Special Input Device Programming for PLCs

Special programming is required for certain PLC circuits or input connections. Programming normally closed input devices is one example. It is important to remember that how a device behaves in a control program affects its programming.
Normally Closed Devices A normally closed input device can be programmed so that it acts as either a normally opened or normally closed device. For normally closed inputs, the same rules apply. The rule is that if a device wired as a normally close input and must act as such, its reference address will be programmed as normally opened. The following example shows that a normally closed device within a hardwired circuit will be programmed as normally open when it is replaced by the PLC control program. The program doesn’t evaluate the device as an input because it isn’t referenced as such.

Sample Problem 2

Draw the PLC ladder program for the circuit shown in Figure 11 and create an I/O address assignment list. Use addresses 108 to 478 for inputs. Start outputs at 50, and internals at 1008.
Electromechanical relay circuit
Figure 11: Electromechanical relay circuit.

Solution to Sample Problem 2

Figure 12 shows an equivalent PLC ladder diagram of the circuit shown in Figure 11. The I/O address assignment table is shown in Table 7. Because internal coil 100 refers to it, the normally closed contact (CR10) must be programmed as such.
PLC ladder diagram of the circuit in Figure 11
Figure 12: PLC ladder diagram of the circuit in Figure 11.

I/O Address









Table 7. I/O address assignment table.
Master Control Relays. A master control relay (MCR) is another circuit that programmers should be aware. An MCR coil is used to control several rungs of an electromechanical circuit diagram by switching ON/OFF power to the rungs. An MCR coil is not able to control the end of a hardwired circuit unless the circuit is fully followed. In Figure 13, for example, the MCR output at line 1 controls power to hardwired elements starting from line 3, which is where the MCR contact is, and ending at line 51.
Electromechanical relay circuit with a master control relay
Figure 13: Electromechanical relay circuit with a master control relay.
When the master control relay (ON), power will flow to these lines (4 through 51). If the master control relay turns off, power will not flow to these rungs (lines 4 through 51). These devices will not be able to execute the control action. This configuration is similar to a hardwired subprogram, or subroutine. If the master control relay is ON, the rungs will be executed. If it is OFF the rungs will not be executed. The circuit’s line 2 contains power branches that connect to other circuits not affected by MCR’s actions. These circuits are the standard hardwired program.
The programmer must place the END MCR instruction following the last rung that the MCR should control during the translation from a hardwired ladder to PLC symbology. Figure 14 shows the location of the MCR instruction in the circuit. Figure 14 shows the placement of the MCR instruction for the circuit in Figure 13. This is how the hardwired circuit works.
PLC ladder diagram with MCR fence
Figure 14: PLC ladder diagram with MCR fence.
The MCR fence is ended by the END1 instruction. After the END1 instruction, the instructions corresponding to hardwired circuits which branch from line 2 of the electromechanical diagram in Figure 13 are found. Figure15 shows a partial ladder of an elaborate circuit with this MCR type. After the rung containing PL3 output, the corresponding PLC program must have an ENDMCR.
Electromechanical relay circuit with an MCR
Figure 15: Electromechanical relay circuit with an MCR.

Sample Problem 3

In Figure 15, highlight the sections that will be controlled by a PLC MCR. What additional steps must be taken in order to bypass or include other hardwired circuits inside the MCR fence

Solution to Sample Problem 3

Figure 16 shows the circuits that need to be fenced according to the MCR instruction. The MCR fencing does not include solenoid SOL1 or part of its driving logic. However, SOL1, CR3, TDR1 and TDR1 may be turned ON using logic before the MCR fence (see Figure 17.). The MCR fence must be properly programmed by the PLC program. It must contain two internal control relays to take out SOL1.

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