I finally found the magazine containing the details of the Micro-comp so I can now give you some more details. The magazine was a one-shot, called Six BD 679 Projects, and the first half contained details of six electronics construction projects based around the BD 679 Darlington power transistor, with the Micro-Comp taking up the remainder of the magazine. (I am at a loss to explain why the Micro-Comp wasn’t featured more prominently on the cover or in the title, but Talking Electronics is a weird mob…) The magazine was published in 1994 and reprinted articles and projects from the Talking Electronics magazine.
Talking Electronics is a local electronics hobbyist company based in Melbourne. Back in the early 1980s (IIRC) TE launched an electronics magazine, which saw some moderate success despite its irregular publishing schedule. In all, fifteen issues of Talking Electronics were published and TE also published a number of one-shot magazines, including six Electronics Notebooks (similar to Forrest M Mims’ notebook series), a two-part Learning Electronics, Digital Electronics Revealed, numerous model railway and FM bug books and a few compilations of popular projects form the Talking electronics magazines. The magazines were generally pretty good and I learned a lot of my electronics chops by studying and building the projects presented therein. On to the Micro-Comp…
When I picked up the BD 679 magazine it caught my eye not because of the cover (a nasty two-tone brown affair) but rather the “free” BD679 taped to the front. I was always the sucker for freebies. 
Flicking through the magazine, the one project that really stood out for me was the Micro-Comp. Ever since I had read about CPU design, I had wanted to try my hand at designing my own computer - the Micro-Comp seemed like a good place to start learning what would be required.
The Micro-Comp uses just three chips - a Z80 CPU, a 2732 EPROM and a 74LS273 as an output buffer. You can’t get much simpler than that! Input is provided by an 8-way DIP switch and two momentary on push buttons. In my earlier post I thought the DIP switches were to select the start address, but that was wrong. The DIP switches are tied to the data bus - the code in the EPROM must read the value of the data bus and use that to jump to the correct start routine on power up.
The clock speed is adjustable, using a trim-pot to adjust an RC network, and the speed can vary from 7.5kHz to 35kHz. As this is really only a “trainer” and not a serious, full-blown computer, adjusting the clock speed can be extremely useful in tracing signals and trying to understand what is happening throughout the system. It is possible to listen to the various bus lines using the audio probe (the fly-lead in the photo I posted in the previous entry) and tweaking the clock speed affects the frequency of the sound.
The programs included in the EPROM are:
- Jump routine - the “boot loader” which jumps to the memory location specified on the DIP switches at boot up.
- Tone - outputs a single tone on the high data line
- Quick Draw - a two-player reaction time game. The first person to press their button when the 7-segment display lights up wins.
- Running Names - a simple text scroller which displays a message on the 7-segment displays.
- Looking at Data - Enter a page number on the DIP switches and the values of data in that page are displayed on the 7-segment display in hex with a small delay.
- Counter - Increments a counter with each press of button ‘A’ and displays the output on the 7-segment displays.
- Auto-increment/Auto-decrement - variable counters.
- 4×4 Display - Displays a series of patterns on the 4×4 LED matrix.
- Dice - Displays a random die roll on the 4×4 matrix.
- Slot Machine - “Rolls” random patterns down the 4×4 LED matrix in a very simple slot machine simulation. 4 lit LEDs in a row wins!
- Binary Clock - Uses the 8 LEDs as a binary clock/counter.
Most of those probably sound quite dull and uninteresting, but if you’ve built the computer and study the code for each of these simple programs, you can learn quite a bit about the workings of a real computer. Studying memory locations, data busses, loops and timers, multiplexing displays and so forth are the real aims of this project and I have to say I did learn a heck of a lot by building this and playing with it. Unfortunately I never had an EPROM burner, so I couldn’t write my own programs to test - maybe that is something I should address when I get some spare time…
A number of extra programs and add-ons were made available through the magazine. Two that I know of that were published in the magazine were a Blackjack program and an add-on board to play noughts and crosses. IIRC, a RAM pack, non-volatile RAM, morse-code trainer and relay driver boards were also available.
I’ll try to be a bit more regular in future.
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