Field variables

To reference field variables (controller tags or variables of an OPC UA server),
FactoryTalk Optix Studio
 uses special project variables (tags) that report the value of the field variables.
TIP: When a tag variable is read,
FactoryTalk Optix Studio
 reads the field variable and updates the value of the project variable. When a tag variable is written,
FactoryTalk Optix Studio
 propagates the write on the field variable.

Read/write field variables in
FactoryTalk Optix Studio
 and in C#

Field variable reading/writing occurs automatically when a tag variable is read/written in
FactoryTalk Optix Studio
. To reproduce the same behavior in C#, you need to use specific APIs in these cases:
  • Reading/writing tag variables synchronized with corresponding controller tags.
  • Reading/writing important variables with an OPC UA Client object from a OPC UA server to synchronize them with the OPC UA server corresponding variables.
  • Reading/writing a variable directly or indirectly connected (using dynamic links or converters) to a variable described in previous cases.
IMPORTANT: When a tag variable is read using the
Get
 method,
FactoryTalk Optix Studio
 does not read the value on the field and can return a value that has not been updated.

Value change events - considerations

  • To subscribe methods to the change value event of the field variables, use a
    RemoteVariableSynchronizer
     object. The
    RemoteVariableSynchronizer
     object updates the project variables referring to field, without repeating readings through simple API reading.
  • The
    RemoteVariableSynchronizer
     object is not required if a link to a tag variable from a graphical object, data logger object, alarm or recipe schema object already exists in
    FactoryTalk Optix Studio
    . In a NetLogic, you can subscribe the change value method directly to the linked tag variable.
  • Management of communication delays with the field. If a field variable is read/written in NetLogic, execute the instructions, and ensure that the current value is read or written .The APIs for reading/writing field variables are synchronous. Their execution must terminate before the next instruction can be ran.
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