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My first computer was a Kontron kit computer. After building a couple of memory boards, I still only had 1.25 kB of RAM, and expanding it would have been really expensive. Then I saw an advertisement in Byte: 32 kB of memory on four boards for only $790! The problem was that it was for the S-100 bus. But that was so much cheaper that I decided to migrate. It wasn't all progress: in those days the S-100 bus was so flaky that it was difficult to run a Z80 faster than 2 MHz—and that where my Kontron CPU managed 4 MHz! But in the course of time I built up a reasonable system. Here's the motherboard:


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The CPU was from SD Systems:


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Image title: SD Sales Z 80 CPU front          Dimensions:          4388 x 2339, 2560 kB
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Here's one of the RAM boards that started the ball rolling:


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This was the first of two S-100 machines, and today it's difficult to be sure which boards I used in which machine, but I think the following is relatively accurate. I bought four 8" floppy disk drives. Here's the controller:


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The 32 kB memory left space for a ROM monitor, which I implemented with this board—I think:


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I bought a half-populated I/O board and put two USARTs on it:


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The pre-populated chips appear to be bus interface logic and a couple of primitive parallel ports. The mess of resistors and transistors on the cable connector are almost certainly a 20 mA current loop adapter.

I also built a ”console“ for the machine, in the days when that meant a set of switches and LED display:


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The switches were on a calculator keyboard that connected to the orange DIP connector at bottom right. The 16 LED display was the address bus, and the 8 LED display the data bus. I had switches that allowed stepping single cycle and single instruction executions.

The 32 kB of memory only half filled the address space. I ultimately increased this, though it's no longer clear by how much, nor with which board. I needed to leave space somewhere for a boot PROM. One candidate could be this board, but even when fully populated it would only have offered 8 kB:


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Image title: Ithaca memory board front          Dimensions:          4457 x 2292, 2384 kB
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Image title: Ithaca memory board rear          Dimensions:          4428 x 2334, 2896 kB
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But the other boards I built had more than 32 kB, so they were probably for the next machine. I have some recollection of a dynamic memory board by SD Sales, which I don't see here. If my memory serves me, I bought it in the hope that its would work with the SD Sales CPU board, but I think a considerable amount of the patch wiring on the CPU board was to get the timing right.

This board may have been a replacement for the previous ROM board, with the added advantage of a PROM burner, though I'm not sure I ever got that to work. The PROMs are 2708s, 1 kB each, so this board needed a 4 kB hole in the address space.


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The next one puzzles me. Clearly it's an I/O board, but I already had one. It has two 8251A USARTs, 2 8255 parallel port chips (a total of 48 bits of I/O; where are the connectors?) and an 8253 timer chip. Did I ever use it?


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