Structured Text (ST)

Structured Text

Structured Text is the language ForgeIEC uses by default for new POUs. Syntax is Pascal-derived: assignment is :=, statements end with ;, control-flow keywords are spelled out (IF, THEN, ELSIF, ELSE, END_IF), and types are declared separately from values.

This chapter covers every ST construct that ForgeIEC accepts. Pages are organised by syntactic category, mirroring the IEC 61131-3 third edition section ordering for ST (clause 6.6 in the 2013 edition).

Sections

Literals

Numeric constants (integer, real, time, duration, date), bit strings, character strings, typed literals (BYTE#1, INT#42, T#100ms).

• Integer & hexadecimal literals — 1, 42, 16#FF, BYTE#1, INT#-3, plus the matiec emit-bug with typed hex inside CASE case lists.

Operators

Arithmetic, bitwise, comparison and assignment operators with precedence and operand-type rules. matiec is strict on type matching — most ported code from C or other vendor toolchains needs explicit conversion calls inserted.

• Arithmetic — +, -, *, /, MOD, **.
• Bitwise & logical — AND, OR, XOR, NOT.
• Comparison — =, <>, <, <=, >, >=.
• Assignment & precedence — := and the full precedence table.

Variables and references

How identifiers resolve in ST. Includes ForgeIEC’s three-namespace pool model (Anvil.<Device>.<Var>, Bellows.<Group>.<Var>, GVL.<Namespace>.<Var>) plus member access, bit access, array indexing.

Function and FB calls

Two different mechanisms — Functions (FUN) are called directly, Function Blocks (FBs) need an instance declaration first. Most common LLM compile-error source in ForgeIEC.

• FB instances vs. function calls — “X is a Function Block — declare instance first” and the canonical TON / CTU / R_TRIG declaration pattern.
• Positional vs. named parameters — “has no declared parameter list” and when to prefer which call form.

Control flow

How execution proceeds through the body of a POU.

• IF / ELSIF / ELSE — conditional branching.
• CASE — multi-way dispatch on integer expressions, including the matiec emit-bug with SHL(BYTE#1, …) and the literal-type pitfall in case lists.

Iteration (loops)

Three loop forms plus the scan-cycle hazard discussion (loops on a PLC must be bounded — they block the cycle).

• FOR — counter-driven iteration.
• WHILE — pre-test condition.
• REPEAT — post-test condition.

Selection and jump

RETURN, EXIT, CONTINUE plus the deprecated/removed GOTO and its modern replacements.

ForgeIEC-specific reference rules

ForgeIEC adds three reference forms on top of the IEC standard that are worth keeping in mind while reading the per-construct pages:

• Anvil.<Device>.<Var> — a bus-bound pool variable. The middle segment is the device hostname (= AnvilVarList POU name).
• Bellows.<Group>.<Var> — a pool variable that carries bellowsExport=true and is mirrored by the Bellows OPC-UA / Modbus-TCP gateway. Same physical pool entry as the matching Anvil.* form (dual-namespace alias) when both apply.
• GVL.<Namespace>.<Var> — a plain global variable (VAR_GLOBAL, no special binding).

POU-local variables are referenced by their bare name without prefix. The ForgeIEC ST resolver matches case-insensitively per IEC 61131-3 §1.4.2.