Electrical – 2 x 74LS181 4 bit ALU’s cascaded to form 8 bit ALU, Cin problem

alu

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.

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