![]() ![]() 3) Allows control to move to the next step when conditions met (basically an if or wait instruction) 4) Initial step: the first step 5) Step: Basically a state of operation. 1) Flow lines - connects steps and transitions (these basically indicate sequence) 2) Transition - causes a shift between steps, acts as a point of coordination. The basic elements of an SFC diagram are shown below. Actions can be described by any of the five languages defined by the IEC 1131-3 standard. Each step is associated with some action, performed while the step is active. When a condition following an active step becomes true, this results in the next step being activated and the active step becoming deactivated. Each transition is associated with a condition. A line between two steps depicts a transition. The steps are connected by transitions. ![]() The initial step is the first step to be activated whenever a SFC is started. At least one of these steps is the initial step, which is depicted by a rectangular box containing vertical bars. Each step represents a particular state of the system being controlled. A number of steps these steps are being depicted by rectangular boxes. A SFC consists of the following elements. It can be used at the top level to show the main phases of a process, but it can also be used at any other lower level. One of the most important aspects of SFC is that it shows the main states of a system, all the possible changes of state and the reasons why those changes would occur. Sequential Function Chart: Sequential function chart (SFC) is a graphical language, which makes it possible to depict sequential behaviour. It is used for defining control sequences that are time- and event-driven. Sequential function chart: Sequential function chart is a graphical language for depicting sequential behaviour of a control system. In function block diagrams function are depicted as some kind of chip, with input ports and output ports. Instruction list: Instruction list (IL) is a kind of assembler language Function block diagram: Function block diagram (FBD) is a graphical language. Structured text: Structured text (ST) looks a lot like Pascal. The first two are textual languages and the other three are graphical. The five languages are structured text, instruction list, ladder diagram, function block diagram and sequential function charts. They can all be connected with each other. Each of these five languages can be used to describe parts of the program. The five languages are a kind of average of all languages used in the PLC industry. Sequential Function Chart (PLC) By Chaitanya Pattapagala (2009ME20659) Satyendra Singh Kardam (2009ME20680) The IEC 61131-3 standard IEC 61131 deals with PLC programming language standards: In the standard five different languages are defined. ![]()
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