Tuesday, July 26, 2011

Languages in PLC Core Application

The languages allowed in the PLC core application will be those defined in IEC 61131-3:
• ST (Structured Text)
• SFC (Sequential function Chart)
• IL (Instruction list)
• LD (Ladder Diagram)
• FBD (Function Block Diagram)
Flow Charts of Siemens CFC will not be permitted in the controllers of conventional. However, CFC will be applied for redundant architectures, organized in the systems of interlock and safety. Siemens HiGraph, Petri Nets, will not be permitted either, because they are not defined in IEC 61131-3.

It is nearly possible to apply anything in any language, nevertheless, each language has its own typical and has been generated in order to solve exacting issues. The below rules must be executed in order to maintain the program of PLC as clean and readable as possible:
• SFC will be utilized to execute sequences (GRAFCET), but output will not be written straightly in GRAFCETS.
• ST will be utilized to execute complicated numerical algorithms, loop algorithms, machines of complex state or Petri nets where a sequence is insufficient. As ST is a structured language, quite similar to Pascal, it is more readable than IL.
• IL will be avoided to use it because assembler is not easy to maintain or read by user who did not create the code. It will be applied only in cases where, for instance, a particular instruction is not existing in ST or performance optimization is required.
• FBD and LD will be utilized to execute the logic Boolean and interlocking. Characteristically all the logic requisite to start or stop a device will be realized in LD. No complicated numerical calculation is allowed in FBD and LD. The option of FBD or LD is left to the programmer. Generally electronic engineers use FBD and electrical engineers use LD. The option is subjective since the representation can be switched LD to FBD in STEP7.

Languages organization in STEP7
The basic language in STEP 7 is IL. All other meta-languages and graphical languages are developed as the STL language abstraction. If a program is built in LD, FBD, ST or SFC, it will lastly end up in blocks of LIST after collection. The same editor is used to program in LD, FBD and IL. It is probable to change very easily from the graphical languages FBD and LD to LIST without recompilation. The other method is not so clear, IL can only be exposed in FBD or LD if certain rules are appreciated. SFC utilizes a specific editor of graphical: S7 Graph. After editing, the code is collected in uncommented blocks of IL. These blocks can be analyzed in IL, but any adjustment will damage the representation of graphical.


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