SetAttrComparePrecision

IVI Library

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Ivi_SetAttrComparePrecision

Usage

ViStatus Ivi_SetAttrComparePrecision(ViSession vi, ViAttr attributeID, ViInt32 comparePrecision);

Purpose

This function changes the degree of decimal precision the default IVI compare callback uses for a specific attribute.

This function is useful only for ViReal64 attributes. You set the initial comparison precision level for an attribute as a parameter to the Ivi_AddAttributeViReal64 function.

Unless you call Ivi_SetAttrCompareCallbackViReal64 to install your own compare callback function, the IVI engine invokes the default compare callback when comparing cache values it obtains from the instrument against new values you set the attribute to. If the values are equal within the degree of precision you specify, the IVI engine does not call the write callback for the attribute.

The IVI engine uses this method instead of strict equality because of differences between computer and instrument floating point representations.

If the compare callback for the attribute is currently VI_NULL, this function installs the default IVI compare callback.

Parameters

Name	Type	Description
vi	ViSession	The ViSession handle that you obtain from Ivi_SpecificDriverNew. The handle identifies a particular IVI session.
attributeID	ViAttr	Pass the ID of an attribute for this parameter. The include file for a specific instrument driver defines constant names for all of the user-accessible attributes that apply to the driver. This includes attributes that the IVI engine defines, attributes that the instrument class defines, and attributes that are specific to the particular instrument. Each defined constant name begins with PREFIX_ATTR_, where PREFIX is the specific instrument prefix. For each IVI engine attribute, the specific driver include file uses the same constant name that appears in ivi.h, except that the specific instrument prefix replaces the IVI prefix. For example, ivi.h defines IVI_ATTR_CACHE, and the Fluke 45 include file, fl45.h, defines the following constant name: #define FL45_ATTR_CACHE IVI_ATTR_CACHE For each instrument class attribute, the specific driver include file uses the same constant name that appears in the instrument class include file, except that the specific instrument prefix replaces the class prefix. For example, the DMM class include file, ividmm.h, defines IVIDMM_ATTR_RANGE, and fl45.h defines the following constant name: #define FL45_ATTR_RANGE IVIDMM_ATTR_RANGE For each specific instrument attribute, the specific driver include file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PUBLIC_ATTR_BASE. For example, fl45.h defines the following constant name: #define FL45_ATTR_HOLD_THRESHOLD \ (IVI_SPECIFIC_PUBLIC_ATTR_BASE + 3L) For each attribute that is private to an instrument driver, the instrument driver source file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PRIVATE_ATTR_BASE. For example, hp34401a.c defines the following constant name: #define HP34401A_ATTR_TRIGGER_TYPE \ (IVI_SPECIFIC_PRIVATE_ATTR_BASE + 1L)
comparePrecision	ViInt32	The degree of precision you want the default IVI compare callback to use for this attribute. The value for this parameter is in terms of decimal digits of precision. The higher the value, the closer the two values must be for the default compare callback to consider them equal. Valid Range: 0, or 1 to 14 If you pass 0, the function sets the precision to the default IVI for this value, which is 14.

Name

Type

Description

ViSession

The ViSession handle that you obtain from Ivi_SpecificDriverNew. The handle identifies a particular IVI session.

attributeID

ViAttr

Pass the ID of an attribute for this parameter.

The include file for a specific instrument driver defines constant names for all of the user-accessible attributes that apply to the driver. This includes attributes that the IVI engine defines, attributes that the instrument class defines, and attributes that are specific to the particular instrument. Each defined constant name begins with PREFIX_ATTR_, where PREFIX is the specific instrument prefix.

For each IVI engine attribute, the specific driver include file uses the same constant name that appears in ivi.h, except that the specific instrument prefix replaces the IVI prefix. For example, ivi.h defines IVI_ATTR_CACHE, and the Fluke 45 include file, fl45.h, defines the following constant name:

#define FL45_ATTR_CACHE IVI_ATTR_CACHE

For each instrument class attribute, the specific driver include file uses the same constant name that appears in the instrument class include file, except that the specific instrument prefix replaces the class prefix. For example, the DMM class include file, ividmm.h, defines IVIDMM_ATTR_RANGE, and fl45.h defines the following constant name:

#define FL45_ATTR_RANGE IVIDMM_ATTR_RANGE

For each specific instrument attribute, the specific driver include file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PUBLIC_ATTR_BASE. For example, fl45.h defines the following constant name:

#define FL45_ATTR_HOLD_THRESHOLD \ (IVI_SPECIFIC_PUBLIC_ATTR_BASE + 3L)

For each attribute that is private to an instrument driver, the instrument driver source file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PRIVATE_ATTR_BASE. For example, hp34401a.c defines the following constant name:

#define HP34401A_ATTR_TRIGGER_TYPE \ (IVI_SPECIFIC_PRIVATE_ATTR_BASE + 1L)

comparePrecision

ViInt32

The degree of precision you want the default IVI compare callback to use for this attribute.

The value for this parameter is in terms of decimal digits of precision. The higher the value, the closer the two values must be for the default compare callback to consider them equal.

Valid Range: 0, or 1 to 14

If you pass 0, the function sets the precision to the default IVI for this value, which is 14.

Return Value

Contains the status code that the function call returns. IVI engine functions can return error and warning values from several sets of status codes. Some status codes are unique to the IVI engine. Other status codes are the same codes that VISA Library functions return. Still others are error or warning values that functions in specific instrument drivers return. Each set of status codes has its own numeric range.

Regardless of the source of the status code, 0 always indicates success, a positive value indicates a warning, and a negative value indicates an error.