XPY — X to the Power of Y

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Description

XPY (X to the Power of Y) writes Source A raised to the power Source B to Dest every scan its rung is true. It is an output instruction with no state and no status bits. Use XPY for a bit weight (2 to the n), a square or cube without a chain of MULs, or a fractional exponent such as a pump affinity law. Do not reach for XPY to square a value that is used once; MUL X X is cheaper on an 8-bit controller, where a power is computed in floating point.

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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 base, X.
Source B	Same as Source A	Same	Any resolvable numeric reference	Yes	The exponent, Y.
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 result is written.

Dest type. The power is computed in floating point and then stored in Dest’s type: a whole-number Dest truncates toward zero and wraps silently past its range. 2 to the 15th already exceeds an INT; use a DINT or a REAL. A negative base with a fractional exponent has no real result and writes 0.

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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, the power is computed and written to Dest every scan the rung is true.

Postscan

Nothing.

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Example

Scenario: A bit mask is built from a bit number: bit n has the weight 2 to the n.

Tags:

• Calc_Pow — Compute the weight, BOOL
• Base — Always 2 here, INT
• Exponent — Bit number, INT (Exp itself is a reserved function name)
• Power — Weight of that bit, DINT

Rung 1:

—] [Calc_Pow———[XPY Base  Exponent  Power]———

Scan 1 — Calc_Pow = 0. Base reads 2 and Exponent reads 10. The rung is false: Power holds 0.

Scan 2 — Calc_Pow = 1. Power reads 1024.

Power is a DINT so that Exponent can go to 30. In an INT the weight would wrap at bit 15.

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See Also

• SQR — Square Root (an exponent of 0.5)
• MUL — Multiply
• CPT — Compute (XPY(base, exp) inside an expression; there is no ** operator)
• Compute Instructions — Category index

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Notes

Inside CPT. Write XPY(X, Y) with a comma. The ** operator is refused by the validator.

Precision. The result is a 32-bit float on most boards, about seven significant digits. 2 to the 30th is exact; 3 to the 20th is not.

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