Introduction to Industrial Automation

Figure 7.1  The generic form of Ladder logic.

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Basic Programming Principles of PLCs 

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circumstances, the term “ladder diagram” expressed a set of similar but incompatible languages 

from all PLC manufacturers, rather than a common programming language for PLCs. On the 

other hand, this situation forced the PLC user to use a specific manufacturer since he needed to 

use specific tools for software development, learning, maintenance, and upgrading. In this unfor-

tunate situation, the International Electrotechnical Commission (IEC) tried to impose standard-

ization by defining the IEC 61131-3 standard, which is described in the next section.