Structured text

Structured text, abbreviated as ST or STX, is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs).^[1]^[2] It is a high level language that is block structured and syntactically resembles Pascal, on which it is based.^[3] All of the languages share IEC61131 Common Elements. The variables and function calls are defined by the common elements so different languages within the IEC 61131-3 standard 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)^[3]
Functions (SQRT(), SIN())

Sample program edit

(* simple state machine *)TxtState := STATES[StateMachine];CASE StateMachine OF   1: ClosingValve();      StateMachine := 2;   2: OpeningValve();ELSE    BadCase();END_CASE;

Unlike in some other programming languages, there is no fallthrough for the CASE statement: the first matching condition is entered, and after running its statements, the CASE block is left without checking other conditions.

Additional ST programming examples edit

// PLC configurationCONFIGURATION DefaultCfg    VAR_GLOBAL        b_Start_Stop  : BOOL;         // Global variable to represent a boolean.        b_ON_OFF      : BOOL;         // Global variable to represent a boolean.        Start_Stop AT %IX0.0:BOOL;    // Digital   input of the PLC (Address 0.0)        ON_OFF     AT %QX0.0:BOOL;    // Digital output of the PLC (Address 0.0). (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_CONFIGURATIONPROGRAM 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 edit

//=======================================================================// 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

References edit

^ Bacidore, Mike (16 May 2018). "Should I limit programming to ladder logic or use all standards within IEC 61131?". Control Design.
^ Stevic, Tom (5 May 2017). "A very short history of PLC programming platforms". Control Design.
^ ^a ^b Roos, Nieke. "Programming PLCs using Structured Text". Department of Computing Science, University of Nijmegen. CiteSeerX 10.1.1.49.2016. {{cite journal}}: Cite journal requires |journal= (help)

[1] Bacidore, Mike (16 May 2018). "Should I limit programming to ladder logic or use all standards within IEC 61131?". Control Design.

[2] Stevic, Tom (5 May 2017). "A very short history of PLC programming platforms". Control Design.

[nlpaper-3] Roos, Nieke. "Programming PLCs using Structured Text". Department of Computing Science, University of Nijmegen. CiteSeerX 10.1.1.49.2016. {{cite journal}}: Cite journal requires |journal= (help)

[1]

[2]

[3]