The use of small cards forces extensibility strategies

The prototyping boards I ordered are relatively small (5.5 x 8 inches) so chances are for some circuits to expand among several cards. This is particularly likely for Registers and the Instruction Decoder and Sequencer (IDS).

Actually I am not unhappy with this reality; on the contrary, what if I wanted to implement more instructions in the future? The natural way would be to develop some kind of "IDS Extender" card. In this sense, the fact that the first IDS card were extensible from day one represents an advantage rather that a disadvantage.

Problem here is the big amount of connections between the IDS and the register cards, so the goal is to implement extensible IDS cards in an elegant, ordered fashion.

Here is the scenario:


The original IDS card (IDS#1) provide all control signals for the two register cards; later on, a second IDS card (IDS#2) is developed to provide more instructions. But the new instructions control the same registers so the new card provides exactly the same control signals. Question is how to interconnect all those wires without making a mess.

One possible solution is to wire all control signals in a bus so I can connect as many IDS cards as needed, as illustrated bellow.


Another solution is to conceive the second card as an extension that connects to the first card only; a third card could be connected to the second and so on. The following figure illustrates the idea.


This arrangement is simple and natural but presents some problems. One is the delay introduced by the AND gates; the other has to do with pins count: Control signals are expected in the order of 70 and now pins would be doubled for each IDS card.