The FCT157T is a quad two-input multiplexer that selects four bits of data from two sources under the control of a common data Select input (S). The Enable input (E) is Active LOW. When (E) is HIGH, all of the outputs (Y) are forced LOW regardless of all other input conditions. Moving data from two groups of registers to four common output buses is a common use of the FCT157T. The state of the Select input determines the particular register from which the data comes. It can also be used as a function generator. The device is useful for implementing highly irregular logic by generating any four of the sixteen different functions of two variables with one variable common. The FCT157T is a logic implementation of a four-pole, two-position switch where the position of the switch is determined by the logic levels supplied to the Select input. The outputs are designed with a power-off disable feature to allow for live insertion of boards.