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Variables

This public entry covers variable blocks, storage classes, direct addresses, and visibility rules for globals, locals, and instance members.

Use the full spec below when you need exact VAR_* forms, direct-address mapping rules, or access-control details.

Related: Data Types, POU Declarations

Variables

IEC 61131-3 Edition 3.0 (2013) - Section 6.5

This specification defines variable declarations and qualifiers for trust-hir symbols.

1. Variable Declaration (Tables 13-14, Section 6.5.1)

Basic Declaration Syntax

VAR
  identifier_list : type_specification;
  identifier_list : type_specification := initial_value;
END_VAR

Declaration Examples (Table 13)

No. Description Example
1 Single variable A: INT;
2 Multiple variables A, B, C: INT;
3 Variable with initial value X: BOOL := TRUE;
4 Array variable Arr: ARRAY[1..10] OF INT;
5 String with length Name: STRING[50];
6 Reference type pInt: REF_TO INT;

Initialization (Table 14)

No. Description Example
1 Elementary type X: INT := 42;
2 Array initialization A: ARRAY[1..3] OF INT := [1, 2, 3];
3 Partial array init B: ARRAY[1..5] OF INT := [1, 2];
4 Repetition count C: ARRAY[1..6] OF INT := [3(1, 2)];
5 Structure init S: MyStruct := (field1 := 1, field2 := 2);
6 FB instance init Timer: TON := (PT := T#1s);

2. Variable Section Keywords (Figure 7, Section 6.5.2)

Input/Output Variables

Keyword Description Scope
VAR_INPUT Input parameters Read-only inside POU
VAR_OUTPUT Output parameters Write inside, read outside
VAR_IN_OUT In-out parameters Read/write both

Local Variables

Keyword Description Scope
VAR Local variables Persistent in FBs; in functions/methods equivalent to VAR_TEMP
VAR_TEMP Temporary variables Non-persistent, fresh each call

Global Variables

Keyword Description Scope
VAR_GLOBAL Global declaration Configuration/resource element or namespace scope
VAR_EXTERNAL External reference Access to VAR_GLOBAL

Special Variables

Keyword Description Scope
VAR_ACCESS Access paths For communication services
VAR_CONFIG Instance-specific Configuration initialization

Rules: - VAR_ACCESS binds a symbolic name to an access path; the declared type must match the target access path type. (IEC 61131-3 Ed.3, Table 13, 6.5.2.2) - READ_ONLY access paths are not assignable in ST code. (IEC 61131-3 Ed.3, 6.5.2.2) - VAR_CONFIG entries shall use the same type as the target variable. (IEC 61131-3 Ed.3, 6.5.2.2) - Instance-specific initialization in VAR_CONFIG is not allowed for VAR_TEMP, VAR_EXTERNAL, VAR_IN_OUT, or VAR CONSTANT targets. (IEC 61131-3 Ed.3, 6.5.2.2) - trust-hir validates simple access paths only; cross-resource/program instance mapping is out-of-scope. (IEC 61131-3 Ed.3, Tables 13-16; DEV-003)

3. Variable Qualifiers

Persistence Qualifiers

Qualifier Description Behavior
RETAIN Retentive Value retained on warm restart
NON_RETAIN Non-retentive Value initialized on warm restart
PERSISTENT Persistent Vendor extension; treated like RETAIN
(none) Default Implementer-specific 
VAR RETAIN
  Counter: INT := 0;  // Retained across power cycles
END_VAR

VAR NON_RETAIN
  TempData: INT;      // Re-initialized on restart
END_VAR

Rules: - RETAIN/NON_RETAIN apply to VAR, VAR_INPUT, VAR_OUTPUT, VAR_GLOBAL, and static VAR sections; not VAR_IN_OUT. (IEC 61131-3 Ed.3, 6.5.6.1-6.5.6.2) - Only one of CONSTANT, RETAIN, NON_RETAIN, or PERSISTENT may appear per VAR section. (IEC 61131-3 Ed.3, Figure 7) - PERSISTENT is accepted as a vendor extension and validated like RETAIN. (DEV-007)

Constant Qualifier

Qualifier Description Behavior
CONSTANT Named constant Cannot be modified 
VAR CONSTANT
  PI: REAL := 3.14159;
  MaxCount: INT := 100;
END_VAR

Rules: - CONSTANT may qualify any VAR_* section keyword listed in IEC Figure 7. - Assignment to a CONSTANT declaration from within the enclosing entity is rejected. - VAR_INPUT CONSTANT is valid and redundant: inputs are already read-only inside the entity. - VAR_IN_OUT CONSTANT preserves normal VAR_IN_OUT call-site binding rules, but the aliased storage is read-only inside the entity. - VAR_OUTPUT CONSTANT is valid and leaves the output unwritable from within the entity. - VAR_TEMP CONSTANT is valid and behaves as a read-only temporary variable. - Function block instances shall not be declared in CONSTANT sections. (IEC Figure 7 footnote *) - Only VAR CONSTANT, VAR_GLOBAL CONSTANT, and VAR_EXTERNAL CONSTANT participate in named compile-time constant-expression evaluation; parameter-block and VAR_TEMP CONSTANT declarations are runtime storage with read-only semantics.

