The Cody Computer : processor board assembly

As a reminder, the Cody computer is a small machine that is, in absolute terms, could be considered no more interesting than any other project of this kind.

Over the past twenty years, hundreds of projects like this one, and probably even more technically interesting projects, have seen the light of day. To my knowledge, all of these projects quickly fell into oblivion. I'm thinking in particular of Geoff Graham's Colour Maximite II (https://geoffg.net/maximite.html) project because I followed this project for years, and then, nothing...

All information about Project Cody is available at this address: https://www.codycomputer.org

But then why be interested in Cody?

The main point that caught my attention about this project is the excellent documentation that the author has produced on his machine. 

Beyond the machine's own characteristics, some of which are unsuitable for me, this documentation makes the assembly, use and programming of the machine completely understandable. All the documentation is in a 'small book' pdf that can be downloaded from the project website: https://www.codycomputer.org/TheCodyComputerBook.pdf

Furthermore, Cody comes in the form of an electronic board that could almost fit inside a ZX81 case. But it is of course more powerful than a ZX81. Equipped with a 6502 processor, it would actually look more like a Commodore C64 with less complex electronics. So, I find that it brings together two interesting aspects, the ease of understanding of the machine, and the power of a Commodore 64. Interesting, right?

So I decided to build this small computer to judge how difficult it was to make. My latest experiment in this area concerns the construction of the Omega computer, type MSX, by Sergey Kiselev : (https://github.com/skiselev/omega/blob/master/Mainboard/images/Omega-Keyboard_and_Mainboard-1.1.jpg)

Omega

Assembling this machine is not complicated for someone with experience, but for a first approach, it is still a different 'story' than assembling Coddy's motherboard :

Cody

As you can see, Cody also contains four 'big' integrated circuits like the ZX81 motherboard. Indeed, Cody also contains 3 other small integrated circuits to solder, which does not complicate the machine further, given that there is no HF modulator to install. On Cody, it is a Parallax circuit that manages the video by directly providing the composite video output (CVBS).

In comparison, the ZX81's board :

Cody therefore looks very much like a ZX81 kit to assemble from the time without more difficulty, but with a more interesting result than the ZX81. So to assemble this little computer, I put myself in the mental conditions that were mine when I was a teenager and I didn't know much about electronics.

I obviously had the printed circuit board made at JLCPCB (that wasn't possible at the time) and bought the four main components, namely the two processors, the RAM and the I/O circuit. I already had the EEPROM. The bus driver, a 541 and the CD4051 were taken from my stock of more or less new components...

I assembled the machine, not with the help of the sheet, but by scrupulously following the instructions and the assembly chronology as indicated in the documentation. It takes barely more than an hour of work to achieve this result:

I almost assembled this board in 'brute force' mode. I didn't have the recommended 3.3V regulator. I put another one in CMS version. The resistors for generating the video signal are also not exactly those recommended. There are a few Ohms of difference. I even created one of the resistors by putting in series two resistors that I had, for a result very close to the original desired value. In short, I tried to make a clean assembly, but with certain tolerances of the type that a beginner could make.

I also didn't buy a original 'Prop Plug' to program the Parallax circuit, I figured the FTDI-style USB/Serial adapter would be enough.

After testing the board's power supply, and initially setting up the Parallax circuit and its EPROM that I programmed using the Parallax tools, the machine did not start correctly, or even did not start at all, and displayed some time parasitic characters on the screen: not a good sign and a slight disappointment. I thought it would start without any problem.

First thing, when programming the Parallax circuit with the Propeller Tool software (https://www.parallax.com/propeller-2/programming-tools/), I had a communication error message telling me that the communication was not going correctly with the processor (/RTS used as RESET signal). In fact, and after several tests, I realized that when acknowledging this error, the application was still programming the processor correctly. I was able to verify with the oscilloscope that I was getting a composite video signal at the CVBS output.

The way Cody is built, assuming the Parallax circuit was programmed correctly, and assuming the 65C02 processor was new and should work, all I had to blame was the 74HC541 bus driver circuit. So I replaced it with another circuit, also from my stockpile, and voila!

As you can see, I went into full DIY mode, with a less than rigorous assembly procedure, but still practicing quality welding, and the machine worked almost the first time. The initial malfunction was only due to a defective component in my stock.

This way of doing things, not entirely according to the rules of the art, allowed me to validate the high feasibility of assembling this small machine, even by a relatively inexperienced person. I expected this result, which is why I 'pushed the vice' to really assemble Cody in beginner mode.

Because there's nothing worse than demotivating someone from the start who doesn't know much but is eager to learn. The possibility of error must be present without it leading to an intellectually catastrophic disaster. This is why I particularly appreciate the approach of the author of this little machine.

And now? Cody doesn't have an operating system but programs itself directly in Basic. It's ideal for getting started in the wonderful world of microcomputing. I haven't written a single line of Basic on this machine yet, and for good reason: I haven't built the keyboard.

Personally, I don't find it very useful to build a specific keyboard for this kind of machine. Obviously, the look loses a bit if you use a standard keyboard but hey, that's my opinion. So I'm going to do what I already did for the Omega, I'm going to create a USB interface allowing you to directly connect a standard USB keyboard to Cody, I find it more practical. The user experience is more comfortable, especially if, as a passionate person, you start spending long hours programming on this little machine...

The Cody Computer : juste for fun.

To get away from the topics I've been working on for several months now, I intend to build a small tube audio amplifier and to 'relax' by making a small 'old-school' microcomputer.

I actually came across a Facebook post, while I'm browsing this social network less and less often due to general interest that is very close to absolute emptiness, on a subject that I find interesting: the construction of a small computer resembling the machines of the 80s, and more precisely the Commodore 64.

Nothing fantastic, you might say. Even the 'advertising' picture of the subject can be quite laughable. You'll probably be right, except that...

For years I have been looking for a small machine to turn into a 'thing', a kind of companion that can be used on a daily basis for various 'basic' type applications.

In fact, almost all attempts at new hardware are very ambitious, since it is often about doing 'better' than what was 'before'. Often it starts strong, and then quickly the interest in the subject wanes. In my point of view, the main problem, beyond the difficulty of realization and its cost, is simply the documentation.

Of course, there are always wikis, forums, etc... where the information is not up to date and 'stuffed' with links pointing to the 'detailed' documentation of this or that subject. In fact, it is extremely 'boring'. Which imposes a very slow and tedious learning curve for any newcomer. I faced this type of problem with the MSX machine that I had the opportunity to assemble a few years ago.

Here, none of that. The machine is simple, even simplistic. It only offers basic and functional functions and above all, the designer took the time to publish a book in PDF format that details absolutely everything you need to know to 'play' effectively with this machine. The book is currently 571 pages long that I forced myself to read to see if there was any 'useless padding' of pages, just to make the counter go up. Well no. Everything is there and very well documented.

Frederick John Milens has done a huge amount of documentation work and it is absolutely brilliant. At least there will be no need to spend hours searching on the Internet, and that is a decisive plus.

And then here, in fact, no totally sterile discussions on the characteristics of the machine, which please or not, no sterile disputes either on the choices of the designer and all these sorts of things. The machine is as it is and that's it! You have to do with it, and that's not so bad. So, notice to the real 'reto(or not)-geeks' able to embellish, improve, create with this little machine... 

So, I already had printed circuits board made for this little computer. The assembly work will be quite simple given that few components are present on the circuit. Almost 'Zx81'.

There is no need for me to specify the characteristics of the Cody Computer here since absolutely all the information can be found on the site dedicated to it:

https://www.codycomputer.org