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Chapter 8: Practical Techniques in Sequence Control Design

By Jawahar Rawtani
From Practical Batch Process Management

8.1 Introduction

There are many different ways of programming PLCs/DCS and a computer. However, there are some standard programming techniques for structured programming, which make program codes more portable, understandable and reusable within and across platforms. In this chapter we will discuss some practical techniques for programming sequence control.

8.2 Programming PLCs/DCS

The International Electrotechnical Commission (IEC) 1131-3 standard on Programmable Controllers Programming languages specifies the syntax and semantics of a unified suite of programming languages for programmable controllers. The standard specifies two textual languages, namely instruction list (IL) and structured text (ST), and two graphical languages, namely ladder diagram (LD) and function block diagram (FBD). The sequential function chart (SFC) elements are defined for structuring the internal organization of programmable controller programs and function blocks. Also, configuration elements are defined which support the installation of programmable controller program into programmable controller systems.

8.2.1 Ladder Diagram (LD)

Ladder diagram is a conventional form of programming the programmable logic controllers (PLCs). Ladder logic was originally developed as replacement for electrical relays in control circuits and motor starter circuits. PLCs were used to implement control circuits similar to wired-control circuits with electrical relays. Over time, many advanced control functions capabilities have been incorporated in the PLCs. Ladder diagrams are best suited for discrete control and interlocking.

8.2.2 Structured Text (ST)

Structured text is very close to traditional computer programming, similar to high-level programming languages like C or Pascal language. It utilizes standard programming constructs. Structured text is used for high-level...

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