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Praxous LadderIDE Docs

LES — Less Than

Category
Compare
Type
Input (condition)
Availability
All platforms

LES instruction block: Source A Level reading 35, Source B 20, dark because 35 is not below 20

Description

Section titled “Description”

LES (Less Than) is true when Source A is strictly less than Source B. It is a condition that writes nothing. Use LES for a low alarm, a low-level fill demand, an “under target” test, or the bottom edge of a band together with GRT. When “equal to” should also count, use LEQ. LES does not accept STRING operands; there is no ordering for text.

Operands

Section titled “Operands”
Operand Type Format Valid Range Required Description
Source A Any numeric tag, member, element or bit, or a literal Tag, Tag.Member, Tag[n], Tag.bit, or a literal Any resolvable numeric reference Yes The value under test.
Source B Same as Source A Same Any resolvable numeric reference; defaults to 0 Yes The threshold.

Literals may be whole numbers (25, -3, 0x1F) or decimals (72.5). Mixed INT and REAL operands compare as numbers. A BOOL reads as 0 or 1.

Scan Behavior

Section titled “Scan Behavior”

Prescan

Section titled “Prescan”

No state. Evaluates from the current values on the first scan.

Rung-condition-in is false

Section titled “Rung-condition-in is false”

The instruction does not pass continuity. The block still colours by its own result on the canvas.

Rung-condition-in is true

Section titled “Rung-condition-in is true”

Both operands are read. If A is less than B the instruction conducts; if A is greater than or equal to B the rung is false from this point.

Postscan

Section titled “Postscan”

Nothing.

Example

Section titled “Example”

Scenario: A tank level in percent. Below 20 % a fill valve opens.

Tags:

  • Level — Tank level, INT (percent)
  • Fill — Fill valve, BOOL

Rung 1:

—[LES Level  20]———(OTE Fill)———

Scan 1 — Level = 35. 35 is not less than 20: the block is dark, Fill = 0.

Figure 1 — LES false: Level reads 35 against Source B 20, the block dark, Fill off

Scan 2 — Level = 12. 12 is less than 20: the block lights, Fill = 1.

Figure 2 — LES true: Level reads 12, the block lit, Fill on

At exactly 20 the valve is closed; LES is strict. If it should open at 20 as well, use LEQ.

See Also

Section titled “See Also”
  • LEQ — Less Than or Equal
  • GRT — Greater Than (the mirror test)
  • LIM — Limit Test (both edges of a band in one instruction)
  • CMP — Compare by expression
  • Compare Instructions — Category index

Notes

Section titled “Notes”

No hysteresis. A level hovering at the threshold will chatter the valve. Open on LES Level 20 and close on GRT Level 30 with an OTL/OTU pair, or a seal-in, when that matters.

Signed values. Every numeric type is signed, so LES Offset 0 is true for negative offsets.

Members as operands. LES Batch.ACC Batch.PRE is true while a counter has not reached its preset, which is XIO Batch.DN written the long way.

Applies to LadderIDE >=1.2.2 · Last reviewed 2026-09-11 · Screenshots verified 2026-09-11