Introduction to Industrial Automation

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  Introduction to Industrial Automation

cycle (first, second, and so on) for the two ways of execution. The logical result of the first rung 

of instructions affects the result of the second rung in the first or second scan cycle depending on 

the way (serial or parallel) the instructions are executed, as shown in Table 7.2. Obviously, after 

the second scan cycle, the result of the program execution will be the same independently of the 

method of execution.

PLC’s “intelligence” restrictions. PLC “intelligence” obviously refers to how “smart” the CPU is 

by means of its capabilities. Let’s consider the LAD one-rung program shown in Figure 7.15 and 

suppose that all the inputs I1.0 to I6.0 are disabled (at logical “0”). During the execution of this 

program, the CPU will produce the following results:

The execution of the instruction 

I1.0

 generates RLO = “1”.

The execution of the instruction 

I2.0

 generates RLO = “0”.

Subsequently, since the CPU realizes that the AND logic is no longer valid and thus inde-

pendent of the state of the other inputs, the final RLO will be a logical “0”. The CPU does not 

execute the other instructions, but proceeds directly to the activation instruction 

Q0.0

 

and then in the next program rung if it exists. Thus, by this functionality it saves time from the 

non-execution of instructions and therefore the scanning cycle of the PLC becomes shorter. A 

PLC whose CPU does not have this ability will execute all the instructions of the rung even if 

there is no need to do so. This ability for PLC smart operations depends on the type of CPU and 

I0.0

Q2.0

Q2.0

Q4.0

Figure 7.14  The logic result of how the first instruction’s row affects the result of the second 

row in the first or second scan cycle, depending on the direction (serial or parallel) of the