Copyright Goodheart-Willcox Co., Inc. 106 Programmable Logic Controllers: Hardware and Programming 0000 0001 Stop Start Control relay Control relay Motor B3:0 B3:0 I:0 I:0 O:0 0 1 0 0 Jog I:0 2 0 Bell O:0 1 End Control relay B3:0 0 Goodheart-Willcox Publisher Figure 6-28. PLC ladder logic diagram for the relay logic diagram shown in Figure 6-26. If the control system does not operate as described above, follow the conditions of the ON/OFF instructions on the screen. Make sure the PLC is in the run/online monitor mode. Then, use problem-solving techniques to find and correct the problem. Notice that when an instruction is energized, its symbol is highlighted. 6.8 Using the PLC Force Instruction for Troubleshooting The force instruction provides a method of testing the PLC system without actually closing or opening input devices, which allows you to test the PLC. When a PLC is in the force mode, you can test the program by pressing keys on the computer keyboard. The force condition must be used with extreme caution. A programmer using the force condition is capable of turning the outputs on and off without actually touching the input/output devices. In this section, you will learn how to use the force instruction to test the ladder logic program for the relay logic diagram displayed in Figure 6-29. The relay logic diagram in Figure 6-29 shows that if the temperature switch is closed, the motors can run in a continuous run mode. The white pilot light indicates that the motors are in continuous run mode. Press the red push- button (RPB) to turn on both motors. Press the green pushbutton (GPB) to turn on only motor #1. Both motors run until the normally closed stop pushbutton is pressed. If the temperature switch is open, motors will be in the jog mode. This means that you must press and hold the pushbuttons in order to run the motors. Figure 6-30 displays the input/output connections of an Allen-Bradley fixed SLC 500 PLC device for the relay logic diagram in Figure 6-29. Table 6-6 illustrates the I/O port assignments. Stop RPB M1 L1 L2 GPB M1 TSW M1 W Stop RPB M2 Stop TSW TSW M2 TSW Goodheart-Willcox Publisher Figure 6-29. Relay logic diagram.