We have many overloads in the .NET API that allow you to easily specify such things without extra coding, but unfortunately COM does not support function overloading, so without polluting the interface with many different functions, we had to select just one overload for each method name. As a result, in COM, we typically have one method that is very simple to use, with minimal arguments, and then a corresponding method that is more complicated to use, but provides all functionality that we have.
In your case, ReadItemValue is the "simple" method, and there is no way to pass it the DAReadParameters in our COM API. But you can switch to the ReadMultipleItemValues method, and that one should allow you to achieve what you want. I do not have an example ready for you, but in essence, you should do the following:
1. Create an instance of DAReadItemArguments object.
2. Set its properties: It has ServerDescriptor and ItemDescriptor properties under which you set what you want to read (in your example, .ServerDescriptor.ServerClass and .ItemDescriptor.ItemId would suffice). And, it has .ReadParameters property as well: There you can set e.g. .ReadParameters.DataSource, .ReadParameters.ValueAge.
3. Create an array with items to read: In this case, an array with one element - and set that element to your DAReadItemArguments object. I am not sure whether you will be able to use some standard Python array, or whether you need to somehow instantiate COM SafeArray-s: that's a Python<->COM binding thing and we are not experts in it here. Hopefulyl it will be possible to pass in the array somehow.
4. Call ReadMultipleItemValue on the EasyDAClient object, passing it the array you created above.
5. You will receive back an array with one element, of type _ValueResult. Test .Exception of that element for null-ness to see if there was an error; if not, the actual value read is in the .Value.
Best regards
ZZ
Note: Values for .DataSource are an enumeration, but some languages (Python?) will need to use an integer. Here is the encoding:
Code:
public enum DADataSource
{
/// <summary>The data source (memory, OPC cache or OPC device) selection will be based on the desired value age and
/// current status of data received from the server.</summary>
ByValueAge = 0 /*OCK_DS_BY_VALUE_AGE*/,
/// <summary>OPC reads will be fulfilled from the cache in the OPC server.</summary>
/// <remarks>Corresponds to OPC_DS_CACHE in the OPC specification.</remarks>
Cache = 1,
/// <summary>OPC reads will be fulfilled from the device by the OPC server.</summary>
/// <remarks>Corresponds to OPC_DS_DEVICE in the OPC specification.</remarks>
Device = 2,
}
