View source for How to subscribe to OPC data changes in PowerShell

[[Category:.NET]] [[Category:Console application]] [[Category:Example]] [[Category:PowerShell]] [[Category:How to]] [[Category:OPC DA]]
{{Note|Beginning with version 2021.3, QuickOPC product installation now contains a large set of PowerShell example scripts, and they are included in the conceptual and reference documentation. OPC "Classic", OPC UA Client-Server, and OPC UA PubSub are covered. The User's Guide also provides more information to QuickOPC usage from PowerShell.}}
The following PowerShell code subscribes to an OPC item and displays the incoming data changes:

<syntaxhighlight lang="ps1">
Add-Type -Path OpcLabs.EasyOpcClassic.dll

# Create EasyOPC-DA component 
$client = New-Object OpcLabs.EasyOpc.DataAccess.EasyDAClient

# Hook events
Register-ObjectEvent -InputObject $client -EventName ItemChanged -Action { Write-Host $EventArgs }

# Subscribe
$client.SubscribeItem("", "OPCLabs.KitServer.2", "Demo.Single", 1000)
</syntaxhighlight>

We have tested this with QuickOPC 5.40 and PowerShell 3.0. {{Note|For some issues and limitations, see [[How to read an OPC item value in PowerShell]].}}
{{Note|A memory leak related to event handling has been observed under some circumstances in PowerShell (and QuickOPC 2017.2). Workaround is to use the "event pull" mechanism in QuickOPC, instead of hooking event handlers.}}


Links related to/used when writing this article:
* https://blogs.technet.microsoft.com/heyscriptingguy/2011/06/16/use-asynchronous-event-handling-in-powershell/
* http://www.get-blog.com/?p=189
* http://serverfault.com/questions/270568/need-to-add-a-wait-command-to-a-powershell-script
* http://stackoverflow.com/questions/1478545/handling-events-with-powershell