The ACS8527 is a highly integrated, single-chip, MUX with PLL solution for protection switching between two SECs (SDH/SONET Equipment Clocks) from Master and Slave SETS (Synchronous Equipment Timing Source) clock cards, for line cards in a PDH, SONET or SDH Network Element. The ACS8527 has fast activity monitors on the inputs and will raise a flag on a pin if there is a loss of activity on the currently selected input. The protection switching between the input reference clock sources is controlled by an external pin.