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Z80 Mark III    In 1981 I was able to purchase a used terminal at the local electronics surplus store. As you can see, it had a very modern look compared to what I had produced so far. What was most appealing to me and the deciding factor in my purchase was that it had empty space inside that was just enough room for me to build in my microcomputer. On top of that, it had a beefy power supply that could power my computer boards with no problem. So the Mark III was born.

The S-100 boards in the Mark I and Mark II would certainly not fit into the new Terminal and so they had to go. I decided to standardize on the 4.25” 44-pin Radio Shack boards. I could fit only five of them in the terminal so board space was critical. The Mark I and II had used the common power supply method of the time in that the power supply provided a voltage higher than needed and a voltage regulator on each board brought down the voltage to the correct value. My new terminal’s power supply provided the correct voltages directly and so I was able to do away with the voltage regulator and its heat sink on each board, thus providing more board real estate. Every board would have to be redesigned and rebuilt.

The interface to the terminal electronics was via a 30-pin edge connector and so this had to be in my design. I also had to study the signals that it required on the edge connector and design the hardware and software to support it. That also meant that all the software dealing with the keyboard and screen had to be rewritten.

This was a considerable amount of work but it was my hobby and so the hours were put in. The result was a self-contained, professional-looking system that was a cut above the Mark I and the Mark II.

Below is a photo of the completed Z80 Mark III computer that I installed inside the terminal. Also shown are photos of the individual boards and the motherboard.

Click on a photo to enlarge it.