Having worked my way through the power supplies, it’s time to take a look at the circuit boards in the card cage.
The original PDP-8 had a number of “Flip-chips” that held basic logic circuitry made from transistors. These boards were plugged into a backplane that routed the signals between the appropriate gates on the flip-chips. Later PDP-8s had the processor built up of 3 large PCBs and the later PDP-8 compatibles from DEC, such as the DECMate had a single chip with the processor.
The CCI machine is none of those. Instead it has six boards that make up its CPU. Each is clearly marked with its function, “Major registers”, “Accumulator” etc.
Here, I’m going to describe working my way through these boards, visually inspecting them and giving them a clean.
Please keep in mind the size of these boards. They are huge by Today’s standard. 40cm x 25.5cm.
Front panel / operator console.
Standing on the front of the machine, the front panel contains the switches and LEDs needed to load simple programs into the computer. PDP-8s of this vontage didn’t have any ROM code so do nothing when first powered up. By using the toggle switches and push buttons, it is possible to load and run simple programs.
This machine is a bit unusual in that it uses 7-segment LEDs to show the register contents rather than the more usual one-LED-per-bit arrangement of genuine PDPs. It also uses push buttons instead of spring loaded toggle switches.
On the right you will see the board after I have removed the red tinted acrylic sheet. There is nothing much wrong there, it just needs a good dust.
Com Seq Gen board
The second board in the cage is labelled “Com Seq Gen”. I’m assuming this mean command sequence generator and I’ll continue to believe that until other evidence comes along. It is clearly labelled Sep 01 1976. Most of the other boards are either 1977 or 1978 with chips with dates anywhere in between.
Interestingly, this is the only board in the first six to have any lacquer on it and it’s only on one side.
Just needs a good clean.
Sequence Input Generator
Another sequence generator. This on is labelled “7807” so I’m thinking July 1977. By now CCI must have given up on lacquering their board as this one is raw fibreglass.
I love the idea that all of the parts that make up the CPU are spread around a number of boards rather than under a plastic or ceramic lid.
Extended Memory Control
The basic PDP-8 architecture can only address 4k words of memory. This machine has 32k as we’ll see later. The original PDP-8 got around this limitation with a K8ME board and I’m assuming that this povides the same functionality.
A small about of patching on this board in red wire (centre left).
Here is the brains of the outfit. The accumulator. Nice isn’t it.
Memory bus terminator
I think this is the last board of the CPU. Alternatively, it’s the first board of the not the CPU.
I wonder if there are enough cards on this page to prove that the CPU is alive?