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Concern with the idea of having a "Control Bus"

11/10/2009

For more than four months the design has kept stable based on certain "primary ideas" that have hold so far. One of those ideas is to wire all control signals together in a bus in "Control Slots" so all Controllers have access to those at the same time. This allows me to have more than one IDS (Instruction Decoder and Sequencer) card, which is good because it represents modularity which is convenient.

There is a price to pay, however: the increase of pin-count. I was been forced to purchase expensive 120-pins connectors for "Control Slots" just because of that. And not just money; another problem is with the Console, which takes many interconnections, that is many ribbon cables plugging to header connectors anywhere within the computer, which is a mess.

That's why I am revising the "primary idea" of the "Control Bus" in the sake of solutions. I wish to have all conations through the Backplane only: not a single ribbon cable plugged to any card at all. The only way I could get to that is by removing pin assignments from the Backplane connectors. The obvious opportunity is with the Control Bus.

I realized that the only cards that really need access to all control signals are the IDS cards. Other controllers such as the Interrupt Controller and the Console Controller only deal with a few control signals. Therefore there is no need (other than simplicity) to wire all 75 control signals to all Control Slots; it is sufficient to wire them to IDS slots. This would free a lot of pins for the Console Controller that could be used to connect to the Console.

The bottom line is to defeat the simple idea of having just two types of Slots (Data and Control). The new approach would be to wire the Backplane more efficiently putting wires where is really needed.

Honestly, I'm not entirely happy with this approach because it will prevent the mess of ribbon cables at the expense of creating a mess in the Backplane wiring. So I'm not making a final decision yet, but just researching the matter very carefully.