Combinational logic

Abstract

COMBINATIONAL LOGIC concerns the assembling of electronic circuits which perform elementary functions in mathematical logic (typically AND, OR and NOT), so that more complex logical expressions may be implemented, to enable us to control, say, a lift, a traffic light, or a computer. The devices have inputs and outputs each of which can only be in one of two states. Electrically, these states are usually a high-voltage state (often +5 V) and a low-voltage state (usually 0 V). If we so choose we can call these two states ON or OFF, TRUE (T) or FALSE (F), or (logical) 1 and (logical) 0, and it makes no difference which we call which, though it is more common to call the high-voltage state ON, TRUE or 1. Because there are only two possible states the algebra governing their combination and association differs rather from the one in everyday use. By using binary numbers any number can be expressed as a string of 1s and 0s and so can be stored in an array of two-state devices. Arithmetic operations can then be carried out with combinations of two-state logic functions.