Introduction to PLC Programming and Implementation (9)

SPECIAL INPUT DEVICE PROGRAMMING

Some PLC circuits and input connections require special programming. One example is the programming of normally closed input devices. Remember that the programming of a device is closely related 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 a normally closed 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 normally closed input, its reference address is programmed as normally open. As the following example illustrates, however, a normally closed device in a hardwired circuit is programmed as normally closed when it is replaced in the PLC control program. Since it is not referenced as an input, the program does not evaluate the device as a real input.

EXAMPLE 2
For the circuit in Figure 11, draw the PLC ladder program and create an I/O address assignment table. For inputs, use addresses 10(8) through 47(8). Start outputs at address 50(8) and internals at address 100(8).

SOLUTION
Figure 12 shows the equivalent PLC ladder diagram for the circuit in Figure 11. Table 7 shows the I/O address assignment table for this example. The normally closed contact (CR10) is programmed as normally closed because internal coil 100 references it and requires it to operate as a normally closed contact.

Master Control Relays. Another circuit the programmer should be aware of is a master control relay (MCR). In electromechanical circuit diagrams, an MCR coil controls several rungs in a circuit by switching ON or OFF the power to those rungs. In a hardwired circuit, there is no definite end to an MCR except when the circuit is followed all the way through. For example, in Figure13, 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 (lines 4 through 51). 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 subprogram or subroutine—if the MCR is ON, the rungs are executed; if it is OFF, the rungs are not executed. At line 2 in the circuit, power branches to other circuits that are not affected by the MCR’s action. These circuits are the regular hardwired program.
During the translation from a hardwired ladder circuit to PLC symbology, the programmer must place an END MCR instruction after the last rung the MCR should control. Figure14 illustrates the placement of the MCR instruction for the circuit in Figure 13. To provide proper fencing for the program’s MCR control section, internal output coil 1000, labeled CR1 (line 1 of PLC program), was inserted so that PL1 would not be inside the fenced MCR area. This is the way the hardwired circuit operates.

The END1 instruction ends the MCR fence. The instructions corresponding to the hardwired circuits that branch from line 2 in the electromechanical diagram of Figure 13 are located after the END1 instruction. Figure15 illustrates a partial ladder rung of a more elaborate circuit with this type of MCR condition. The corresponding PLC program should have an END MCR after the rung containing the PL3 output.

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