HP 3U Front Panel each £7.22
174-39389 5HP Handle pack of 10 £8.98
957-235349 5HP Panel each £7.22
957-235351 6HP Panel each £7.98
950-232662 Card Guides pack of 10 £4.35
950-249489 DIN 41612 Mounting Extrusion 84HP pair £14.26
174-39373 M2.5 Front Panel Bush pack of 100 £21.55
174-39374 M2.5 Slotted Screw pack of 100 £13.46
173-12525 Plastic Bracket pair £1.25
173-232669 Plastic Bracket pack of 10 £3.75
950-229781 Rear Tie Bar (DIN 41612) each £8.70

Most of the components used on the boards are still available though you may have to take a bit of a gamble sometimes with Chinese suppliers, one good UK source is Mark at RetroClinic who seems to have all sorts of useful parts squirreled away if you ask. 

 

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