OR Operation (10th-Physics-Lesson-19.13)

If we connect two switches A and B in parallel combination and make a circuit as shown in fig. 19.22. the value of the output current X in this circuit will depend upon the state of the switches A and B, i.e., upon the value of the input variables A and B.

In this case, the Boolean variables A and B will have a set of four possible values as shown in table 19.4. According to the values of A and B, in each line of this table a circuit has been drawn in the fig. 19.23 (a, b, c, d). It can be seen in this figure that the value of is 0 only in the state when A= 0 and B= 0. In all other states, the current flows through the circuit and the value of X is 1. In the table 19.4, the value of output X is written for all possible values of A and B. this is the truth table of OR operation. We can define OR

Operation to be that in which the output has a value 1 when at least one of its inputs is at 1. The output is 0 only when all the inputs are 0.

OR operation is represented by the plus sign (+). The value of X in the table 19.4 would be written as

X = A +B

Table 19.4

It is read as X equals to A OR B. by using the symbol of OR operation, the various lines of the truth table of OR operation can be written as in table 19.5.

Table 19.5

The electronic circuit which implements the truth table of OR operation is known as OR gate. It is symbolically shown in fig. 19.24. It has two or more than two inputs and has only one output. It operates in such fashion that its output is at 1 (5 volts) even when one of its input is at 1 (5 volts). The value of its output would be 0 only when all of its inputs are at 0. In other words the value of its output is in accordance with the truth table of OR operation.

Note that although the symbol of OR operation is the same as the plus sign, yet its operation is quite different from the action of plus sigh. For example, we would write the fourth line of the truth table of the OR operation as 1 + 1 = 1