I got the ALU wired up and ready to go...
There were a few issues to be resolved that were discovered when "integration testing" the whole unit.
- The first was easy. XOR was not working as expected. It turned out I had messed up the Encoder Pegs for XOR. Rearranged pegs. Done.
- On a more serious note both ADD and SUB, the two operations that I actually need for a Working Digital Computer were not working either. Checked the Encoder Pegs but no joy I had installed them correctly. Turned out there were two distinct issues.
- For ADD, when creating the 4-Bit ALU PCB I had crossed the Q and Co pins. Needed a little surgery to fix.
The operation of SUB as defined isn't quite right. A + NOT B is not the whole story, there needs to be a +1 in there to get the two's complement. Fortunately I noticed that the GS2 control line was not being used by any of the operations I had defined. So I was able to tie that input on the PCB to ground and use the GS2 switch on the encoder to control the Ci input on the PCB. So now the SUB operation (only) has a Peg in the GS2 position which makes the Carry In bit high introducing the required +1.
All operations are now working as expected.