These properties are mandatory and need to be implemented by all OPC DA Servers.
Note: The property IDs 2, 3 and 4 may not provide actual values in some cases. Refer to the Note column below for more information.
|
Property ID |
Description |
Variant data type |
Note |
|---|---|---|---|
|
1 |
"Item Canonical DataType" (VARTYPE stored in an I2) |
VT_I2 |
Variant data type of the item. |
|
2 |
"Item Value" (VARIANT) Note: the type of value returned is as indicated by the "Item Canonical DataType" above and depends on the item. This will behave like a read from DEVICE. |
Depends on DATATYPE |
This value might not be valid at the time the properties were obtained and it can only be considered if Property ID 4 (Quality) is set to GOOD. |
|
3 |
"Item Quality"
This will behave like a read from DEVICE. |
VT_I2 |
Quality of the value. In the majority of cases it will be set to BAD_WAITING_FOR_INITIAL_DATA and in that case Property ID 2 (Value) should not be considered as useful. |
|
4 |
"Item Timestamp"
This will behave like a read from DEVICE. |
VT_DATE |
Timestamp of the value. If Property ID 2 is set to BAD_WAITING_FOR_INITIAL_DATA, this value might represent the timestamp obtained by the OPC DA Server process and that might be on fixed offset to the actual timestamp provided by the I/O server. Actual data provided by the Read or OnDataChange will bring correct timestamp as stamped by the device. |
|
5 |
"Item Access Rights" |
VT_I4 |
Access rights. |
|
6 |
"Server Scan Rate" in milliseconds. This represents the fastest rate at which the server could obtain data from the underlying data source. This value generally represents the optimum fastest RequestedUpdateRate which could be used if this item were added to an OPCGroup. The accuracy of this value can be greatly affected by system load and other factors. |
VT_24 |
Scan rate |
See Also
Published June 2018