The 74157 is an integrated circuit logic device or chip used in digital logic circuitry. It is a quad 2-input multiplexer with four 3-state outputs. A multiplexer is a device that allows digital signals to be selectively switched between inputs, depending on the state of the control lines. The 74157 has four 2-input multiplexers, which can be controlled with two separate select signals. It has four active-low, 3-state output buffers, meaning that when in the 3-state mode, the outputs remain disconnected from the bus lines until a valid logic level is applied to the Output Enable pins. The 74157 is typically used for data selection within a digital system, such as selecting the data output from a CPU or selecting certain data lines from a ROM or RAM chip.