BCD

Binary-Coded Decimal (BCD) are decimal numbers where
each digit is represented by a four-bit binary number. BCD is
commonly used with input and output devices. A thumbwheel
switch is one example of an input device that uses BCD. The
binary numbers are broken into groups of four bits, each group
representing a decimal equivalent. A four-digit thumbwheel
switch, like the one shown here, would control 16 (4 x 4) PLC
inputs.