SUB — Subtract

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Description

SUB (Subtract) writes Source A minus Source B to Dest every scan its rung is true. It is an output instruction with no state and no status bits. Use SUB for a remaining quantity, an error between setpoint and measurement, or an offset removed from a raw reading. Do not use SUB to count down; an ungated SUB Left 1 Left subtracts one every scan. Use CTD for events, or gate the SUB with an ONS.

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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 subtracted from.
Source B	Same as Source A	Same	Any resolvable numeric reference	Yes	The value subtracted.
Dest	A writable numeric tag, member or element	Tag, Tag.Member, Tag[n]	SINT, INT, DINT or REAL; not a literal, not an input pin	Yes	Where the difference is written.

Literals may be whole numbers (25, -3, 0x1F) or decimals (72.5). A BOOL source reads as 0 or 1. Dest may be the same tag as a source.

Dest type. A whole-number Dest truncates a fractional result toward zero and wraps silently past its range (INT ±32767, DINT ±2,147,483,647); a REAL keeps the fraction. Every type is signed, so a result below zero is stored as a negative number, never clamped.

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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

Both sources are read, Source B is subtracted from Source A and the result is written to Dest, every scan the rung is true.

Postscan

Nothing.

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Example

Scenario: A day tank holds 500 litres when full. A flow meter accumulates what has been drawn off, and the operator display shows what is left.

Tags:

• Update — Recalculate, BOOL
• Tank_Full — Full capacity in litres, INT
• Used — Litres drawn, INT
• Remaining — Litres left, INT

Rung 1:

—] [Update———[SUB Tank_Full  Used  Remaining]———

Scan 1 — Update = 0. Tank_Full reads 500 and Used reads 135. The rung is false, so Remaining still shows 0.

Scan 2 — Update = 1. 500 − 135 is written: Remaining reads 365.

If Used ever exceeds Tank_Full, Remaining goes negative. Test for that with LES if the display should clamp at zero.

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See Also

• ADD — Add
• NEG — Negate (0 − Source in one operand)
• ABS — Absolute Value (the size of a difference regardless of sign)
• CPT — Compute
• Compute Instructions — Category index

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Notes

Control error. SUB Setpoint Measured Error is the classic use. Follow it with ABS when only the size of the error matters, or use LIM on Error for a deadband.

Counting down. SUB Left 1 Left behind an ONS decrements once per event. Without the ONS it runs every scan.

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