CLR — Clear

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Description

CLR (Clear) writes zero to Dest every scan its rung is true. It is an output instruction with a single operand, no state and no status bits. Use CLR to zero a running total, a shift-register word, or a fault code at the start of a cycle; on a STRING it empties the text. Do not use CLR on a timer or counter to reset it; that is what RES is for, and CLR on a struct is not accepted.

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Operands

Operand	Type	Format	Valid Range	Required	Description
Dest	A writable tag, member or element	Tag, Tag.Member, Tag[n]	SINT, INT, DINT, REAL, STRING or a BOOL; not a literal, not an input pin	Yes	The value to zero.

A REAL Dest becomes 0.0, a STRING Dest becomes the empty string, a BOOL Dest becomes 0. A timer or counter member such as Batch.ACC is a valid Dest; the whole struct is not.

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Scan Behavior

Prescan

Nothing. Dest keeps its power-up value.

Rung-condition-in is false

The instruction does nothing. Dest holds its last value.

Rung-condition-in is true

Zero is written to Dest every scan the rung is true. Anything else writing Dest on a later rung in the same scan wins, because rungs execute in order.

Postscan

Nothing.

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Example

Scenario: The combined count from the ADD example is zeroed at the start of each shift.

Tags:

• Reset_Total — Shift start, BOOL
• Total — Combined count, INT

Rung 1:

—] [Reset_Total———[CLR Total]———

Scan 1 — Reset_Total = 0. Total reads 65 from the previous shift. The rung is false: nothing changes.

Scan 2 — Reset_Total = 1. Total reads 0.

If Reset_Total is a pushbutton held for a second, the CLR runs for the whole second. That is harmless here, but a CLR on a value another rung is accumulating will fight it; put an ONS on the button.

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See Also

• MOV — Move (write any value, not only zero)
• RES — Reset (zero a timer or counter properly)
• ADD — Add (the running sum this example clears)
• Compute Instructions — Category index

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Notes

Clearing a member. CLR Batch.ACC zeroes the accumulator but leaves DN as it was. RES clears the accumulator and the status bits together.

Clearing a STRING. The text becomes empty, length zero. Compare with EQU Msg "" to detect it.

The block never colours. Watch the rung wire and the Dest value line.