The 74S20 is a quadruple 2-input Exclusive-NOR gate, which is a digital logic IC from the TTL (Transistor-Transistor-Logic) family. It has four independent 2-input Exclusive-NOR gates integrated into the same chip, each with Schmitt-trigger inputs for improved noise immunity. Each gate has two Schmitt inputs, A and B, and one output, Y (active low). The outputs are logically equivalent to the result of an Inverter gate with an Exclusive-OR gate chained together in series. The 74S20 has an output enable (OE) which can be used as a logic-level enable, so when OE = 0 all outputs are high-impedance. All four gates share the same power supply,there is no logic level conversion, and it can operate at different speed grades.