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_VARFUNCTION (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_TYPEVariable 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 |