Programmable Logic Control Systems
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Programming of PLCs: Sequential Function Charts
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After learning the lesson students should be able to A. Describe the major features of the IEC 1131-3 standard for PLC programming B. Describe the major syntax conventions of the SFC programminglanguage C. Identify valid and invalid SFC segments D. Develop SFC programs for simple sequence control problems
We have studied the RLL in a previous lesson. There are also other languages to program a PLC in, than the RLL. Most of the significant manufacturers support about 3 to 5 programming languages. Some of these languages, such as the RLL, have been in use for a long time.While most manufacturers used similar languages, these were not standardised in terms of syntactic features. Thus programs developed for one would not run in another without considerable modifications, often mostly syntactic. In the last few years there has been effort to standardise the PLC programming languages by the International Electrotechnical Commission (IEC). One of the languages, namely theSequential Function Chart, which offers significant advantages towards development of complex structured PLC programs for concurrent industrial processes, is studied in detail. Know other languages are introduced in brief.
IEC 1131-3: The Language Standard
IEC 1131 is an international standard for PLCs formulated by the InternationalElectrotechnical Commission (IEC). As regards PLC programming, it specifies the syntax, semantics and graphics symbols for the following PLC programming languages:
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Ladder diagram (LD) Sequential Function Charts (SFC) Function Block Diagram (FBD) Structured Text (ST) Instruction List (IL)
IEC 1131 was developed to address the industry demands for greater interoperability and standardisationamong PLC hardware and software products and was completed in 1993. A component of the IEC 1131, the IEC 1131-3 define the standards for data types and programming. The goal for developing the standard was to propose a programming paradigm that would contain features to suit a large variety of control applications, which would eliminate proprietary barriers for the customer and their associatedtraining costs. The language specification takes into account modern software engineering principles for developing clean, readable and modular code. One of the benefits of the standard is that it allows multiple languages to be used simultaneously, thus enabling the program developer to use the language best suited to each control task. Version 2 EE IIT, Kharagpur 3
Major Features of IEC 1131-3The following are some of the major features of the standard. 1. Multiple Language Support: One of the main features of the standard is that it allows multiple languages to be used simultaneously, thus enabling the program developer to use the language best suited to each control task. 2. Code Reusability: The control algorithm can include reusable entities referred to as "program organizationunits (POUs)" which include Functions, Function Blocks, and Programs. These POUs are reusable within a program and can be stored in user-declared libraries for import into other control programs. 3. Library Support: The IEC-1131 Standard includes a library of pre-programmed functions and function blocks. An IEC compliant controller supports these as a "firmware" library, that is, the library ispre-coded in executable form into a prom or flash ram on the device. Additionally, manufacturers can supply libraries of their own functions. Users can also develop their own libraries, which can include calls to the IEC standard library and any applicable manufacturers' libraries. 4. Execution Models: The general construct of a control algorithm includes the use of "tasks", each of which can have...