IEC 1131 Structured Text Overview

This high-level language resembles Pascal or Basic, and, in fact, people trained in computer programming languages often find it the easiest language to use for programming control logic. When symbolic addressing is used, ST programs resemble sentences, making it highly intelligible to the novice user as well. ST is ideal for tasks requiring complex math, algorithms or decision-making. Its concise format allows a large algorighm to be displayed on a single page (vs multiple pages of ladder logic).

The IEC 1131-3 standard is extensible. I.E. Vendors may augment their offerings to meet the needs of specific markets. As an example of this extensibility Rockwell Software augments ST with an exclusive feature called "PowerText�". It supplements standard ST with real-time display of discrete status, force status, analog values and floating-point values. This PowerText information is automatically integrated into the source code, and is invaluable for debugging and application commissioning.

Benefits of Structured Text

People trained in computer languages can easily program control logic
Symbols make the programs easy to understand
PowerText facilitates system debugging and application commissioning
Programs can be created in any text editor
Runs as fast as ladder logic

Structured Text Constructs

Bit / Word assignment
IF-THEN-ELSE
CASE
FOR-NEXT
WHILE
REPEAT
Ladder equivalent instructions

Structured Text Examples

Example 1 (Sorting machine)

IF (LIMIT_SWITCH_1 AND BOX_PRESENT) THEN GATE1 := OPEN; GATE2 := CLOSE; ELSIF ((LIMIT_SWITCH_2 OR (WEIGHT <> SETPOINT))) THEN GATE1 := CLOSE; GATE2 := OPEN; ELSIF (LIMIT_SWITCH_3 XOR LIMIT_SWITCH_4) THEN GATE1 := OPEN; GATE2 := OPEN; ELSE GATE1 := CLOSE; GATE2 := CLOSE; END_IF;

Example 2 (Split-range temperature control, with deadband)

CASE (TEMPERATURE) OF 0 .. 120 : HEAT_VALVE := OPEN; COOL_VALVE := CLOSE; 150 .. 32767 : HEAT_VALVE := CLOSE; COOL_VALVE := CLOSE; ELSE HEAT_VALVE := CLOSE; COOL_VALVE := CLOSE; END_CASE; PID (CONTROL_BLOCK, TANK_#27_TEMPERATURE, TIEBACK, COOLANT_VALVE);

Example 3 (Computational examples)

POWER := (CURRENT ** 2.0) * RESISTANCE; F8:1 := ((N7:1 * 3.1428571) + (N7:3 / N7:4)); F8:0 := ( SIN (ANGLE)) MOD 6.0; JSR (3, 0);

Example 4 (If-Then-Else example)

IF I:000/0 AND !I:001/5 THEN N7:0 := 1; ELSIF i:000/2 OR (N7:5 <> (N7:6 * N7:2)) THEN N7:0 :=2; ELSIF !I001/4 THEN N7:0 := 3; ELSE N7:0 :=4; END_IF;

Example 5 (For-Next examples)

FOR N7:0 := 10 TO 0 BY -1 DO N7:4 := N7:4 + 1; END_FOR;
FOR N7:1 := 0 TO 10 DO N7:1 := N7:1 + 1; END_FOR;

Example 6 (While example)

WHILE I:000/0 AND (I:001/0 OR !I:002/1) DO N7:0 := N7:0 + 1; END_WHILE;

Example 7 (Case example)

CASE N7:12 OF 0: N7:1 := 1; 1, 9: N7:1 := 2; 2..4: N7:1 := 3; 10, 5..8: N7:1 := 4; ELSE N7:1 :=5; END_CASE;

Example 8 (Using Ladder Diagram instructions in Structured Text programs)

IF (!I:000/00) THEN ARL(0, ST15:3, R6:6, 82,0); END_IF;
R6:6.DN AND ONS(B3/4) MOV(ST15:3.LEN, N17:85);


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