OPC Item Specific Errors

If an OPC item cannot be read, written, is unknown or is in any other way erroneous, the OPC server has to inform the client. These errors do not regard the whole operation, instead the response message, which transports the OPC items, implements the two following options that denote the item specific error:

ResultID:

Item specific errors are always outlined by the ResultID. The OPC XML-DA specification distinguishes between so-called error and success codes, denoting if the transported item data is valid or not. The ResultID is of the type QName and contains a unique ID of the error. [OPCXMLDA] defines various errors, which are in the format of E_FAIL or E_ACCESS_DENIED. A complete description of these error and success codes can be found in [OPCXMLDA].

In case the provided OPC errors do not suffice, custom errors can be defined. PyOPC defines a few errors in its namespace, which can also be utilized.

ErrorText:

This non-mandatory option may provide descriptive error text, which will make the reason of an error understandable to humans.

Despite of an item specific error, the response message may transport invalid item data. Therefore OPC XML-DA applications always have to check for the ResultID, so that item specific errors are detected.