Definitions

# Digital comparator

A digital comparator is a hardware electronic device that compares two numbers in binary form and generates a one or a zero at its output depending on whether they are the same or not.

Comparators can be used in a central processing unit (CPU) or microcontroller in branching software. A comparator can be simulated by subtracting the two values (A & B) in question and checking if the result is zero. This works because if A = B then A - B = 0.

The analog equivalent is the comparator. Many microcontrollers have analog comparators on some of their inputs that can be read or trigger an interrupt.

The operation of a single bit digital comparator can be expressed as a truth table:

Inputs Outputs
$A$ $B$ $A=B$ $A>B$
0 0 0 1 0
0 1 1 0 0
1 0 0 0 1
1 1 0 1 0

The operation of a two bit digital comparator can be expressed as a truth table:

Inputs Outputs
$A_1$ $A_0$ $B_1$ $B_0$ $A=B$ $A>B$
0 0 0 0 0 1 0
0 0 0 1 1 0 0
0 0 1 0 1 0 0
0 0 1 1 1 0 0
0 1 0 0 0 0 1
0 1 0 1 0 1 0
0 1 1 0 1 0 0
0 1 1 1 1 0 0
1 0 0 0 0 0 1
1 0 0 1 0 0 1
1 0 1 0 0 1 0
1 0 1 1 1 0 0
1 1 0 0 0 0 1
1 1 0 1 0 0 1
1 1 1 0 0 0 1
1 1 1 1 0 1 0

Examples include the CMOS 4063 and 4585 and the TTL 7485 and 74682-'89.