Hello.

This is doable, but we do not have ready-made example for that in Python. The number of examples theoretically needed is huge (because there are many tools an languages that can use QuickOPC), so we do not have them all. But the list keeps growing.

OPC XML-DA is handled by the same object as OPC DA "Classic" in QuickOPC; the difference is just in the "server descriptor" which for COM/DCOM normally contains the computer name and ProgID, but for OPC XML-DA it contains the URL of the server endpoint.

So the code needs to be made by combining knowledge from multiple related examples (and not all of them are in Python but they show the idea). I will to to send them to you or point you to them. Hopefully that will help. If not, let me know, and I will put it together myself.

Here is the basic event pull in Python for OPC DA:

 

# This example shows how to subscribe to item changes and obtain the events by pulling them.

 

import time

import win32com.client

 

# Instantiate the client object

client = win32com.client.Dispatch('OpcLabs.EasyOpc.DataAccess.EasyDAClient')

# In order to use event pull, you must set a non-zero queue capacity upfront.

client.PullItemChangedQueueCapacity = 1000

 

print('Subscribing item changes...')

client.SubscribeItem('', 'OPCLabs.KitServer.2', 'Simulation.Random', 1000)

 

print('Processing item changes for 1 minute...')

endTime = time.time() + 60

while time.time() < endTime:

  eventArgs = client.PullItemChanged(2*1000)

  if eventArgs is not None:

    # Handle the notification event

    print(eventArgs)

 

print('Unsubscribing item changes...')

client.UnsubscribeAllItems()

 

print('Finished.')

 

 

This may be the starting code snippet. The problem with it is that is for OPC DA and not OPC XML DA. And, the EasyDAClient.SubscribeItem does not support OPC XML-DA; you need to use SubscribeMultipleItems instead. An example for it - in VBScript - is:

 

Rem This example shows how to subscribe to changes of multiple items and display the value of the item with each change.

 

Option Explicit

 

Dim ItemSubscriptionArguments1: Set ItemSubscriptionArguments1 = CreateObject("OpcLabs.EasyOpc.DataAccess.OperationModel.EasyDAItemSubscriptionArguments")

ItemSubscriptionArguments1.ServerDescriptor.ServerClass = "OPCLabs.KitServer.2"

ItemSubscriptionArguments1.ItemDescriptor.ItemID = "Simulation.Random"

ItemSubscriptionArguments1.GroupParameters.RequestedUpdateRate = 1000

 

Dim ItemSubscriptionArguments2: Set ItemSubscriptionArguments2 = CreateObject("OpcLabs.EasyOpc.DataAccess.OperationModel.EasyDAItemSubscriptionArguments")

ItemSubscriptionArguments2.ServerDescriptor.ServerClass = "OPCLabs.KitServer.2"

ItemSubscriptionArguments2.ItemDescriptor.ItemID = "Trends.Ramp (1 min)"

ItemSubscriptionArguments2.GroupParameters.RequestedUpdateRate = 1000

 

Dim ItemSubscriptionArguments3: Set ItemSubscriptionArguments3 = CreateObject("OpcLabs.EasyOpc.DataAccess.OperationModel.EasyDAItemSubscriptionArguments")

ItemSubscriptionArguments3.ServerDescriptor.ServerClass = "OPCLabs.KitServer.2"

ItemSubscriptionArguments3.ItemDescriptor.ItemID = "Trends.Sine (1 min)"

ItemSubscriptionArguments3.GroupParameters.RequestedUpdateRate = 1000

 

Dim ItemSubscriptionArguments4: Set ItemSubscriptionArguments4 = CreateObject("OpcLabs.EasyOpc.DataAccess.OperationModel.EasyDAItemSubscriptionArguments")

ItemSubscriptionArguments4.ServerDescriptor.ServerClass = "OPCLabs.KitServer.2"

ItemSubscriptionArguments4.ItemDescriptor.ItemID = "Simulation.Register_I4"

ItemSubscriptionArguments4.GroupParameters.RequestedUpdateRate = 1000

 

Dim arguments(3)

Set arguments(0) = ItemSubscriptionArguments1

Set arguments(1) = ItemSubscriptionArguments2

Set arguments(2) = ItemSubscriptionArguments3

Set arguments(3) = ItemSubscriptionArguments4

 

Dim Client: Set Client = CreateObject("OpcLabs.EasyOpc.DataAccess.EasyDAClient")

WScript.ConnectObject Client, "Client_"

 

Client.SubscribeMultipleItems arguments

 

WScript.Echo "Processing item changed events for 1 minute..."

WScript.Sleep 60*1000

 

Sub Client_ItemChanged(Sender, e)

  If Not (e.Succeeded) Then

    WScript.Echo "*** Failure: " & e.ErrorMessageBrief

    Exit Sub

  End If

 

 WScript.Echo e.Arguments.ItemDescriptor.ItemId & ": " & e.Vtq

End Sub

 

What matters is that an array of objects needs to be prepared upfront and passed to SubscribeMultipleItems; the rest of the Python code can remain the same.

The last difference is in how - in the above code - the ServerDescriptor is configure for XML-DA. What you need to do is instead of assigning to the ProgID ServerDescriptor.ServerClass property, you take the server URL (something like "opcxml.demo-this.com/XmlDaSampleServer/Service.asmx") and assign it to the ServerDescriptor.UrlString property.

Best regards