MicroKit09

What is it?

Some month ago I was pointed to the remake of the French MicroKit09 and I got interested in it.

Background

My first computer was a Commodore 64 but on the HTS, technical university (?), I worked with the Labbus, a computer equiped with the 6809. And deep inside of me I had the ise of working with this CPU again. I obtained a Tandy Colour Computer 2 But in the end I couldn't do anything with it because I hadn't a floppy drive. Then Fred Jan showed me his rebuild of the MicroKit09, the one seen on the page mentioned above, and we agreed to exchange the Coco 2 for his rebuild.

To document everything, I decided to create my own Eagle version of the schematic of the MicroKit09. Schematic of the CPU board:

Schematic of the IO board:

It is not an exact copy. The original board enables you to choose between 2716 or 2732 EPROMs and between 2 KB and 8 KB SRAMs using jumpers. I choose for the 2732 and 8 KB SRAM only version to simplify things a bit.

My own version

But.... I'm a tinkerer and I couldn't resist to create my own version of the MicroKit09.

and the PCB:

Main differences:
- Support up to 8 KB of EPROM
- 128 KB of RAM
- GAL instead of 74138s
OK, the schematics says 27256 but the highest address lines, A13 and A14, are noy used and have been connected to +5V. In this way I even can insert a 2732, if needed.
The reason for choosing a 128 KB SRAM is very simple: I only need one IC.
The GAL enables me to configure my version of the MicroKit09 in more than one way. For example, the MicroKit09 has the I/O mapped at $A000-$AFFF. And although it uses a 4 KB 2732, the binary starts at $E000. The GAL enables me to let the EPROM start at $F000 and the I/O at $E000. Which leaves me a whopping 56 KB of SRAM, if needed.
And then a bonus: the MicroKit09 does not use the output CA2 for anything. An idea of Fred Jan: why not use it for signaling the system to replace the EPROM with SRAM once it has started up?
Eagle doesn't like unconnected inputs and so I connected CA1 to CA2 and did the same for CB1 and CB2. The last connection enables me to check the presence of the 6821, if needed.

FLEX09

In the past the operating system FLEX was writen for systems with the Motorola 6800 processor. It was later ported to the 6809 and was called FLEX09. And I had the vague idea to see if it was possible to run on the MicroKit09.

FLEX09 has some requirements:

1) Computer system with 8K of RAM at $C000 and at least 12K of RAM beginning at location $0000.
2) A system console or terminal such as a CRT terminal or printer terminal.
3) A single 8 or 5 1/4 inch disk drive with controller capable of running soft-sectored format with 256 byte sectors.
4) A monitor ROM or some program affording the ability to begin execution at any desired point and to enter code into the system.

The MicroKit09 cannot meet these requirements but both the original and Fred Jan's version have an expansion bus. Requirements 1, 2 and 4 shouldn't be a problem, requirement 3 will be a challenge. Page 22 of the 6809 FLEX Adaptation Guide describes how the FLEX09 can be loaded but doesn't say explicitely that it has to be a disk where the operating system is loaded from. So that leaves us room for other ways to load FLEX09.

I designed an expansion card that IMHO should meet above requirements:

and the PCB:

I can only add maximal 44 KB of SRAM to the original MicroKit09 and Fred Jan's version. 44 KB means using a 128 KB IC again. The main purpose of the GAL is to "smear" those 44 KB into the various free address ranges. The second purpose is to take care of the extra I/O.

Using a terminal means I need a serial interface and the obvious IC to use is the 6850 ACIA. A 2.4576 MHz oscillator and a 74293, 4-bit binary counter, enables us to choose from a big range of baud rates. I intend to use a RS232-USB interface and that one only needs the original TTL levels so a 3-pin access is sufficient.

I'm quite sure that I could load FLEX09 using the ACIA but I'm also thinking about using an Arduino with SD card or even an old PC with LPT port. For that reason I added a 6821 PIA. A 26-pin header enables me to connect the card to the Arduino or PC.

OS9

While searching for FLEX09, I ran into OS9, another operating system for computers equiped with the 6809. But I have found some requirements, at least 12 KB of RAM but nothing like where it is expected.

The future...

I even started to create a version that combined my version of the MicroKit09 with the above card but I soon realiesed that that was pure overkill. So I ommited more and more parts and at the end I realised that the design that was left over could not be called a MicroKit09 anymore. But I also owned a Eurocom-1 computer with a 6802 CPU. Eltec, the creator of this computer, also created a small card containing a 6809 CPU. I decided to use this as base for my FLEX9 and OS9 adventures. Please see: Eurocom-I and Eurocom6809

Having questions or comment? You want more information?
You can email me here.