AssignmentAttempt: Attempt assignment to a reference (S7-1500) - STEP 7

Description

With the "AssignmentAttempt" instruction, you attempt an assignment to a reference tag.

The following assignment attempts are possible:

• Assignment attempt of a VARIANT to a reference

• Assignment attempt of a DB_ANY to the reference of a technology object

Assignment attempt of a VARIANT to a reference

The data type of a reference tag is determined at the time of declaration. However, the data type of a VARIANT tag is determined during runtime. An implicit data type conversion is not permitted with reference tags. To assign the two data types to each other therefore, use the assignment attempt. With the assignment attempt, a check is made at runtime to find out whether the assigned tag is of the correct data type. If this is the case, the instruction is executed. If the instruction is executed successfully, there is a valid reference in the target tag, otherwise a NULL.

After the assignment attempt, you can check if the attempt was successful and, depending on the result, continue processing the program. In LAD and FBD, you can use the enable output "ENO" for the check. "ENO" returns the signal state "1" if the assignment attempt was successful. Only then are the subsequent instructions executed in the network.

In STL and SCL, you can use the instructions "IS_NULL" or "NOT_NULL", for example, to check the success of an assignment attempt.

The following rules apply to the assignment attempt of VARIANT. VARIANT tags that do not match these rules return the value "NULL" at runtime.

• The VARIANT must point to an address in an optimized memory area.

• The VARIANT must not point to an address in a temporary memory area.

• If you want to assign a VARIANT to a reference to an ARRAY, the following rules apply:

  • The VARIANT tag must point to an ARRAY whose limits exactly match those of the declared reference. A VARIANT tag which points to an ARRAY [0..9] does not match a tag REF_TO ARRAY[1..10].

  • Moreover, you should compile the blocks that form the value of the VARIANT tag once in a CPU of the S7-1500 series, firmware version V2.5.

• The assignment attempt of a VARIANT to the reference of a technology object is not possible.

• In SCL, assignment attempts cannot be used in multiple assignments (a := b := c;).

Assignment attempt of a DB_ANY to the reference of a technology object

A reference to a technology object always points to a specific technology object, e.g. REF_TO TO_SpeedAxis. If you want to assign a technology object via a tag of the type DB_ANY during runtime, you need to check whether the technology object matches the declared reference. To do this, use the assignment attempt. With the assignment attempt, a check is made during runtime to find out whether the technology object has the declared type. If this is the case, the assignment is performed. If this is executed successfully, there is a valid reference in the target tag, otherwise a NULL.

The following rules apply to the assignment attempt of a DB_ANY to the reference of a technology object:

• The DB_ANY must point to a technology object in an optimized memory area.

• Two technology objects of the same type can be assigned to each other.

• A derived type can be assigned to its base type.

• A basic type cannot be assigned to its derived type

Parameter

The following table shows the parameters of the instruction:

You can find additional information on valid data types under "See also".

Example

The following example shows how the instruction works:

The interface of the block was programmed as follows:

In the program code an attempt is made to assign "myVariant" to the reference tag "myReference". If "myVariant" has the data type "Int" at runtime, "myReference" contains a valid reference to the source tag of the VARIANT, otherwise NULL. When the assignment was successful, the enable output "ENO" has the signal state "1" and the value "10" can be written to the target tag.

In another network, an attempt is made to assign "myDB" to the reference tag "myReferenceTO". If "myDB" points to a technology object of the "Positioning Axis" type in runtime, "myReferenceTO" contains a valid reference, otherwise a NULL. When the assignment was successful, the enable output "ENO" has the signal state "1" and the value "1" can be written to the target tag within the technology object.