Edge Detection Qualifiers (Table 14)

Qualifier Description Use
R_EDGE Rising edge VAR_INPUT only
F_EDGE Falling edge VAR_INPUT only 
FUNCTION_BLOCK MyFB
VAR_INPUT
  Trigger: BOOL R_EDGE;  // Rising edge detection
END_VAR
// Body sees Trigger=TRUE only on 0->1 transition
END_FUNCTION_BLOCK

4. Access Specifiers (Section 6.5.2.3)

For variables within CLASS and FUNCTION_BLOCK:

Specifier Description Access
PUBLIC Public access Anywhere class is visible
PROTECTED Protected access Own class and derived classes
PRIVATE Private access Own class only
INTERNAL Internal access Within same NAMESPACE only 
CLASS MyClass
  VAR PUBLIC
    PublicVar: INT;     // Accessible everywhere
  END_VAR
  VAR PRIVATE
    PrivateVar: INT;    // Only within MyClass
  END_VAR
  VAR PROTECTED
    ProtectedVar: INT;  // MyClass and derived classes
  END_VAR
END_CLASS

Default Access: - PROTECTED is the default access specifier for variables - PROTECTED members are visible to the declaring class and derived classes

5. Directly Represented Variables (Table 16, Section 6.5.5)

Directly represented variables map to physical I/O or memory locations. See 10-runtime-semantics.md §9 for the language/runtime I/O contract and 11-runtime-engine.md §6.4 for the process image/runtime engine view.

Syntax

%<Location><Size><Address>

Location Prefixes

Prefix Description
I Input location
Q Output location
M Memory location

Size Prefixes

Prefix Size Type
X 1 bit BOOL
B 8 bits BYTE
W 16 bits WORD
D 32 bits DWORD
L 64 bits LWORD

Examples (Table 16)

No. Variable Description
1 %IX1 Input location 1, single bit
2 %IW6 Input word at location 6
3 %QB17 Output byte at location 17
4 %MD48 Memory double word at location 48
5 %QX7.5 Output bit 5 of byte 7

Hierarchical Addressing

%IX1.2.3.4     // Hierarchical address (leftmost = highest level)
%QW2.5.7.1    // Additional levels are implementer-specific

Symbolic Mapping with AT

VAR
  StartButton AT %IX0.0: BOOL;     // Maps to physical input
  MotorSpeed  AT %QW10:  INT;      // Maps to physical output
END_VAR

Incomplete Address Specification

VAR
  LocalAddr AT %I*: BOOL;  // Location determined by VAR_CONFIG
END_VAR

Rules: - Incomplete direct addresses (%I*, %Q*, %M*) are not allowed in VAR_INPUT or VAR_IN_OUT sections. (IEC 61131-3 Ed.3, 6.5.5.4) - Each incomplete direct address must be fully specified in a VAR_CONFIG entry using AT and a concrete address (no *). (IEC 61131-3 Ed.3, 6.5.5.4)

6. Variable-Length Arrays (Tables 15-16, Section 6.5.4.5)

Declaration

Variable-length arrays are declared using * for bounds:

FUNCTION Sum: INT
VAR_INPUT
  Values: ARRAY[*] OF INT;  // Variable-length input array
END_VAR
VAR
  i: INT;
  result: INT := 0;
END_VAR
FOR i := LOWER_BOUND(Values, 1) TO UPPER_BOUND(Values, 1) DO
  result := result + Values[i];
END_FOR;
Sum := result;
END_FUNCTION

Bound Functions

Function Description
LOWER_BOUND(arr, dim) Lower bound of dimension
UPPER_BOUND(arr, dim) Upper bound of dimension

Rules: - Only allowed in VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT - Only usable in FUNCTIONs and METHODs - Dimensions must match at call site - Multiple dimensions: ARRAY[*, *] OF INT

7. Variable Scope Rules (Section 6.5.2.2)

Scope Hierarchy

  1. Local variables (VAR, VAR_TEMP) - Visible within declaring POU
  2. Parameters (VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT) - Part of POU interface
  3. Global variables (VAR_GLOBAL) - Accessible via VAR_EXTERNAL in strict IEC form, or via vendor-parity bare/qualified access in truST

Name Resolution

  1. Local names take precedence over global names
  2. Qualified names can disambiguate namespace-backed globals: GVL.shared
  3. THIS.member for class/FB member access
  4. Directly represented variables are globally unique

