Structured text is one of the 5 languages supported by the IEC 61131-3

IEC 61131-3

IEC 61131-3 is the third part of the open international standard IEC 61131 for programmable logic controllers, and was first published in December 1993 by the IEC...

 standard. It is designed for programmable logic controller

Programmable logic controller

A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...

s (PLCs). It is a high level language that is block structured and syntactically resembles Pascal

Pascal (programming language)

Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...

. All of the languages share IEC61131 Common Elements. The variables and function calls are defined by the common elements so different languages can be used in the same program.
Complex statements and nested instructions are supported:

• Iteration loops (REPEAT-UNTIL; WHILE-DO)
• Conditional execution (IF-THEN-ELSE; CASE)
• Functions (SQRT, SIN)

Sample Program

(* simple state machine *)
TxtState := STATES[StateMachine];
 
CASE StateMachine OF
1: ClosingValve;
ELSE
;; BadCase;
END_CASE;

Additional ST Example Programming examples

// PLC configuration
CONFIGURATION DefaultCfg
VAR_GLOBAL
Start_Stop : BOOL; // Global variable to represent Operator Input
ON_OFF : BOOL; // Global variable to represent Output Coil
END_VAR

// Schedule the main program to be executed every 20 ms
TASK Tick(INTERVAL := t#20ms);

PROGRAM Main WITH Tick : Monitor_Start_Stop;
END_CONFIGURATION

PROGRAM Monitor_Start_Stop // Actual Program
VAR_EXTERNAL
Start_Stop : BOOL;
ON_OFF : BOOL;
END_VAR
VAR // Temporary variables for logic handling
ONS_Trig : BOOL;
Rising_ONS : BOOL;
END_VAR

// Start of Logic
// Catch the Rising Edge One Shot of the Start_Stop input
ONS_Trig := Start_Stop AND NOT Rising_ONS;

// Main Logic for Run_Contact -- Toggle ON / Toggle OFF ---
ON_OFF := (ONS_Trig AND NOT ON_OFF) OR (ON_OFF AND NOT ONS_Trig);

// Rising One Shot logic
Rising_ONS := Start_Stop;
END_PROGRAM

Function Block example

//

// Function Block Timed Counter : Incremental count of the timed interval
//

FUNCTION_BLOCK FB_Timed_Counter
VAR_INPUT
Execute : BOOL := FALSE; // Trigger signal to begin Timed Counting
Time_Increment : REAL := 1.25; // Enter Cycle Time (Seconds) between counts
Count_Cycles : INT := 20; // Number of Desired Count Cycles
END_VAR

VAR_OUTPUT
Timer_Done_Bit : BOOL := FALSE; // One Shot Bit indicating Timer Cycle Done
Count_Complete : BOOL := FALSE; // Output Bit indicating the Count is complete
Current_Count : INT := 0; // Accumulating Value of Counter
END_VAR

VAR
CycleTimer : TON; // Timer FB from Command Library
CycleCounter : CTU; // Counter FB from Command Library
TimerPreset : TIME; // Converted Time_Increment in Seconds to MS
END_VAR

// Start of Function Block programming
TimerPreset := REAL_TO_TIME(in := Time_Increment) * 1000;

CycleTimer(
in := Execute AND NOT CycleTimer.Q
,pt := TimerPreset);

Timer_Done_Bit := CycleTimer.Q;

CycleCounter(
cu := CycleTimer.Q
,r := NOT Execute
,pv := Count_Cycles);

Current_Count := CycleCounter.cv;
Count_Complete := CycleCounter.q;

END_FUNCTION_BLOCK