Some PLC circuit and inputs connections require special programming. On example is normally programming closed input devices. Remember that the programming of a device is related closely to how that device should behave in the control program.
Normally closed devices
An input device that is wired as a normally open input can be programmed to act as either a normally open or normally closed device. The same rule applies for normally closed inputs. Generally, if a device is wired as normally closed input and it must act as a normally closed input, its reference address is programmed as normally open. As the following illustrates, however, a normally closed device in a hardwired circuit is programmed as normally closed when it is replaced in the PLC control program. The programs do not evaluate the device as a real input since it is not referenced as an input.
Master control relays
The other circuit the programmer should be aware of is a MCR (master control relay). An MCR coil controls several rungs by switching ON or OFF the power to those rungs in electromechanical circuit diagrams. For instance, the MCR output in line 1 controls the power to the hardwired elements from line 3, where the MCR contact is located, to the last element in line 51. If the master control relay is ON, power will flow to these rungs. If the master control relay is OFF, power will not flow and these devices will not implement the control action. This configuration is equivalent to a hardwired subroutine or subprogram, if the MCR is ON, the rungs are executed, if it is OFF, the rungs are not executed. Power branches to other circuits which are not affected by the MCR’s action at line 2 in the circuit. Theses circuits are the regular of hardwired program.