Saturday, November 27, 2010

Structured Text PLC Programming

Structured Text language resembles closely a high level computer programming language such as C or Pascal with its IF…THEN loops, CASE selectors, and line ending in semicolons. Structured Text has seen the greatest increase in adoption, the aforementioned control engineering survey indicated that of all the IEC 61131 defined programming languages.

This language perhaps best embraces the growing complexity of Programming PLC, such as the functions of process control involved in chemical or plastic manufacturing. Calculus, trigonometry, and data analysis can be implemented far easier in this language than in Instruction List or Ladder. Pointers (variables used to do indirect addressing) and decision loops allow for a more compact program implementation than can be achieved in Ladder.

The flexible Structured Text editor that is common in most packages of programming makes it easy to insert comments throughout a program, and to use line spacing and indents to emphasize related sections of code. This makes the structuring task a complex program easier. The text based, Structured Text non graphical nature is similar to Instruction List also runs much faster than Ladder programming.

An additional Structured Text benefit is that it comes closer than most of the other language in achieving the IEC 61131 standard of transferability promise. Copying and pasting Structured Text from one programming package editor to another can often be done with just a few changes, emancipating a programmer from the hardware platform. A final benefit is that recently many students graduating from engineering studies have a better background in computer languages than in the electrical wiring basics, and therefore can be more proficient in Structured Text than Ladder programming.

A disadvantage is that for many experienced previously maintenance or programmers and service personnel, the Structured Text environment is somewhat unsuitable and unfamiliar for troubleshooting. In many ways, the structure and code necessary to make this code maintenance friendly can reduce some of the advantages gained from its compactness.

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