Guest Blog Post: Building the Cosmac ELF


Image Details: ‘darelfii’ by airship, via Flickr

Given the overwhelming success of the Unnycount, Mr Shinybees has been busy beavering away in the Shinybees skunkworks on a new project that I am struggling to even vaguely connect with knitting. I am reliably informed that the electronic connections utilise a process known as wire wrapping, which I guess provides a somewhat tenuous link to my particular creative flavours, so I shall let him off this time and allow him to occupy some valuable blog space to tell you all about his Cosmac Elf.

Apart from a drought that affected most parts of South West England (which would go on to last for 45 days), not a whole lot was happening in the UK during August 1976. There was, however, an article in Popular Electronics titled ‘Build The COSMAC “ELF” A Low-Cost Experimenter’s Microcomputer’. Low cost meant around $100; that equates to around $800 in todays money if you use the relative share of GDP as a comparison. That may not seem low-cost to you, but when you consider the Apple II, which was released a year later, had a starting price of $1298, or $10400 in today’s money, you can start to see where they were coming from. I think its true to say that the home computing market had yet to really kick off, and for many people the closest they were going to come to a computer was when they used an ATM, the first of which was introduced in Enfield Town, North London on the 27th of June 1967. So, it was left to the hobbyist electronics enthusiast, alone in his (the feminist movement had yet to really take effect) room, soldering iron in one hand and copy of Popular Electronics in the other, to help pioneer one of the most exciting technical revolutions since a cave man rolled a twig in his fingers and invented the wheel. One is left wondering if those brave forefathers, shunned as geeks and condemned to a life of poor hygiene and questionable fashion, could possibly have realised the enormity of what they were starting? To the layman, that dull beige box with the flashing lights and splattering of switches may not have looked like much. Taken on it’s own they would probably have been right. However, it would go on to spawn smaller and better things such as the ZX Spectrum, which would first go on sale in 1982 and continue to be produced in various guises until 1992 (an ice age in the world of computers). We can also thank those brave, socially dyslexic, technology warriors for the Commodore 64, the Nintendo Entertainment System, the Sony Playstation and the IBM Personal Computer, all of which took genesis from those halcyon days, when a handful of like-minded individuals dreamed ideas and turned them into reality at the home computer club.

Price was not the only difference between the Cosmac Elf and the Apple II, there were many in fact, the Apple II had a full QWERTY Keyboard and could display colour graphics on a Video Display Unit, the Elf had a bank of eleven switches and a single button, outputting its calculations on 2 small LED hexadecimal displays. The Apple II had 4Kb of RAM compared with the paltry 256 bytes in the Elf, the Apple II was bundled with its own Operating System that included a simplified BASIC programming language, the Elf came with no Software or Firmware at all. But probably the biggest difference, other than the price, was that you had to build the Elf, and for many people you also had to go out and find all the components as well.

View from above...

At least in the 1970’s the components that comprised the Elf were more readily available. This was the first stumbling block I encountered when I tried to build my own. In this situation eBay is definitely your friend, although Mrs Shinybees certainly wasn’t mine when she started sifting through my e-mail after a large number of suspect packages were delivered by our long-suffering Postlady, none of which contained yarn for her. And whilst it may have cost $100 in 1976, you should expect to pay more today to get those hard to find ‘vintage’ components. There is an exception to this though. What does a Cosmac Elf and the Galileo spacecraft have in common? No witty punchline here I’m afraid – the Answer is the 1802 Microprocessor, originally manufactured by RCA in 1976 and still built today, 35 years later, by Intersil Corporation. Obviously NASA didn’t go onto eBay and pay $3.98 for the brains behind their guidance computer, they opted for the version fabricated using Silicon on Sapphire technology, giving it a hardened resistance to radiation and electrostatic discharge…yawn. The important thing for us to bear in mind is that this chip, old as it is, is still readily available to buy, unlike the two hexadecimal displays! The original design called for a couple of Hewlett-Packard 5082-7340’s that many swapped for TIL311’s as they are very similar and easier to find. Both feature a built in decoder that will convert the 4 digital inputs into a hexadecimal digit on the LED segmented display and as some of the HP units were available on eBay I decided to stick with originality (although there were also some TIL311’s for sale so I grabbed them too….).

