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:
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:
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:
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:
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.
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?
