PLC Example 1 :
A signal lamp is required to be switched on if a pump is running and the pressure is satisfactory, or if the lamp test switch is closed.
For the inputs from the pump and the pressure sensors we have an AND logic situation since both are required if there is to be an output from the lamp. We, however, have an OR logic situation with the test switch in that it is required to give an output of lamp on regardless of whether there is a signal from the AND system.
Also Read: PLC Latching Function Logic
The function block diagram and the ladder diagram are thus of the form shown in Figure 1.37. Note that with the ladder diagram we tell the PLC when it has reached the end of the program by the use of the END or RET instruction,
PLC Example 2 :
As another example, consider a valve which is to be operated to lift a load when a pump is running and either the lift switch is operated or a switch operated indicating that the load has not already been lifted and is at the bottom of its lilt channel. We have an OR situation for the two switches and an AND situation involving the two switches and the pump. Figure 1.38 shows a possible program.
PLC Example 3 :
As another example, consider a system where there has to be no output when any one of four sensors gives an output, otherwise there is to be an output. One way we could write a program for this is for each sensor to have contacts that are normally closed so there is an output. When there is an input to the sensor the contacts open and the output stops. We have an AND logic situation. Figure 1.39 shows the functional block diagram and the ladder diagram of a system that might be used.
Also Read: Introduction to PLC Programming