Hi,
this is snippet from "Data Access Automation Interface Standard" v2.02 by OPC Foundation. Hope this helps.
BTW, you can download this document form
OPC Foundation site.
SyncRead Description
This function reads the value, quality and timestamp information for one or more items in a group.
Syntax SyncRead(Source As Integer, NumItems As Long, ServerHandles() As Long, ByRef Values() As Variant, ByRef Errors() As Long, Optional ByRef Qualities As Variant, Optional ByRef TimeStamps As Variant)
Source - The ‘data source’; OPC_DS_CACHE or OPC_DS_DEVICE
NumItems - The number of items to be read.
ServerHandles - Array of server item handles for the items to be read
Values - Array of values.
Errors - Array of Long’s indicating the success of the individual item reads. This indicates whether the read succeeded in obtaining a defined value, quality and timestamp. NOTE any FAILED error code indicates that the corresponding Value, Quality and Time stamp are UNDEFINED.
Qualities - Variant containing an Integer Array of Qualities.
TimeStamps - Variant containing a Date Array of UTC TimeStamps. If the device cannot provide a timestamp then the server will provide one.
The function runs to completion before returning. The data can be read from CACHE in which case it should be accurate to within the ‘UpdateRate’ and percent deadband of the group. The data can be read from the DEVICE in which case an actual read of the physical device is to be performed. The exact implementation of CACHE and DEVICE reads is not defined by this specification. When reading from CACHE, the data is only valid if both the group and the item are active. If either the group or the item is inactive, then the Quality will indicate out of service (OPC_QUALITY_OUT_OF_SERVICE). Refer to the discussion of the quality bits later in this document for further information. DEVICE reads are not affected by the ACTIVE state of the group or item.
