The Logic OR Function function states that an output action will occur or become TRUE if either one “OR” more events are TRUE, but the order at which they occur is unimportant as it does not affect the final result. For example, A + B = B + A. In Boolean algebra the Logic OR Function follows the Commutative Law the same as for the logic AND function, allowing a change in position of either variable.
The OR function is sometimes called by its full name of “Inclusive OR” in contrast to the Exclusive-OR function we will look at later in tutorial six.
The logic or Boolean expression given for a logic OR gate is that for Logical Addition which is denoted by a plus sign, (+). Thus a 2-input (A B) Logic OR Gate has an output term represented by the Boolean expression of: A+B = Q.
Here the two switches A and B are connected in parallel and either Switch A OR Switch B can be closed in order to put the lamp on. In other words, either switch can be closed, or at logic “1” for the lamp to be “ON”.
Then this type of logic gate only produces and output when “ANY” of its inputs are present and in Boolean Algebra terms the output will be TRUE when any of its inputs are TRUE. In electrical terms, the logic OR function is equal to a parallel circuit.
Again as with the AND function there are two switches, each with two possible positions open or closed so therefore there will be 4 different ways of arranging the switches.
|Switch A||Switch B||Output||Description|
|0||0||0||A and B are both open, lamp OFF|
|0||1||1||A is open and B is closed, lamp ON|
|1||0||1||A is closed and B is open, lamp ON|
|1||1||1||A is closed and B is closed, lamp ON|
|Boolean Expression (A OR B)||A + B|
Logic OR gates are available as standard i.c. packages such as the common TTL 74LS32 Quadruple 2-input Positive OR Gates. As with the previous AND Gate, OR can also be “cascaded” together to produce circuits with more inputs such as in security alarm systems (Zone A or Zone B or Zone C,etc).