The Acorn Atom - Uncovered

TheCorfiot

Out in the Wild
VIP
Joined
Mar 28, 2008
Posts
8,698
Country
Greece
Region
Corfu
Good Morning Forum Friends.

As Promised, We have a full Acorn Atom Photoshoot and write-up.

Huge Thanks must go to Iainjh from ALL of us as the system belongs to him and these are just photos of the full repair / refurb and upgrade which I undertook for him.. ;)

These Systems are incredibly rare and quite possibly the only chance we will get on Amibay to have a good look inside..

Let's go back to 1980 when I was a 13yr old schoolboy and was enjoying dabbling with Electronic circuits, when asked what I wanted for my 12th Birthday I replied "A Soldering Iron Please Ma", no PS3's then I suppose :LOL:

I was completely oblivious to computer's, didnt know what they were and had no interest to find out.
My Best Friend at School got bitten by the bug & saved all his pocket money up and ordered himself a nice Basic Acorn Atom..
Then I had a phonecall one friday afternoon by a very upset friend who told me that his computer had arrived but it was a bag of bits & a circuit board.....Armed with soldering Iron and basic tools I set off.

I disappeared for the whole weekend as we carefully followed the little blue technical manual & built the board up step by step...my eyes were red raw from the solder fumes...I was still unconvinced but helping my friend out.

After carefully inserting all the chips and triple checking everything, On the Sunday morning we hooked it up to his TV and powered on...
Nothing......I was now bored.
Finally we tuned the TV in and were greeted by a rolling Screen with the prompt "ACORN ATOM" in the top left...
At this point my Interest shot up & we adjusted the Frame hold on the TV to stabilize the Picture...
Now I was hooked, this box of circuits would respond to our written english commands and communicate back via the TV .....And so my journey began...
The System we built was the Basic 2K RAM and 8K ROM Atom and evrey other saturday we would run into the town centre to buy another 1K of RAM and begin to unleash the HiRes Graphics Modes....

That my friends is why the Acorn Atom is so special to me and I didn't hesitate when Iain asked me to restore this beauty...

Note --- The little Blue Technical Manual in some of the Pix, that's the original one that I used and followed back in 1980 and it served it's use again this time around too...

Anyway,,,,, Onto the Technical stuff :LOL:

Acorn began designing computers based on the old Eurocard System, all mounted in a rack...
The System 2,3,4,5

By the time of the System 3 The ZX80 was looming on the Horizon & Chris Curry (MD of Acorn) gave his team the task of building a cut down system 3 into a home Microcomputer.
Roger / Sophie Wilson (Designer of BBC Basic - the Beeb & the Arc's & the ARM chip) was the chief designer and this is what they came up with.
Now bear in mind the footprint & design of the Atom's Aesthetics was one of the first of it's kind, and indeed most Home computer's (apart from Sinclair) borrowed and expanded on this type of enclosure, just look at the C64, Dragon, Amiga....etc

The Atom has some really wierd design constraints but this was the day before ULA's and PAL's etc..everything is built up of discrete Logic and off the shelf processors....

Basic Spec

CPU - 1 Mhz 6502 processor...

RAM - Max 12K, split into 6K Lower Text or CPU only RAM and 6K Upper Text or Video RAM which could be accessed by the CPU & VDG.

ROM - Max 16K, split into 8K OS&Basic, 4K Floating point and Colour ROM, and 4K socket available for Utility or 3rd party ROMS's

Video - The Motorola 6847 VDG as used by the Dragon / CoCo later on, This provides 256x192 max resolution using 6K Upper Text Ram.
This was an odd choice as this VDG is a 60Hz NTSC device (remember the rolling screen I had in 1980)
When used in a PAL CoCo or Dragon, The VDG is stalled at the end of every frame & clever descrete logic adds an extra 24/25 lines to the display to produce a Pseudo PAL Signal.
Although the Atom did not have Colour as std, Acorn did produce a Colour Board for it which provides exactly the same colour / res facilities as the CoCo & dragon...
Another nasty side effect was Noise on the Screen when the CPU needed prolonged access to the Upper Text RAM (See Later), effectively blocking the 6847..
Still in those day's there was not really any choice regarding VDG availability.

All the RAM on the Atom is Static Ram, it does not need refreshing, it uses 2114 SRAM's which are 1K by 4bits so two are needed for an additional 1K of RAM.
The older version of these chips was also extremely power hungry, ie 100mA per 1K, or 1.2Amps for 12K
The later devices cut this down to 35mA per 1K.
You can see Iain's board has a mixture of old & new type SRAMS...

Atom development on the Stardot forums is certainly active and some of the guy's notably Prime & SirMorris have developed some nice addons...

Prime has designed and produced the ROM-RAM board...
It provides full ROM capacity for the Atom but also 32K of RAM, this means the Lower Text 6K can be removed from the main board, but the 6K Upper or Video RAM must stay.
Most importantly this board contains a noise killer circuit which effectively doubles the clock speed of the Video RAM and removes ALL noise from the screen, it also speeds up processing too..

Sir Morris produced the MMC2 Interface which has superb firmware on board and performs amazingly.
The firmware is flashed ready on the Rom-RAM board...

As for bringing the ole girl back to life, it really was a labour of love as so many individual faults had manifested over the years...

Dead 6502
Short Transistor & Dead Caps in the Video Output Circuit.
Corrosion on some of the RAM socket pins causing failure to startup.
4MHz System clock unstable, so I took the liberty of replacing this & the 3.58MHz VDG clock with modern low profile Oscillators, this also helped the Rom-Ram board fit flush.

