The CY74FCT399T is a high-speed quad 2-input register that selects four bits of data from either of two sources (ports) under control of a common select (S) input. Selected data are transferred to a 4-bit output register synchronous with the low-to-high transition of the clock (CP) input. The 4-bit D-type output register is fully edge triggered. The data inputs (I0X, I1X) and S input must be stable only one setup time prior to, and hold time after, the low-to-high transition of CP for predictable operation. The CY74FCT399T has noninverted outputs. This device is fully specified for partial-power-down applications using Ioff The Ioff circuitry disables the outputs, preventing damaging current backflow through the device when it is powered down.