When trying to add two numbers on my two cascaded 74LS181's I find that the result seems to be right when I tie the carry (Cn) of the least significant one to Vcc, contra my expectation that Cn should be low.
Trying to add zero and zero yields b'00000001'…
I did tie the Cn+4 of the LSB to the Cn of the MSB.
Any thoughts?
Gerard
Best Answer
When using Active High operands, Cn (carry in) and Cn+4 (carry out) are active Low (indicated by the circles on pins 7 and 16 in the logic symbol below).
The least significant ALU's carry input should be set to No Carry, ie. Cn = High.