Chapter 6 PLC Programming 123
PLC should be in the run and online monitor mode. Then, you will
use problem-solving techniques to find and correct the problem. Notice
that when an instruction is energized, its symbol is highlighted. Using
the force instruction to find and correct the problem is very important.
In the next section, the force instruction and its use is described.
6.9 Using PLC Force Instruction for
Troubleshooting
A method of testing the PLC system without actually closing or
opening input devices is to test the PLC in the forced condition mode.
When a PLC is placed in the force mode, keys on the computer key-
boards are pressed to test the program. The force condition must be used
with extreme caution. A programmer using the force condition is capa-
ble of turning the outputs on and off without actually touching the
input/output devices. In this section, you learn how to use the force
instruction to test the ladder logic program for the relay logic diagram
displayed in Figure 6-29.
Figure 6-29 illustrates 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
pushbutton (RPB) to turn on both motors. Press the green pushbutton
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
Figure 6-28. PLC ladder logic diagram for the relay logic diagram shown in Figure 6-26.