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TN Hist270 Resolving NULL values with Historian IO Server Service aahIOSvrSvc




This article from InSource shows...

  • Author: Peter Farrell
  • Published: 04/04/2017
  • Applies to: Historian all versions


Wonderware  Historian includes an IO Server service called aahIOSvrSvc.  This service acts as an IO Server to live Historian values and is pre-configured with a single topic called Tagname. The Historian aahIOSvrSvc IO Server will listen for clients (such as WWClient or WindowViewer, etc.) that are attempting to establish a connection using the preconfigured topic. After a client connects with the The Historian aahIOSvrSvc IO Server, a link is established between the client and Historian aahIOSvrSvc IO Server. For example, the InSQL I/O Server could be used by InTouch WindowViewer to access system tag values provided by the IndustrialSQL Server historian to monitor system health. You could configure WindowViewer to generate an alarm when abnormal behavior is detected within the Historian.

By default, Historian IO Server service aahIOSvrSvc runs as a Windows service and can be started and stopped using the System Management Console. You can also monitor the InSQL I/O Server from within the System Management Console. 


Note that Historian IO Server service aahIOSvrSvc is a read-only server; clients cannot update data in Historian Runtime database. 

The Historian aahIOSvrSvc is a useful IO service, but there are events where it will return NULL such as during recovery after a network interruption, with slow changing values, or when values are received out of time synchronization.  If your client side logic does not have a way to deal with a NULL value, you can prevent aahIOSvrSvc from returning a NULL as follows.

To improve aahIOSvrSvc, particularly with slow changing values, create the registry key shown below on every historian:

In HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ArchestrA\Historian\Programs\aahIOSvrSvc create a Dword called SendOPCQuality and set it to a value of 1.