An external reference allows you to refer to any input, output, or parameter available in the DeltaV system. External references are best configured using the parameter browser available in all DeltaV expression dialogs. The browser opens a graphical list of areas, blocks, and parameters. By constructing a parameter path from the browser, you can avoid the potential of typographical errors and case sensitivity when referencing block parameters.
Enter #IGNORE as the path to disable the verification warning that the external reference parameter is incomplete. The path is not downloaded but is converted to an empty path in the download. The runtime behavior of an external reference with the parameter path configured as #IGNORE is exactly like one with an empty path.
External reference parameters are denoted in expressions by surrounding them in single quotes (' '). For example, an external reference (EXT_REF1) could be constructed to point to the MODE parameter in a PID block (PID1) in another module (FIC_501). When EXT_REF1 is created, the path would be defined as:
FIC_501/PID1/MODE
When referenced in a phase logic statement, the expression could be something like the following:
IF '/EXT_REF1.ACTUAL' = MAN THEN OUT1 := 5.0 END_IF;
The fields for an external reference parameter are described in the following table.
|
Name and Purpose |
Type |
Configurable |
Readable |
Writeable |
|---|---|---|---|---|
|
.$REF - Use to read the current path |
String |
Yes |
Yes |
No |
|
.CST (Connection Status) - Use to see if the reference has been resolved (that is, the value has been found and can be read). This is particularly useful if the external parameter is in another node. |
Integer |
No |
Yes -3 means 'external reference not resolved' -2 means 'parameter not configured' -1 means 'module not configured' 0 means 'good' 1 means 'not communicating' |
No |
|
.AWST (Asynchronous Write Status) - Indicates if the last attempt to write the referenced parameter was successful |
Integer |
No |
Yes -4 means 'write rejected' -3 means 'external reference not resolved' -2 means 'parameter not configured' -1 means 'module not configured' 0 means 'success' 1 means 'not communicating' 2 means 'write pending' |
No |
|
.ST - The status of the referenced parameter. Use this field to copy the ST value from the externally referenced field to a local parameter's ST field for use in subsequent ST sensitive calculations. |
Integer |
No |
Yes |
No |
When using a reference field other than the .CV field, you will need to type the field name.
External references can be bound to module parameters or node parameters (including I/O parameters) in another node.
Writes from one node to an I/O reference in another node are not supported. No messages or errors appear to indicate the write was not successful.
Establishing an external reference to a parameter in a unit module phase that is not loaded will not cause the phase to be loaded. Therefore, the phase attribute will not be read.