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Structured Plc Programming With Sequential Function Charts

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2001 Annual Conference


Albuquerque, New Mexico

Publication Date

June 24, 2001

Start Date

June 24, 2001

End Date

June 27, 2001



Page Count


Page Numbers

6.898.1 - 6.898.6

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James Rehg

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NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract

Session 2648

Structured PLC Programming with Sequential Function Charts James A. Rehg Pennsylvania State University


Programmable logic controllers (PLCs) have been programmed using ladder logic since their introduction in the early 1970s. The programming technique most often used follows an empirical process that begins with entering the first rung to satisfy the first output requirement. Rungs continue to be entered leading toward a solution to the problem. At some point in the programming process the target system fails to respond as desired when the program is tested, and corrections are made to rungs in order to fix the problems. This process continues until a program solution is reached. The result is a program without sequential structure that is difficult to analyze and understand. With large PLC programs, troubleshooting the program when system problems develop is difficult.

This paper describes how a technique called sequential function charts (SFC) is used in a two- course sequence covering PLC programming. The SFC process adds structure to the PLC programming process and produces programs that are easy to analyze and troubleshoot. The paper also includes a description of the SFC process; types of problems assigned for student work; and the benefit derived from using SFC in a PLC programming laboratory or class.


PLCs have been used for industrial control since their development in the early 1970s. The PLC programming language, called ladder logic, was designed to look like two-wire relay control logic schematics. This similarity was necessary at the start of PLC applications because the electricians who were responsible for the maintenance of the systems were familiar with relay diagrams. As a result, the integration of PLCs into industrial control applications was so successful that PLCs are used in industrial plants of all sizes in most automated manufacturing plants around the globe. However, the overwhelming success of PLCs and the ladder logic language has caused the following problems:

• No industry-wide standards for ladder logic program syntax exists • Programs are not interchangeable among PLCs from different vendors • Programs written by different programmers for the same problem rarely are the same • Ladder logic programs are difficult to read and troubleshoot • Ladder logic is not well suited to some of the current high-end automation applications

“Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright  2001, American Society for Engineering Education”

Rehg, J. (2001, June), Structured Plc Programming With Sequential Function Charts Paper presented at 2001 Annual Conference, Albuquerque, New Mexico.

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