This is issued during the certification test.

• OPC Foundation requests as below;
  In OPC UA it is possible to write Substrings to a variable of the DataType String by specifying an IndexRange. For example by specifying an IndexRange of “1” and specify the value “a”, it is possible to update the current value (“abc”) from a variable of the DataType String to “aac”.
  In the actual version of the client, it is not possible to specify an IndexRange for a scalar string variable. It is expected that this is possible. Same rules apply for arrays.

• Our opinion is below;
  From the specification, this looks like ability to perform string manipulations and update specific characters or substrings in a field. I think the use case for this is for programmed applications that need to parse strings or trim them before processing.

Theoretically this is possible using the PROCESS service. If an item is read as a string, you can apply sub-string functions to filter out parts you need, and then maybe change them and write them back. However I think this is only in specific cases where there is tight relationship between client and server or for very specific functions.

Even then, the OPC spec may provide a mechanism for writing back to a specific character in a string, but that means the client application has to use that specific mechanism to perform an update. This doesn’t fit in an architecture when data is first acquired, then processed in different software layers. In practice an entire string would be written down from the processing application to the write layer, which would then need to figure out the difference, and then apply the operation on specific characters or substrings. This feels very impractical.

I can’t think of any case that would do that at the moment. I certainly don’t see this is part of the common engineering or operator use case.