ST Main Syntax

A Structured Text (ST) program is a list of ST statements. Each statement ends with a semi-colon (";") separator. Inactive or active separators separate names used in the source code (variable identifiers, constants, language keywords...). Inactive separators are space character, end of line or tab stops. Active separators have a well-defined significance, for example, the ">" separator indicates a "greater than" comparison.
Comments enable the inclusion of non-executed information throughout code. Insert comments anywhere in an ST program. Comments can run multiple lines and must begin with "(*" and end with "*)". Do not use interleave comments, for example, comments within comments.
When typing statements, a drop-down combo box lists the available items such as identifiers, operators, functions, and function blocks. Filter the listed items by typing letters, digits, and specific special characters: !, #, $, %, &, \, *, +, -, ,/ <, :, =, >, ?, @, \, ^, _, `, |, and ~.
These are basic types of ST statements:
  • Assignment statement (variable := expression;)
  • Function call
  • Function block call
  • Selection statements (IF, THEN, ELSE, CASE...)
  • Iteration statements (FOR, WHILE, REPEAT...)
  • Control statements (RETURN, EXIT...)
  • Special statements for links with other languages
    TIP: Use wired variables instead of directly represented variables. The compiler does not support a directly represented variable or a default wired variable after wiring the directly represented variable to a user-defined variable. For example, use _IO_QW1_1_10 instead of %QW1.1.10.
Basic coding is black while other items display using customizable colors for ST syntax. The default colors for ST elements are the following:
  • Comments are green
  • The Editor background is white
  • Identifiers are black
  • Numbers are firebrick
  • Operators are black
  • Program organization units (POUs) are blueviolet
  • Punctuation marks are black
  • Reserved words are fuchsia
  • Strings of text are gray
Inactive separators between active separators, literals, and identifiers increase ST program legibility. The ST inactive separators are space (blank), tabs, and end of line. Place end of lines anywhere in a program. These rules apply to inactive separators:
  • Write one statement on one line
  • Use tabs to indent complex statements
  • Insert comments to increase legibility of lines or paragraphs
Examples
Low Readability
imax := max_ite; cond := X12;

if not(cond (* alarm *)

then return; end_if;

for i (* index *) := 1 to max_ite

do if i <> 2 then Spcall();

end_if; end_for;

(* no effect if alarm *)
High Readability 
(* imax : number of iterations *)

(* i: FOR statement index *)

(* cond: process validity *)

imax := max_ite;

cond := X12;

if not (cond) then

return;

end_if;

(* process loop *)

for i := 1 to max_ite do

if i <> 2 then

Spcall ();

end_if;

end_for;
Task
Procedure
Customize the default display settings for ST programs
  1. From the
    Tools
    menu, click
    Options
    .
  2. From the
    Options
    dialog box, expand
    IEC Languages
    , and then click
    Structured Text (ST)
    .
  3. Expand the respective category, customize the required setting, then click
    OK
    .
Customize the display settings for current ST program
  1. From the
    View
    menu, click
    Properties Window
    . The Properties window displays.
  2. Select the Structured Text (ST) Container.
  3. From the
    Properties Window
    :
Customize the font for the required item by clickingstx_ms. The
Font
dialog box displays allowing customization of the font, text size, bold, italic, strikeout, and underline styles.
  • Customize the text color for the required items. The possible colors are custom, web, and system colors.
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