I have used today to get my old 4-bit adder working with the rest of this computer. I am having some logic level voltage issues between the adder and the rest. The adder was designed and wired a year ago as a stand alone project to operate at 9 to 12 volts and most of the rest of this computer operates at 5 volts. I have been able to get the adder output to latch into my new register. However, The lower level outputs from the other stages are not high enough for the adder/alu inputs. I will try using buffers at the alu inputs to boost to the higher level voltage that the alu is looking for. It's either that or rebuild the alu and I really do not want to do that.