Introduction to structure text (ST)

-

The Structured Text (ST) language defined by IEC 61131-3 is a high-level language
whose syntax is similar to Pascal. In general, ST is useful for implementing calculation intensive functions and other functions that are difficult to implement in the other languages. The ST language has a complete set of constructs to handle variable assignment, conditional statements, iteration, and function block calling.

Expressions

An expression is a construct which, when evaluated, yields a value corresponding to one of the data types.
Expressions are composed of operators and operands. An operand shall be a, a variable, a function invocation, or another expression.
The operators of the ST language are summarized in below table. 
The evaluation of an expression consists of applying the operators to the operands in a sequence defined by the operator precedence shown in below table. 
The operator with highest precedence in an expression shall be applied first, followed by the operator of next lower precedence, etc., until evaluation is complete. 
Operators of equal precedence shall be applied as written in the expression from left to right

For example, if A, B, C, and D are of type INT with values 1, 2, 3, and 4, respectively, then
A+B-C*ABS(D) shall evaluate to -9, 
and (A+B-C)*ABS(D) shall evaluate to 0 .

When an operator has two operands, the leftmost operand shall be evaluated first. For example, in the expression SIN(A)*COS(B), the expression SIN(A) shall be evaluated first, followed by COS(B), followed by evaluation of the product.

Boolean expressions may be evaluated only to the extent necessary to determine the resultant value. For instance, if A<=B, then only the expression (A>B) would be evaluated to determine that the value of the expression (A>B) & (C<D) is Boolean zero.

Functions shall be invoked as elements of expressions consisting of the function name followed by a parenthesized list of arguments.



Statements

The statements of the ST language are summarized in below table. Statements shall be terminated by semicolons


Comments

Post a Comment

Popular posts from this blog

Control system documentation

-
Image
One of the basic activity in any project is to ensure it is well documented.  Automation and control projects are no exception, though they have different and unique requirements to properly capture the requirements, outcomes, and deliverable of  the work being performed.  The International Society of Automation (ISA) has developed  standards that are broadly accepted and used across the industry as the preferred method for  documenting control systems. The purpose of this post is to highlight and present briefly the major instrumentation and control documentation required for successful project execution. The information presented in this post is according to relevant standards from ISA (International society of automation). Why do we document? Automation and control solutions become more complex and involve the inputs of and coordination with different disciplines such as process, Mechanical, Electrical,... Documentation i...
Read more

Introduction to IEC61131-3 programming languages

-
Image
IEC 61131-3 is set to standardize programming languages for industrial automation that is independent of any platform brand or automation vendor IEC 61131-3 is the third part of the IEC 61131 family which consists of nine parts as follows: Part 1: General Overview Part 2 Equipment Requirements & Tests Part 3 Programming Languages Part 4 User Guidelines Part 5 Communications Part 6 Functional Safety Part 7 Fuzzy Control Programming Part 8 Application Guidelines Part 9 Communication Interface IEC 61131-3 defines five programming languages for industrial automation: Ladder Diagram (LD) Function Block Diagram (FBD) Structured text (ST) Instruction List (IL) Sequential Function Chart (SFC) Even though ladder diagram (LD) was initially developed for programmable logic controllers (PLC) and Function block diagram (FBD) was originally developed for distributed control system (DCS), a PLC is no longer limited to Ladder Diagram (LD) and DCS is...
Read more