Not only does this project involve using vintage components, but it also involves using vintage techniques to actually build, as in the 1970’s fast prototyping services were unheard of. Production equipment would feature Printed Circuit Boards (PCB’s) that were only economical for high volume production and therefore out of the price bracket of your everyday hobbyist. Instead the erstwhile electronic inventors of the 70’s used a process called wire wrapping that involved ‘cold soldering’ the legs of the components to wire by a process of wrapping using an appropriate tool. This actually proved very useful because it was both reliable and easy to change should you make a mistake. Of course all the components were completely recyclable and there was no lead based solder involved so I guess you could say it was pretty environmentally friendly too.

You can obtain all the necessary part lists and build instructions from the internet, but I will mention a few of the techniques I used that maybe differ slightly from those instructions.

Firstly, after cutting the wood used to support the board to the correct dimensions I decided to give it a couple of coats of wood varnish. You are unlikely to want to write your latest novel or browse the internet on this beast, it is probably more likely to become a show piece to be proudly displayed on the mantle piece so its worth making the extra effort with the computer’s appearance.

Secondly, and again to improve the look of the machine, I covered the aluminium control panel (after cutting the necessary holes for the switches) with a couple of coats of black spray paint (I used automotive paint from Halfords hoping it will be more scratch resistant). I also decided to mount the single LED on the panel with the switches, not exactly authentic, but it looked better and since I’d already spent quite a bit of time pimping my Elf, I thought it was worth it.

Finally, I found it very useful to place labels on the reverse of the board where the IC sockets are. This makes IC and pin identification very simple and vastly reduces the possibility of incorrectly wiring the board.

More like spaghetti junction...

The instructions will have you believe that you can complete the wiring in a few hours, but I found it took a few days. This was mainly because I was picking it up and putting it down quite a bit with little Miss Shinybees occupying our time so much, so in the absence of a newborn, you should complete it quicker than me. I used a manual wire-wrapping tool that I purchased off eBay for about £12, but told my wife it only cost £5. Whilst the automatic wire wrapping tool may have been easier and quicker it is expensive and I didn’t think I would be able to sneak that one under my wife’s finely tuned radar. Of note, wire wrapping tools bear no useful function in the field of nappy changing a newborn but it doesn’t stop you trying (ahh the things you can say when you 1500 miles away from child services).

After double-checking the wiring one more time it was finally time to apply power and see the fruits of my labour. With Mrs Shinybees at my side and looking almost as excited as the time I pumped her full of rohypnol, wheeled her down the aisle and got her to say ‘I Do’, I hesitantly connected a 9v supply to the board whilst crossing my fingers behind my back (my wife had her fingers in her ears). Considering the amount of Chianti I had drunk during the wirewrap process (nappy related stress relief) I was pleasantly relieved to find it didn’t bring down the street lighting, in fact it didn’t even blow one of the house fuses, it worked first time!

Programming the computer requires you to convert the hexadecimal instruction code into binary and flick the necessary toggle switches on the front panel, before pressing the load button, which moves the instruction code into memory and increments the pointer. After a bit of practise I have become quite nifty at programming the computer, although with a single LED and two hexadecimal displays my repertoire is somewhat limited. Like my chat-up lines, according to Mrs Shinybees.

Unlike my chat, the Elf offers plenty of expansion opportunities, including a variety of keyboards from hexadecimal to full size QWERTY, a video display driver, more memory, the list goes on. Unfortunately it’s difficult to please some people and Mrs Shinybees seemed a little underwhelmed by my flashing LED programme, even though I had it loaded in under a minute, but managed to muster a couple of reasonably theatrical ‘oooohs’. It seemed like too much effort and also expense when more powerful and smaller computers fall out of cornflake packets these days and probably do a lot more. Many people will say that she has a point, those people may also feel that they have wasted their time reading this particular blog entry. To those people, I would like to say that I am happy to have wasted a little bit of your life. And if you are waiting for me to give you a good reason for building this miracle of almost modern electronics in the first place then you will be waiting a long time, if this doesn’t make sense to you now then it never will no matter what I write.

But the Unnycount was pretty awesome, wasn’t it?!

8 comments on “Guest Blog Post: Building the Cosmac ELF

  1. socrates says:

    Simply the finest, and most interesting blog post I have ever read. Keep it up.

    1. Shinybees says:

      No prizes for guessing who wrote this comment…. Not Socrates that’s for sure!

    2. i’ve got 3 under 2yrs….. perfect time for me to undertake this…. uh….. undertaking…..

      1. Shinybees says:

        It’s important to keep yourself occupied with these kinds of crucial projects to balance the challenge of parenting. When you’re not face down in a bowl of porridge…

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