Writing to write-only variables fails

[dirk-zimoch]

We have an OPC UA server with some write-only variables.
Writing to them causes the error message: unsupported conversion for outgoing data

opcuaShow CRIO-9046:BO-1 1
item ns=2;s=Data.Node1 record=CRIO-9046:BO-1 state=up status=BadNotReadable dataDirty=n context=SUB1@OPS1
sampling=0(-1) qsize=1(1) cqsize=3 discard=old timestamp=server bini=read output=y monitor=y registered=-(n)
leaf=[ROOT] record(bo)=CRIO-9046:BO-1 type=OpcUa_Null timestamp=server bini=read monitor=y

The data type is Null. This type is deduced from incomingData, which of course had not been set during startup, because the variable is not readable. See also: status=BadNotReadable.

[ralphlange]

Interesting.

I have never seen a write-only variable before. That will need some effort...

Things I see in your configuration:

• bini=read cannot work if the node is not readable
• monitor=y neither

How should we get the right definition of the OPC UA data type, if the node is not readable? Does a client like UA Expert still show the type?

[dirk-zimoch]

bini=read cannot work if the node is not readable
monitor=y neither

I think those are simply the defaults. The OUT link reads:
field (OUT, "@SUB1 ns=2;s=$(SetParam)")

This is what uaexpert sees. The type is "Boolean":

[ralphlange]

I know these are the defaults - just saying that these defaults will not work for write-only variables, as they configure the client to read at connection-up and set up a monitor.

Ok.
So UaExpert can read the type without having read access to the data. I need to find out how I can get to that information.

[dirk-zimoch]

The only things uaexpert logs for this are:

Attribute Plugin | OPC UA Server | Read attributes of node 'NS2|String|Data.Node2' succeeded [ret = Good].
Reference Plugin | OPC UA Server | Browse succeeded.

[ralphlange]

"Read attributes" - sounds good.

[ralphlange]

Found it. I need to use the browse (introspection) interface of the server to do that. That gets me access to all the attributes of a node without reading the value.

Up to now I was just reading the value, which also gets me the data type.

[dirk-zimoch]

Maybe do that only if initialRead fails because of BadNotReadable?

[ralphlange]

Yes. Otherwise I would have to always add a lot of OPC UA traffic just to cover that rare case.