Backplane Design and Construction
Design-wise there's not much to it (also available as a PDF on my Google Docs):
Something you might have noticed on this diagram and on other cards is the label 'Type:Z'. This refers to the type of Control bus (which is the third connector from the left) which is the 20 pin connector that carries the various control signals that pass between the cards. So far all the control signals required by the ALU cards fit within the 20 pin connector. What hasn't been covered in this blog yet are the general purpose registers A, B, C and D which use most of the remaining pins on this connector. The other cards that make up the computer will need other control signals but won't necessarily need to talk directly to the 'Type:Z' cards and so will have their own 'Type:Y' or 'Type:X' connectors. The new control lines, additional to the ones already introduced on the ALU cards, are:
- RLA ... Register Load A
- RLB ... Register Load B
- RLC ... Register Load C
- RLD ... Register Load D
- RSA ... Register Select A
- RSB ... Register Select B
- RSC ... Register Select C
- RSD ... Register Select D
It turns out that that everything therefore works out quite nicely as the five cards (ALU Logic, ALU Arithmetic, ALU Control, Register AD, Register BC) all use the same connectors but also fit nicely if slotted into the backplane side by side with plenty of head room on each card for the relays. This seems to be yet another one of those happy coincidences that I keep coming across with these pad board cards where everything comes together and just fits.
The initial construction of the backplane is fairly trivial ... just a case of soldering down all the connectors ... and it looks like this:
Right, with that small diversion out of the way I can get back to constructing the ALU cards.