(A) Write the Boolean logic expressions for the push-button switch of Example 9.2 using th...

(A) Write the Boolean logic expressions for the push-button switch of Example 9.2 using the following symbols: X1 = START, X2 = STOP, P = POWER–TO–MOTOR, and M = MOTOR. Hint: See Figure 9.4(b)

Example 9.2

Example 9.2 Push-Button Switch

A push-button switch used for starting and stopping electric motors and other powered devices is a common hardware component in industrial control systems. As shown in Figure 9.4(a), it consists of a box with two buttons, one for START and the other for STOP. When the START button is depressed momentarily by a human operator, power is supplied and maintained to the motor (or other load) until the STOP button is pressed momentarily, which breaks the power to the POWER-TO-MOTOR. POWER-TO-MOTOR is the output of the push-button switch, but it also serves as a contact to power the motor. (a) Define the initial values of the variables. Construct (b) the truth table for the push button and (c) the logic network diagram.

Figure 9.4 (a) Push-button switch of Example 9.2 and (b) its logic network diagram.

Solution: (a) The variables are defined as follows:

(b) The truth table for the push button is presented in Table 9.5. From an initial motor off condition (MOTOR = 0), the motor is started by depressing the start button (START = 1), which momentarily closes the contact. If the stop button is in its normally closed condition (STOP = 1), power is supplied to the motor (POWER–TO–MOTOR = 1), which turns on the motor (MOTOR = 1). When the motor is running (MOTOR = 1), it can be stopped by depressing the stop button (STOP = 0), which breaks the power supply to the motor.

Table 9.5 Truth Table for Push-Button Switch of Example 9.2