difference gate
A logic gate whose output is TRUE when the two inputs are different; a Boolean logic XOR gate.
difference gate: XOR logic that fires when inputs don't match
A difference gate is a two-input logic circuit that outputs a HIGH signal (logic 1) only when its inputs differ: one HIGH and one LOW. When both inputs are the same (both HIGH or both LOW), the output goes LOW. This behavior makes it functionally identical to the exclusive-OR gate, or XOR, and the terms are used interchangeably in digital electronics.
The gate performs a simple Boolean operation: output is TRUE when (A AND NOT B) OR (NOT A AND B). In truth table form, a difference gate with inputs A and B produces output only in two of four possible states. This asymmetry distinguishes it from the standard OR gate, which outputs TRUE when either or both inputs are TRUE, and from the NAND and NOR gates, which invert other functions.
Implementation and construction
Difference gates are built from combinations of more primitive gates (AND, OR, NOT) or manufactured as single integrated circuit packages. Early discrete implementations used diodes and resistors; modern versions appear as standard logic ICs in TTL (transistor-transistor logic) or CMOS families. The SN7486 is a common 14-pin DIP package containing four independent XOR gates, each with two inputs and one output. CMOS variants like the CD4070 are available for applications requiring lower power consumption.
The gate's practical value lies in detecting inequality and in arithmetic circuits. Adder and subtractor circuits rely on XOR gates to generate sum bits. In comparators, arrays of difference gates determine whether two binary numbers match bit-by-bit. Serial communication protocols use XOR gates in parity checkers to verify data integrity. Multiplexing circuits and phase detectors also depend on this logic function.
The name "difference gate" reflects its core function: it responds to difference between inputs, not to their absolute values or magnitudes. Although XOR is the standard term in digital logic design, "difference gate" remains useful in educational and explanatory contexts where the intuitive meaning clarifies the gate's role in a larger system.