74LS181 4-Bit Arithmetic Logic Unit IC

The 74LS181 designed as a 4-bit slice arithmetic logic unit (ALU), implemented as a 7400 series TTL microcircuit. This 4-bit Arithmetic Logic Unit (ALU) capable of performing all the possible 16 logic operations on two variables and range of arithmetic operations. The 74LS181 controlled by the four Function Select inputs (S0–S3) and therefore the Mode Control input (M). Thus performs all the 16 possible logic operations or 16 different arithmetic operations on active HIGH or active LOW operands.

When the Mode Control input (M) made HIGH, all internal carries inhibited. And therefore the device performs logic operations on the individual bits. When the Mode Control input remains LOW, the carries made enabled. And therefore the device performs arithmetic operations on the two 4-bit words. The device incorporates full internal carry lookahead and provides for either ripple carry between devices using the Cn+4 output, or for carry-lookahead between packages using the signals P (Carry Propagate) and G (Carry Generate). Moreover, within  ADD mode, P indicates F is 15 or more, while G indicates that F is 16 or more. Whereas, within the SUBTRACT mode, P indicates that F is zero or less. While G indicates that F is a smaller amount than zero.

P and G aren’t stricken by carry-in. When speed requirements aren’t stringent, these often utilized in an easy ripple carry mode by connecting the Carry output (Cn+4) signal to the Carry input (Cn) of the subsequent unit. For high-speed operation, the device employed in conjunction with the 9342 or 93S42 carry-lookahead circuit. One carry-lookahead package required for every group of 4 DM74LS181 devices.