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Array Tags

An array tag holds a list of values of the same data type under a single name. Reference an element with Tag[index].

Creating an array​

In the tag editor, create a tag with usage Local and set its array size to a positive number. Rungs stores the elements as Tag[0], Tag[1], …, Tag[size - 1].

Every data type can be an array element — BOOL, DINT, REAL, TIMER, COUNTER, FBD_TIMER, FBD_COUNTER. For example, DelayTimer with size 12 gives you 12 independent TIMER structures reachable as DelayTimer[0].PRE, DelayTimer[0].DN, DelayTimer[1].PRE, …

Referencing an element​

Use Tag[index] anywhere a tag name is accepted — instruction operands, expressions, comparisons:

  • Sensors[0] — the first element of Sensors
  • Samples[3] — the fourth element of Samples
  • DelayTimer[2].PRE — the PRE member of the third TIMER in DelayTimer
  • Samples[i] — index with a DINT tag to read or write the element at runtime

In Ladder Logic, array references drop directly into instruction operands:

XIC(Sensors[0])GT(Samples[3],100)OTE(Alarm)

Every reference must include an integer or integer tag in square brackets. Missing the index on an array tag produces:

Tag 'Samples' is an array and requires an index, use Samples[0]

Using an index on a scalar (non-array) tag produces:

Tag 'Count' is not an array and cannot be indexed

Index expressions​

The index inside […] must produce an integer. The compiler accepts these forms:

  • integer literals — Samples[0], Samples[3]
  • DINT tags — Samples[i]
  • DINT struct members — Samples[Tmr.ACC]
  • integer arithmetic with +, -, *, /, MOD — Samples[i + 1], Samples[(i - 1) MOD 8]
  • ABS() over an integer argument — Samples[ABS(offset)]

DINT division truncates toward zero, so Samples[i / 2] with i = 7 reads index 3 (not 3.5).

The compiler rejects:

  • REAL values — Samples[1.5], Samples[realTag]
  • BOOL values — Samples[boolTag]
  • whole-array indexes — Samples[OtherArray] (read an array element into a DINT tag first, then use that tag)
  • nested array access in the index — Samples[Inner[i]]
  • bit access in the index — Samples[i.0]
  • non-integer math functions — SQRT(), SIN(), COS(), TAN()
  • unary + — write Samples[i], not Samples[+i]
  • statically out-of-range literals — Samples[5] when Samples has 3 elements

If the index is a tag or expression the compiler cannot evaluate, an out-of-range access at runtime raises Runtime error: Array index out of bounds and halts the scan.

Restrictions​

  • Single-dimension only. Multi-dimensional arrays (Tag[row][col]) are not supported.
  • Local usage only. Input and Output tags cannot be arrays — set usage to Local.
  • Indices must be in range. Writing Samples[5] when Samples has only 3 elements is a compile-time error. If the index is itself a tag (Samples[i]), the compiler cannot check it — an out-of-range access at runtime raises Runtime error: Array index out of bounds and halts the scan.
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