CPT — Compute

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Description

CPT (Compute) evaluates a free-form expression and writes the result to Dest every scan its rung is true. One instruction replaces a chain of ADD, MUL and DIV blocks and the temporary tags between them. Use CPT for any calculation with more than one operation, for a function call such as SQR or SIN, or when the formula reads better as text than as three blocks. Do not use CPT for a plain two-operand sum or a single MOV; the dedicated block is clearer on the rung and costs less code. Compare by expression is CMP, which uses the same language and gates the rung instead of writing a value.

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Operands

Operand	Type	Format	Valid Range	Required	Description
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.
Expression	Text	Tags, numbers, operators, functions, parentheses	Must not be empty	Yes	The formula.

Vocabulary:

Kind	Accepted	Notes
Tags	Temp, TMR[0].ACC, Speeds[3], Status.3	Any numeric tag, member, element or INT/DINT bit. STRING tags cannot be used.
Numbers	25, -3, 72.5, 0x1F	Hexadecimal for masks.
Arithmetic	+ - * | /	| is the SLC 500 divide symbol; / also divides. No **; use XPY(base, exp).
Comparison	< > <= >= == !=	Each yields 1 or 0, usable as a number.
Bit operators	AND OR XOR NOT	Bitwise, on whole numbers.
Functions	SQR ABS SIN COS TAN ASN ACS ATN LN LOG EXP DEG RAD XPY TRN	Trig in radians. TRN truncates to a whole number. EXP and TRN have no block of their own.
Grouping	( )	Use them freely; * and | bind tighter than + and -.

Division is floating point. Every | or / in an expression divides as a REAL, whatever the operand types: 7 | 2 is 3.5. A whole-number Dest then truncates the final result toward zero, so 7 | 2 into an INT is 3, and 7 | 2 * 2 into an INT is 7, not 6. This differs from the DIV block, which divides whole numbers as whole numbers when Dest is a whole-number type.

Dest type. The expression is evaluated in floating point when any operand or division is involved. A REAL Dest keeps the fraction; a whole-number Dest truncates toward zero and wraps silently past its range. Trig and log results into an INT are 0 or ±1; give them a REAL.

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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

The expression is evaluated with the current tag values and the result is written to Dest, every scan the rung is true. A function given an out-of-range argument (SQR of a negative, LN of 0, ASN of 2) yields 0 with no fault.

Postscan

Nothing.

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Example

Scenario: A pressure transmitter feeds an analog input that reads 0 to 1023. The live zero of the transmitter sits at 512, and full scale is 1023, so the useful span is 512 counts. The operator display wants percent of span.

Tags:

• Scale — Compute percent, BOOL
• Raw — Analog input counts, INT
• Percent — Percent of span, REAL

Rung 1:

—] [Scale———[CPT Percent  (Raw-512)*100|512]———

Scan 1 — Scale = 0. Raw reads 768. The rung is false: Percent holds 0. The face shows Dest with its live value beneath it, and the expression as typed; the tags inside the expression do not paint their values, so read them in the Tag Browser.

Scan 2 — Scale = 1. (768 − 512) × 100 ÷ 512 is written: Percent reads 50.

Type 640 into Raw and Percent reads 25. Had Percent been an INT, 25.0 would still be 25, but 700 counts would give 36 instead of 36.7188.

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See Also

• CMP — Compare by expression (the same language as a rung condition)
• SCP — Scale with Parameters (this example without writing the formula)
• DIV — Divide (whole-number division when that is what you want)
• Math Functions — the functions as standalone blocks
• Compute Instructions — Category index

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Notes

Reserved names. A tag cannot be named after a function or keyword (Log, Exp, And), so an expression never confuses the two. The Tag Creator refuses such names.

Spaces. Optional. (Raw-512)*100|512 and (Raw - 512) * 100 | 512 are the same expression; the block shows what you typed.

Bits as values. Status.3 reads as 0 or 1, so Status.3 * 100 is a quick way to turn a bit into a full-scale number.

Comparison as arithmetic. (Temp > 60) * 5 is 5 when the test holds and 0 otherwise; a way to select between two values without a second rung.

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