Description

Function description

The COMP_DB function block enables two numerical values IN1 and IN2 to be compared.

Depending whether IN1 is greater than, equal to or less than IN2, the function blocks sets one of the outputs GREATER, EQUAL or LESS to 1.

The function block takes any dead zone or hysteresis into account.

and can be configured as additional parameters.

COMP_DB/COMP_DB_DFB

Creating new programs, always use the COMP_DB function block.

For technical reasons the COMP_DB_DFB may be implemented during converting legacy programs. But the functionality of COMP_DB and COMP_DB_DFB is exactly the same.

Representation in FBD

Representation:

Representation in LD

Representation:

Representation in IL

Representation:

CAL COMP_DB (IN1=:InputValue1, IN2:=InputValue2,
        DBAND=:DeadBand, HYST:=Hysteresis,
        GREATER=>GreaterValueIndicator,
        EQUAL=>EqualValueIndicator, LESS=>LessValueIndicator)

Representation in ST

Representation:

COMP_DB (IN1=:InputValue1, IN2:=InputValue2,
        DBAND=:DeadBand, HYST:=Hysteresis,
        GREATER=>GreaterValueIndicator,
        EQUAL=>EqualValueIndicator,
        LESS=>LessValueIndicator);

Parameter description

Description of input parameters:

Parameter	Data type	Description
IN1	REAL	Input No. 1
IN2	REAL	Input No. 2
DBAND	REAL	Dead zone
HYST	REAL	Hysteresis

Description of output parameters:

Parameter	Data type	Description
GREATER	BOOL	Greater-than marker
EQUAL	BOOL	Equals marker
LESS	BOOL	Less-than marker

Runtime error

An error is displayed in the Diagnostics display (View → Diagnostics View) if a non floating point value is determined at an input, or if a problem occurs when calculating a floating point value. In this case the GREATER, EQUAL and LESS outputs remain unchanged.

NOTE: For a list of all block error codes and values, see Mathematics.

Warning

A warning is displayed in the Diagnostics display (View → Diagnostics Display) if,

the DBAND parameter is negative: the procedure then uses the value DeadBand = 0 for calculation.
The HYST parameter is outside the [0, DeadBand] range: the procedure then uses the closest correct value for calculation, i.e. 0, if HYST is less than 0 and DBAND, if HYST is greater than DBAND.

REAL

The REAL type is encoded in a 32 bit format.

The possible value ranges are shown in the figure below:

When a result is:

between -1,175494e-38 and 1,175494e-38, it is considered to be a DEN;
less than -3.402824e+38, the symbol -INF (for - infinity) is displayed;
greater than +3.402824e+38, the symbol INF (for + infinity) is displayed;
undefined (square root of a negative number), the symbolNAN is displayed.

NOTE: The IEC 559 standard defines two classes of NAN: the silent NAN (QNAN) and the signaling NAN (SNAN). A QNAN is a NAN with a most significant fraction bit while an SNAN is a NAN without a most significant fraction bit (bit number 22). QNANs can be propagated via most arithmetic operations without throwing an exception. As for SNANs, they generally indicate n invalid operation when they are used as operands in arithmetic operations (see %SW17 and %S18).

NOTE: When a DEN (non-standardized number) is used as an operand, the result is not significant.