Attribute-Based Resource Matching
VISA can also search for a resource based on the values of the resource's attributes. The viFindRsrc() search expression is handled in two parts: the regular expression for the resource string and the (optional) logical expression for the attributes. Assuming that a given resource matches the given regular expression, VISA checks the attribute expression for a match. The resource matches the overall string if it matches both parts.
Attribute matching works by using familiar constructs of logical operations such as AND (&&), OR (||), and NOT (!). Equal (==) and unequal (!=) apply to all types of attributes, and you can additionally compare numerical attributes using other common comparators (>, <, >=, and <=).
You are free to make attribute matching expressions as complex as you like, using multiple ANDs, ORs, and NOTs. Precedence applies as follows:
- The grouping operator () in an attribute matching expression has the highest precedence.
- The NOT ! operator has the next highest precedence.
- The AND && operator has the next highest precedence.
- The OR operator || has the lowest precedence.
The following table shows three examples of matching based on attributes.
| Expression | Meaning |
|---|---|
| GPIB[0-9]*::?*::?*::INSTR {VI_ATTR_GPIB_SECONDARY_ADDR > 0 && VI_ATTR_GPIB_SECONDARY_ADDR < 10} | Find all GPIB devices that have secondary addresses from 1 to 9. |
| ASRL?*INSTR{VI_ATTR_ASRL_BAUD == 9600} | Find all serial ports configured at 9600 baud. |
| ?*VXI?INSTR{VI_ATTR_MANF_ID == 0xFF6 && !(VI_ATTR_VXI_LA ==0 || VI_ATTR_SLOT <= 0)} | Find all VXI instrument resources with manufacturer ID of FF6 and which are not logical address 0, slot 0, or external controllers. |
Notice that only global VISA attributes are permitted in the attribute matching expression.
The following example is similar to the finding resources example, except that it uses a regular expression with attribute matching. Notice that because only the first match is needed, VI_NULL is passed for both the retCount and findList parameters. This tells VISA to automatically close the find list rather than return it to the application.
Example
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Note This example shows C source code. There is also an example in Visual Basic syntax. |
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#include <stdio.h>
/* Find the first matching device and return a session to it */
status = viOpenDefaultRM(&defaultRM); /* Error initializing VISA ... exiting */ } /* Error finding resources ... exiting */ } viClose(defaultRM); } } |