![]() Sequential Function Chart(SFC) - The "Everything" Language The following is the calculation shown in Instruction List and Structured Text above, as a Function Block Diagram: Inputs and outputs between the blocks are wired graphically using a mouse. These function blocks can be written in any of the five languages, including the Function Block Diagram language again. 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. 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. If this algorithm were to be written in Structured Text, it would be:įunction Block Diagram (FBD) - The Graphical Language This allows graphical representation of logic in a form easily understood by electrical technicians and engineers alike. Ladder Diagram is probably the most popular language for situations that involve relay logic with AND and OR gates. Using Structured text, the code above is written as:Įrror_Calc := ABS(IN1-IN2) Ladder Diagram(LD) - The Electrical Technicians' Language 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. It has operators to allow logical branching (IF), multiple branching (CASE), and looping (FOR, WHILE, REPEAT). Structured Text is a Pascal-like language that generally allows greater flexibility, and less tedium, in writing control algorithms. Structured Text (ST) - The High-Level Language ST Error_Calc (* Store the value in the register into the variable called Error_Calc *) SUB IN2 (* Subtract IN2 from that register, storing the result into that register *)ĪBS (* Perform the Absolute Value function on that register *) LD IN1 (* Load IN1 into the calculations register *) The following is the code used to calculate the absolute value of the difference between two variables named IN1 and IN2 (comments are contained between the (* and *) symbols): Assembler language is relatively tedious to program, but is supposedly faster to execute. Instruction List is most popular for relatively simple, yet frequently used, algorithms. THE FIVE LANGUAGES Instruction List (IL) - The Assembler-style Language A task can be assigned a cyclic rate, can be event driven, or be triggered by specific system functions, such as startup. The general construct of a control algorithm includes the use of "tasks", each of which can have one or more Program POUs. ![]() A function block program, for example, can incorporate ladder diagram logic in it. Under some circumstances, a POU can have a combination of languages. Users can also develop their own libraries, which can include calls to the IEC standard library and any applicable manufacturers' libraries.Īll user-declared POUs, regardless of type, can be written in any of the five languages. An important consideration here is that, if a firmware library is used, the device that receives the program must support that library. ![]() Typically, these would also be firmware libraries. 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.Īdditionally, manufacturers can supply libraries of their own functions. The IEC-1131 Standard includes a library of pre-programmed functions and function blocks. These POUs are reusable within a program and can be stored in user-declared libraries for import into other control programs. The control algorithm can include reusable entities referred to as "program organization units (POUs)" which include Functions, Function Blocks, and Programs. The basic principle of IEC-1131 is that a programmer can develop a control algorithm (referred to as a "Project") using any combination of five control languages Instruction List, Structured Text, Ladder Diagram, Function Block Diagram, and Sequential Function Chart. IEC-1131 - The First Universal Process Control Language INTRODUCTIONĭESCRIPTION OF THE FUNDAMENTAL CONCEPTS OF IEC-1131 By using IEC-1131, a programmer can develop a control algorithm for a particular brand of controller, and import that same program to another brand with minimum modifications, primarily to process input/output subsystems. IEC-1131 is the first international standard for process control software.
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