Introduction to Industrial Automation

7.5.1   General Highlights and Restrictions in PLC Programming

Parallel or serial execution of LAD instructions. For a program written in the LAD language, there 

are two ways to execute its instructions. The CPU can execute the instructions either serially in a 

row-by-row (rung) approach or parallel in a column-by-column approach, as presented in Figure 

7.13. In serial execution, all the instructions are first executed in a row, and then the program 

proceeds to the next row. In parallel execution, the program executes the first instruction initially 

from all the rows (= column) of the program, and then it proceeds to execute the second instruc-

tion from all the rows and so on, while the activation instructions of the last column are executed 

at the end. In general, there has not been a standardized way to execute the instructions of a LAD 

program, while on the contrary, each manufacturer has followed the approach that is considered as 

most suitable for its digital system. For example, AEG Modicon follows parallel execution, while 

Allen-Bradley follows serial execution.

What matters for the parallel or serial execution of LAD instructions is whether the end result 

is the same. The answer to this question will be given with an example. Consider the LAD pro-

gram with two rungs of instructions, shown in Figure 7.14. Suppose also that the digital input I0.0 

is energized, then the execution of the instruction 

I0.0

 gives RLO = “1”. Subsequently, we can 

follow the evolution and change of the RLO per executed instruction and sequentially per scan 

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Basic Programming Principles of PLCs 

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