Extended descriptor attribute (exdesc)

The extended descriptor (exdesc) attribute is a multi-purpose field.

If you use the exdesc attribute to specify more than one setting, put a comma between the definitions.

By default, leading and trailing spaces are stripped from entries in the exdesc attribute. To preserve leading and trailing spaces, enclose your entry in double quotes.

PI Interface for OPC DA uses the exdesc attribute in the following ways:

• Event-based data collection

  To define an event tag, set this attribute to event=tag. When the specified tag has an exception event, the tags for which it is the trigger are read from the OPC server.

• Dzero for scaled tags

  When the device returns values that must be scaled to fit the range of values stored by the tag, store the device zero in the exdesc attribute. The format for specifying the device zero is Dzero=nnnnn.nnn. To specify the device span, use the convers attribute.

• Target OPC item for output tag time stamp

  To direct the time stamp of an output point to an OPC item, specify the target ItemID in the exdesc attribute.

  The format of the time stamp depends on the data type of the ItemID that is to receive the time stamp, as follows:

  • Tim=ItemID

    Setting the Tim option causes the time stamp to be written as a string (VT_BSTR), formatted for the interface instance using the time stamp format (tf) parameter in the batch file. The time stamp comes from the Data Archive server and is not adjusted for differences in time zone or daylight savings time setting.

  • Dat=ItemID

    Setting the Dat causes the time stamp to be written as a VT_DATE. VT_DATE is an automation type that does not specify time zone. VT_DATE values are usually treated as local time.

    In error messages related to this time stamp ItemID, the interface reports a generated tag name of the form TS:pointname, where pointname is the name (tag attribute) of the output PI point.

• OPC ItemID

  Because of limitations to the maximum length of the instrumenttag attribute, you might need to specify the OPC ItemID in the exdesc attribute.

  Use the syntax instr=ItemID, where ItemID exactly matches the ItemID defined on the OPC server. If the ItemID contains a comma or space, enclose it in double quotes.

  If PI API is version 1.6.0.2 or later and Data Archive is 3.4.370.x or later, the maximum length of the instrumenttag attribute is 1023 characters. For all earlier versions, the maximum is 32. If you are running earlier versions and require more than 32 characters to specify the ItemID, you must enable the AF SDK or use the exdesc attribute to specify the OPC ItemID.