Programmable Controllers: Theory and Implementation

Introduction to PLC Programming and Implementation—from relay logic to PLC logicremote systems also have fault contacts incorporated into the remote controllers.This allows subsystem failures to be signaled promptly, so that theproblem can be fixed without endangering personnel.SPECIAL INPUT DEVICE PROGRAMMINGSome PLC circuits and input connections require special programming. Oneexample is the programming of normally closed input devices. Rememberthat the programming of a device is closely related to how that device shouldbehave in the control program.Normally Closed Devices. An input device that is wired as a normallyopen input can be programmed to act as either a normally open or a normallyclosed device. The same rule applies for normally closed inputs. Generally,if a device is wired as a normally closed input and it must act as a normallyclosed input, its reference address is programmed as normally open. As thefollowing example illustrates, however, a normally closed device in ahardwired circuit is programmed as normally closed when it is replaced in thePLC control program. Since it is not referenced as an input, the program doesnot evaluate the device as a real input.EXAMPLE 2For the circuit in Figure 11, draw the PLC ladder program and createan I/O address assignment table. For inputs, use addresses 10 8through 47 8 . Start outputs at address 50 8 and internals at address 100 8 .L1LS14PS1CR10L2CR10CR10SOL7LS15Figure 11. Electromechanical relay circuit.SOLUTIONFigure 12 shows the equivalent PLC ladder diagram for the circuit inFigure 11. Table 7 shows the I/O address assignment table for thisexample. The normally closed contact (CR10) is programmed asnormally closed because internal coil 100 references it and requiresit to operate as a normally closed contact.Industrial Text & Video Company www.industrialtext.com 1-800-752-839817