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). It is a high level language that is block structured and syntactically resembles Pascal, on which it is based. 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)
* Functions (SQRT(), SIN())

Sample program

(* simple state machine *)
TxtState := STATES tateMachine

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

// PLC configuration
CONFIGURATION 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_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

References

{{reflist

IEC standards
Programmable logic controllers
Programming languages