Iec61131

IEC 61131-3 Programming Languages Reference

Overview

IEC 61131-3 defines 5 programming languages for PLCs, all interchangeable:

Program Organization Units (POUs)

PROGRAM

Top-level executable, associated with a task (scan cycle).

FUNCTION_BLOCK (FB)

Reusable block with persistent internal state (memory between calls). Used for motors, PID, timers, counters.

FUNCTION_BLOCK MotorControl
VAR_INPUT
    StartCmd : BOOL;
    StopCmd  : BOOL;
    FbkRun   : BOOL;
    Interlock: BOOL;
END_VAR
VAR_OUTPUT
    StartOut : BOOL;
    Running  : BOOL;
    Fault    : BOOL;
END_VAR
VAR
    MonTimer : TON;
    State    : INT;
END_VAR

FUNCTION (FC)

Stateless, no memory. Always returns same output for same input.

Data Types

Elementary

BOOL, BYTE, WORD, DWORD, LWORD, SINT, INT, DINT, LINT, USINT, UINT, UDINT, ULINT, REAL, LREAL, TIME, DATE, TIME_OF_DAY, DATE_AND_TIME, STRING, WSTRING

Derived (User-Defined)

TYPE MotorData :
STRUCT
    Running  : BOOL;
    Fault    : BOOL;
    Speed    : REAL;
    Runtime  : TIME;
END_STRUCT
END_TYPE

Variable Scopes

Language Details

ST (Structured Text) - Most Relevant for Code Generation

IF Motor.FbkRunning AND NOT Motor.Fault THEN
    Motor.Status := RUNNING;
    Motor.Runtime := Motor.Runtime + T#1s;
ELSIF Motor.Fault THEN
    Motor.Status := FAULTED;
    Motor.AlarmActive := TRUE;
END_IF;

CASE State OF
    0: (* IDLE *)
        IF StartCmd AND NOT Interlock THEN State := 1; END_IF;
    1: (* STARTING *)
        StartOut := TRUE;
        MonTimer(IN:=TRUE, PT:=T#10s);
        IF FbkRun THEN State := 2; END_IF;
        IF MonTimer.Q THEN State := 99; END_IF;
    2: (* RUNNING *)
        IF StopCmd THEN State := 3; END_IF;
    3: (* STOPPING *)
        StartOut := FALSE;
    99: (* FAULT *)
        StartOut := FALSE;
        Fault := TRUE;
END_CASE;

SFC (Sequential Function Chart)

Elements: Steps, Transitions, Actions

• Steps contain actions (in any language)
• Transitions are boolean conditions
• Action qualifiers: N (non-stored), S (set), R (reset), L (time limited), D (delayed), P (pulse)
• Parallel branches (simultaneous), Alternative branches (selective)

FBD (Function Block Diagram)

Signal flow left-to-right. Blocks represent functions. Wires connect outputs to inputs. Primary language in DCS (PCS 7 CFC, DeltaV Control Studio, ABB Freelance).

LD (Ladder Diagram)

Based on relay logic. Contacts (NO/NC), coils, timers, counters. Left/right power rails. Rungs top-to-bottom.

PLCopen Standards

PLCopen XML Format (TC6)

<project xmlns="http://www.plcopen.org/xml/tc6_0201">
  <types>
    <pous>
      <pou name="MotorControl" pouType="functionBlock">
        <interface>
          <inputVars>
            <variable name="Start"><type><BOOL/></type></variable>
          </inputVars>
          <outputVars>
            <variable name="Running"><type><BOOL/></type></variable>
          </outputVars>
        </interface>
        <body>
          <ST><xhtml><!-- Structured Text --></xhtml></ST>
        </body>
      </pou>
    </pous>
  </types>
</project>

Supported by: CODESYS, ABB AC500, Schneider, Beckhoff, B&R. NOT supported by: ABB Freelance (uses PRT), Emerson DeltaV (uses FHX), Siemens (uses SimaticML).

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