Lifetime

Section Initialization Persistence
VAR (in FB) Once at instantiation Persists across calls
VAR (in Function) Each call Lost after return
VAR_TEMP Each call Lost after return
VAR_INPUT Each call (from caller) -
VAR_OUTPUT Each call -
VAR_IN_OUT Each call (from caller) -

8. External Variable Declaration (Figure 8, Section 6.5.6)

VAR_GLOBAL and VAR_EXTERNAL Relationship

// In CONFIGURATION or RESOURCE
VAR_GLOBAL
  GlobalCounter: INT := 0;
  GlobalTimer: TON;
END_VAR

// In PROGRAM, FUNCTION_BLOCK, or CLASS
VAR_EXTERNAL
  GlobalCounter: INT;    // Must match type exactly
  GlobalTimer: TON;
END_VAR

VAR_EXTERNAL CONSTANT

VAR_GLOBAL CONSTANT
  MaxItems: INT := 100;
END_VAR

// Reference as constant
VAR_EXTERNAL CONSTANT
  MaxItems: INT;
END_VAR

Rules: - VAR_EXTERNAL creates a reference to VAR_GLOBAL declared in the associated program, configuration, or resource. (IEC 61131-3 Ed.3, 6.5.2.2, Figure 8) - Type must exactly match the VAR_GLOBAL declaration. (IEC 61131-3 Ed.3, 6.5.2.2) - VAR_EXTERNAL cannot declare an initial value. (IEC 61131-3 Ed.3, 6.5.1.3) - VAR_EXTERNAL CONSTANT is required when the referenced VAR_GLOBAL is CONSTANT. (IEC 61131-3 Ed.3, Figure 8) - Error if VAR_GLOBAL doesn't exist. (IEC 61131-3 Ed.3, 6.5.2.2) - Modification of VAR_EXTERNAL CONSTANT is an error. (IEC 61131-3 Ed.3, 6.5.2.2)

truST vendor-parity note: - VAR_EXTERNAL remains supported and type-checked, but it is optional in truST for vendor-parity global access paths. See docs/IEC_DEVIATIONS.md. - NAMESPACE ... VAR_GLOBAL ... END_NAMESPACE is accepted as a vendor-style namespaced GVL extension, and qualified access such as GVL.shared resolves directly.

9. Declaration Rules Summary

truST vendor-parity note: - In addition to the IEC-declared locations below, truST accepts top-level file-scope VAR_GLOBAL ... END_VAR and namespaced VAR_GLOBAL blocks as vendor-style GVL extensions. See docs/IEC_DEVIATIONS.md.

What Can Be Declared Where

Section Function FB Program Class Config
VAR Yes Yes Yes Yes -
VAR_TEMP Yes Yes Yes Yes -
VAR_INPUT Yes Yes Yes - -
VAR_OUTPUT Yes Yes Yes - -
VAR_IN_OUT Yes Yes Yes - -
VAR_EXTERNAL Yes Yes Yes Yes -
VAR_GLOBAL - - Yes - Yes
VAR_ACCESS - - - - Yes
VAR_CONFIG - - - - Yes

Qualifier Combinations

Qualifier VAR VAR_INPUT VAR_OUTPUT VAR_GLOBAL
CONSTANT Yes - - Yes
RETAIN Yes Yes Yes Yes
NON_RETAIN Yes Yes Yes Yes
PERSISTENT (DEV-007) Yes Yes Yes Yes
R_EDGE - Yes - -
F_EDGE - Yes - -

Implementation Notes for trust-hir

Symbol Table Requirements

  1. Track variable name, type, and location
  2. Record scope (local, parameter, global)
  3. Store qualifiers (CONSTANT, RETAIN, NON_RETAIN, PERSISTENT, access specifier)
  4. Maintain reference to initial value expression
  5. For AT: store direct address mapping (IEC 61131-3 Ed.3, Table 16)

Semantic Checks

  1. Undefined variable: Reference to undeclared identifier
  2. Duplicate declaration: Same name in same scope
  3. Type mismatch: Initial value type vs declared type
  4. Constant assignment: Attempt to modify CONSTANT
  5. Input modification: Attempt to modify VAR_INPUT
  6. Missing VAR_GLOBAL: VAR_EXTERNAL without corresponding global
  7. Invalid qualifier: Wrong qualifier for variable section
  8. Scope violation: Access specifier violation
  9. Missing VAR_CONFIG mapping: Incomplete AT address without a concrete VAR_CONFIG entry

Error Conditions

  1. Variable used before declaration
  2. Variable declared multiple times in same scope
  3. Assignment to CONSTANT or VAR_INPUT
  4. Type mismatch in initialization
  5. Invalid direct address format
  6. VAR_EXTERNAL without matching VAR_GLOBAL
  7. Access specifier violation (PRIVATE member access)
  8. Array bounds out of range