iec61131

IEC 61131-3 Programming Languages Reference

Overview

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

Language	Type	Best For	Status
LD (Ladder Diagram)	Graphical	Discrete logic, motor interlocks	Active
FBD (Function Block Diagram)	Graphical	Continuous control, PID, analog	Active (DCS primary)
ST (Structured Text)	Textual	Complex calculations, algorithms	Active (most expressive)
IL (Instruction List)	Textual	Low-level, assembly-like	Deprecated (3rd ed. 2013)
SFC (Sequential Function Chart)	Graphical	Sequences, batch, state machines	Active

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

Scope	Keyword	Description
Local	VAR	Internal to POU
Input	VAR_INPUT	Read-only input parameter
Output	VAR_OUTPUT	Output parameter
In/Out	VAR_IN_OUT	Bidirectional parameter
Global	VAR_GLOBAL	Accessible by all POUs
External	VAR_EXTERNAL	Reference to global
Temp	VAR_TEMP	No persistence
Config	VAR_CONFIG	Set at download
Constant	CONSTANT	Named constant

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

TC	Topic	Description
TC1	Standards	IEC 61131-3 compliance testing
TC2	Motion Control	MC_MoveAbsolute, MC_Stop, MC_Home, etc.
TC3	Software Engineering	Coding guidelines
TC4	Communications	OPC UA mapping
TC5	Safety	IEC 61508 extensions
TC6	XML Exchange	Vendor-neutral program interchange format

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).

Other Exchange Formats

Format	Standard	Used By
PLCopen XML	IEC 61131-10	CODESYS, Schneider, Beckhoff
SimaticML	Siemens proprietary	TIA Portal
AutomationML	IEC 62714	Siemens PCS 7 V9+, multi-vendor
FHX	Emerson proprietary	DeltaV
PRT/CSV	ABB proprietary	Freelance
DEXPI	ISO 15926	P&ID exchange