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IEC is the first international standard for process control software. The control algorithm can include reusable entities referred to as "program organization units 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. The IEC Standard includes a library of pre-programmed functions and function blocks. Any controller that is IEC compliant supports these as a "firmware" library, that is, the code for these is pre-written into a prom or flash ram on the device.
Additionally, manufacturers can supply libraries of their own functions. Typically, these would also be firmware libraries. An important consideration here is that, if a firmware library is used, the device that receives the program must support that library.
Users can also develop their own libraries, which can include calls to the IEC standard library and any applicable manufacturers' libraries. All user-declared POUs, regardless of type, can be written in any of the five languages. Under some circumstances, a POU can have a combination of languages. A function block program, for example, can incorporate ladder diagram logic in it.
The general construct of a control algorithm includes the use of "tasks", each of which can have one or more Program POUs. A task can be assigned a cyclic rate, can be event driven, or be triggered by specific system functions, such as startup.
Instruction List is most popular for relatively simple, yet frequently used, algorithms. Assembler language is relatively tedious to program, but is supposedly faster to execute. Structured Text is a Pascal-like language that generally allows greater flexibility, and less tedium, in writing control algorithms. Typically, a programmer would create his own algorithms as Functions or Function Blocks in Structured Text and use them as callable procedures in any of the five languages.
This allows graphical representation of logic in a form easily understood by electrical technicians and engineers alike. A brief example would be:. The Function Block Diagram Language allows control algorithms to be developed graphically by inserting the program units called Functions and Function Blocks into a control program.
These blocks can be called from a library of functions specified by the IEC standard, or can be called from manufacturer-supplied or user-created libraries. These function blocks can be written in any of the five languages, including the Function Block Diagram language again.
Inputs and outputs between the blocks are wired graphically using a mouse. Sequential Function charts allow complex algorithms to be executed using a series of "steps" and "transitions".
This could be done by anything within the program. Complex algorithms can be developed using multiple branching techniques. Also actions can be directed to continue running, run once, or terminate, instead of running continuously. Functions are pre-programmed calculations that accept numerous inputs, but return only one output.
The Function must be declared as a variable type, can be created in any of the five languages, and can be used in any of the five languages. It is always referred to by its created name. The standard library of IEC consists mostly of Functions.
A Function, when used, does not consume additional memory. It is simply a procedure call, which uses an existing equation. The following is an example of the Function shown on the previous page, in a Structured Text equation:.
Function Blocks are pre-programmed calculations that accept numerous inputs, and can return several outputs. The Function Block can be created in any of the five languages, and can be used in any of the five languages. Any use of a Function Block is referred to as an "instance" of that block.
Each instance must be given a name that is unique to the POU in which the block resides. What distinguishes a Function Block from a Function is that each instance contains a unique set of values that are retained with every execution of the instance.
Its purpose is to generate a Boolean pulse the coil "Output" at regular intervals when started by a Boolean variable the relay "Run" going high, and it can be stopped by another Boolean variable the relay "Reset" going high:.
Programs are simply POUs created in any of the languages, which can incorporate unique code, or can include any Functions or Function Blocks, created locally to a Project, or referenced from external Libraries. Tasks are the devices that execute Program POUs. System tasks are triggered to execute once on specific events within the running program. These include cold and warm program starts, floating point errors, and stack overflows.
Cyclic tasks run at programmer-declared intervals. IEC allows multiple tasks, and these can be assigned priorities from 0 to 31 0 being highest. All variables used within a project must be declared, either locally to a POU or globally to the project. Regardless of the type of POU or Language used, all variables must be declared. Additionally, user-declared variable types can be created that are "structures", or combinations of several variable types.
Also, arrays can be created that are combinations of structures. An example is:. The variable's name can be used in other POUs with no conflict within the project. An example:. When a Function or Function Block is created, it must have input and output terminals. Notice that Input and Output variables can be assigned initial values.
This declaration would occur in the "System Resource" section of the project. Once variables have been declared globally, they can then be used in any POU by being re-declared as "External". IEC is a powerful, flexible, and adaptable standard that includes something for every programming taste and style.
Users can create programs in any combination of the five languages, and can develop code as POUs that are re-usable within a project, and can be stored as libraries for use in other projects.
The manufacturers who support this standard are free to use their own on-line interface software, and future developments will allow more powerful communications capabilities between controllers of different manufacturers.
This article was written and provided by Bristol Babcock. Bristol Babcock is a leading supplier of measurement and control instruments and systems with offices throughout the world. Bristol Babcock is a supplier of measurement and control instruments and systems with offices throughout the world. News Facebook LinkedIn Twitter. About The Author Bristol Babcock is a supplier of measurement and control instruments and systems with offices throughout the world.
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IEC-1131 - The First Universal Process Control Language
Safe Comp 97 pp Cite as. Programmable Logic Controllers PLCs are playing an increasing role in the construction of safety critical systems. Here we consider a subset of the IEC —3 languages, that of function blocks, and present an assessment of the standard from the viewpoint of providing a formal semantics to that subset. In doing so, we also provide justification for our decisions in resolving ambiguities in the standard. Finally, we comment on the overall structure of the standard and, particularly, on how it relates function blocks to the other programming elements.
IEC is the first international standard for process control software. The control algorithm can include reusable entities referred to as "program organization units 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. The IEC Standard includes a library of pre-programmed functions and function blocks. Any controller that is IEC compliant supports these as a "firmware" library, that is, the code for these is pre-written into a prom or flash ram on the device. Additionally, manufacturers can supply libraries of their own functions. Typically, these would also be firmware libraries.
An Assessment of the IEC 1131-3 Standard on Languages for Programmable Controllers