74LS138 RAM select for the Upper Text RAM was also dead, causing the screen display to be totally wrong in Hi-Res modes.

Mix and match to find compatible SRAM's when used with the boosted clock of the noise killer circuit, if you notice on the finished board, 2 of the Upper Text Rams on the far right are the old type, as the last two modern varients couldn't cope....

I have fully defluxed the board too, repaired so many joints, the wiring mods on the keyboard side are twofold.... to support the NoiseKiller on the RAM-ROM board & to add 8-bit printer capability..

The Empty sockets on the board, far left are for buffers to support the fitted Eurocard connector on the back, this feature is now redundant thanks to the ROM-RAM board & the MMC.
You would need to fit them to use a std Atom Disk Pack.
The Eurocard socket is of course compatible with Acorn System expansion cards...

The only final part I am waiting for now is the Proper PSU socket to mount on the board & lose the awful trailing wires.

OK, chap's I am out of breath and need a coffee.

I hope you enjoy this article & the pictures, it's certainly one of the most satisfying projects I have undertaken....

Again Iain many thanks for this opportunity and allowing us all to share your Atom, I bet there are many of us quite filled with envy now :LOL:

TC :coffee:
 

Attachments

  • AcornAtom.jpg
    AcornAtom.jpg
    182 KB · Views: 2
  • Stock Mainboard.jpg
    Stock Mainboard.jpg
    211.5 KB · Views: 3
  • Main Components.jpg
    Main Components.jpg
    214.7 KB · Views: 4
  • LowerRam+Via.jpg
    LowerRam+Via.jpg
    213.1 KB · Views: 4
  • UpperRam+Main chipset.jpg
    UpperRam+Main chipset.jpg
    212.2 KB · Views: 3
  • The Other side.jpg
    The Other side.jpg
    209.7 KB · Views: 2
  • RomRam Board + MMC2.jpg
    RomRam Board + MMC2.jpg
    213.7 KB · Views: 3
  • Upgraded Atom.jpg
    Upgraded Atom.jpg
    214 KB · Views: 3
  • Closeup.jpg
    Closeup.jpg
    210.1 KB · Views: 2
  • Wiring Mods.jpg
    Wiring Mods.jpg
    212.3 KB · Views: 2
Last edited:
And the Final Piccy's

PS - Bryce, I hope it was worth the wait :p

TC :LOL:
 

Attachments

  • Full System.jpg
    Full System.jpg
    190.7 KB · Views: 1
  • First Test.jpg
    First Test.jpg
    125.8 KB · Views: 1
  • With the MMC2.jpg
    With the MMC2.jpg
    118 KB · Views: 2
  • Acornsoft Snapper.jpg
    Acornsoft Snapper.jpg
    180.7 KB · Views: 2
What a nice system, good thing that you bought it back from the grave.

Thanks for sharing :)
 
Well worth the wait. Great report and "RetroPr0n" pictures :) Makes me want to get myself one of these, but I've seen what they go for on the other bay, so I doubt I'll ever have one :(

Bryce.
 
Now bear in mind the footprint & design of the Atom's Aesthetics was one of the first of it's kind, and indeed most Home computer's (apart from Sinclair) borrowed and expanded on this type of enclosure, just look at the C64, Dragon, Amiga....etc

not sure what your pointing at given the machnes like the apple ii, ti-99 and atari 400 that predates it.
 
Amazing work Bas! Did you find many mistakes that your 13-Year old self made during the initial assembly? I'm very happy you were able to restore and improve this wonderful computer!

Heather
 
Let's go back to 1980 when I was a 13yr old schoolboy and was enjoying dabbling with Electronic circuits, when asked what I wanted for my 12th Birthday I replied "A Soldering Iron Please Ma", no PS3's then I suppose :LOL:

Why when you were 13 did your Mum ask what you wanted for your 12th birthday? Does your Mum do timetravelling? :p
 
Amazing work Bas! Did you find many mistakes that your 13-Year old self made during the initial assembly? I'm very happy you were able to restore and improve this wonderful computer!

Heather

I don't think that's the machine he built back then. It's a different one, just the manual was the one he originally used.. I think?

Bryce.
 
Why when you were 13 did your Mum ask what you wanted for your 12th birthday? Does your Mum do timetravelling? :p

If Bas could build an Atom at 13 it stands to reason that his Mum had a flux capacitor in her Hoover.
 
Pure, disgusting retro - nekkid filth. I feel oddly violated having my nearest and dearest stripped , fixed (!) and put on show!

Hehe

Nice one bas.

Public thanks to bas thecorfiot for his skills and work on this. It was one thing after another and thanks to bas IT LIVES!

Gold star to bas please :)

Ps Take a pic of the heavy- gage case. It's moulded like a rack station's separate keyboard- as that's what it originally was.
 
Last edited:
Nice work Bas! On both occasions! (y)

The Atom is rather a nice and neat design considering its age. All the components are nicely set up.
 
This is sufficiently epic to warrant being made a sticky. :mrgreen:
 
This is sufficiently epic to warrant being made a sticky. :mrgreen:


Yes!! ..... By jove it does!!!







I hope to god (that I don't believe in) that you mean to make the thread a forum sticky.....
 
More close up filth please bas. You've got it for a bit longer!! :) :( :(

2 banks of ram, the via, the naff sockets, the 65c02, the heat marks on the board from the voltage regulator, the perfectly logical but odd keyboard /components on opposite sides ... All character from the acorn genius of Wilson et al, it's such a landmark machine!

:)
 
Back
Top Bottom