Mapping Arguments and Phases

✔p>When a mapping operation, such as Read, Write is performed, the execution is made in certain phases. First, the arguments for the operation are obtained from the targets, the actual operation then takes places, and after that, the results are stored to the targets again. For operations such as Subscribe, when the mapping source generates an asynchronous event, similar phases exist during the processing of the notification as well.

For the purpose of clarity, we will call the individual phases as follows:

Input: Before the mapping operation is executed, values are obtained from the mapping target (and then send to the mapping source).
Output: After the mapping operation has been executed, values (received from the mapping source) are stored to the mapping target.
Notification Input: When an asynchronous notification arrives from the mapping source, the values contained in the notification are stored to the mapping target.
Notification Output: When an asynchronous notification from the mapping source is finalized, it is updated with the values obtained from the mapping target. Not currently used.

You can map some of Outputs and Notification Inputs, or none, or all of them – all these approaches are valid, because it is up to you how you handle the data that comes from the mapping source. With Inputs (and Notification Outputs), you must, however, obey the rules that exist for every mapping operation, and map the proper combination of them, so that the mapping source can receive the data it expects.

The following table shows how (depending on the item mapping kind) the Read, Write and Subscribe operations treat their mapping arguments for OPC-DA item mappings.

	Mapping Operation
Mapping Kind	Read	Write	Subscribe
Result	Output	Output	Notification Input
ErrorId	Output	Output	Notification Input
Exception	Output	Output	Notification Input
ErrorMessage	Output	Output	Notification Input
StatusInfo	Output	Output	Notification Input
Vtq	Output	Input	Notification Input
Value	Output	Input	Notification Input
Timestamp	Output	Input	Notification Input
TimestampLocal	Output	Input	Notification Input
Quality	Output	Input	Notification Input

The Write operation in OPC-DA can either write just a value, or a value/timestamp/quality triple. Only following combinations of inputs are therefore valid for the OPC-DA Write operation:

Vtq	Value	Timestamp or TimestampLocal	Quality	Outcome
✔				Value/timestamp/quality triple is written.
	✔	✔	✔	Value/timestamp/quality triple is written.
	✔			Just the value is written.

For OPC-DA property mappings, there is just one mapping operation (Get), and the corresponding table is trivial:

	Mapping Operation
Mapping Kind	Get
Result	Output
ErrorId	Output
Exception	Output
ErrorMessage	Output
StatusInfo	Output
Value	Output

The following table shows how (depending on the item mapping kind) the Read, Write and Subscribe operations treat their mapping arguments for OPC-UA data mappings.

	Mapping Operation
Mapping Kind	Read	Write	Subscribe
Result	Output	Output	Notification Input
ErrorId	Output	Output	Notification Input
Exception	Output	Output	Notification Input
ErrorMessage	Output	Output	Notification Input
StatusInfo	Output	Output	Notification Input
AttributeData	Output	Input	Notification Input
Value	Output	Input	Notification Input
ServerTimestamp	Output	Input	Notification Input
ServerTimestampLocal	Output	Input	Notification Input
SourceTimestamp	Output	Input	Notification Input
SourceTimestampLocal	Output	Input	Notification Input
StatusCode	Output	Input	Notification Input

The Write operation in OPC-UA can either write just a value, or a value/server timestamp/source timestamp/status code quadruple. Only following combinations of inputs are therefore valid for the OPC-UA Write operation:

AttributeData	Value	ServerTimestamp or ServerTimestampLocal	SourceTimestamp or SourceTimestampLocal	StatusCode	Outcome
✔					Value/server timestamp/source timestamp/status code quadruple is written.
	✔	✔	✔	✔
	✔				Just the value is written.