Opc interface to the pi system

I/O Rate Point

An I/O Rate point measures the rate at which the Interface writes data to its input tags. The value of an I/O Rate point represents a 10-minute average of the total number of values per minute that the Interface sends to the PI Server.

When the Interface starts, it writes 0 to the I/O Rate point. After running for ten minutes, the Interface writes the I/O Rate value. The Interface continues to write a value every 10 minutes. When the Interface stops, it writes 0.

The ICU allows you to create one I/O Rate point for each copy of this Interface. Select this Interface from the Interface drop-down list, click IO Rate in the parameter category pane, and check Enable IORates for this Interface.

As the preceding picture shows, the ICU suggests an Event Counter number and a Tagname for the I/O Rate Point. Click the Save button to save the settings and create the I/O Rate point. Click the Apply button to apply the changes to this copy of the Interface.

You need to restart the Interface in order for it to write a value to the newly created I/O Rate point. Restart the Interface by clicking the Restart button:

(The reason you need to restart the Interface is that the PointSource attribute of an I/O Rate point is Lab.)

To confirm that the Interface recognizes the I/O Rate Point, look in the pipc.log for a message such as:

PI-ModBus 1> IORATE: tag sy.io.etamp390.ModbusE1 configured.

To see the I/O Rate point's current value (snapshot), click the Refresh snapshot button:

Interface Status Point

The PI Interface Status Utility (ISU) alerts you when an interface is not currently writing data to the PI Server. This situation commonly occurs if

• 
  the monitored interface is running on an Interface Node, but the Interface Node cannot communicate with the PI Server; or
  
• 
  the monitored interface is not running, but it failed to write at shutdown a System state such as Intf Shut.
  

The ISU works by periodically looking at the timestamp of a Watchdog Tag. The Watchdog Tag is a tag whose value a monitored interface (such as this Interface) frequently updates. The Watchdog Tag has its excdev, excmin, and excmax point attributes set to 0. So, a non-changing timestamp for the Watchdog Tag indicates that the monitored interface is not writing data.

Please see the Interface Status Interface to the PI System for complete information on using the ISU. PI Interface Status runs only on a PI Server Node.

If you have used the ICU to configure the PI Interface Status Utility on the PI Server Node, the ICU allows you to create the appropriate ISU point. Select this Interface from the Interface drop-down list and click Interface Status in the parameter category pane. Right click on the ISU tag definition window to bring up the context menu:

Click Create to create the ISU tag.

Use the Tag Search button to select a Watchdog Tag. (Recall that the Watchdog Tag is one of the points for which this Interface collects data.)

Select a Scan frequency from the drop-down list box. This Scan frequency is the interval at which the ISU monitors the Watchdog Tag. For optimal performance, choose a Scan frequency that is less frequent than the majority of the scan rates for this Interface's points. For example, if this Interface scans most of its points every 30 seconds, choose a Scan frequency of 60 seconds. If this Interface scans most of its points every second, choose a Scan frequency of 10 seconds.

If the Tag Status indicates that the ISU tag is Incorrect, right click to enable the context menu and select Correct.

The PI Interface Status Utility – and not this Interface – is responsible for updating the ISU tag. So, make sure that the PI Interface Status Utility is running correctly.

Appendix A:
OPC Server Issues

The OPC specification allows a great deal of flexibility in how OPC Servers are designed and in what features they will support. This is an outline of how that may affect users of this interface.

Browsing

Point browsing is not a requirement of the OPC specification. If the OPC Server does not support browsing, there must be access to a list of the points which it will accept or the format of point names it will allow to be used. If browsing is allowed, the PI OPCClient can be used to see the points which the OPC Server recognizes.

Timestamps

There has been much discussion of what the timestamp value should be when the OPC Server sends a timestamp with the data. Some vendors send the timestamp for the last time the data value and quality were read from the device so the timestamp will change even if the value does not. Others send the timestamp of the last read where the value or quality changed so if the data remains the same, the timestamp will not change no matter how many times, or in what way, it is read. If the OPC Server sends timestamps for when the data last changed, the /TS=N parameter should generally be used on the startup command line.

Disconnecting

If the interface disconnects improperly from an OPC Server (for instance, the network connection goes down or the windows system crashes), then the server may not clean up the connection on its side. The symptoms for this will probably be that the interface cannot reconnect with the server. Use the PI OPCClient to verify that this is occurring and the solution will probably be to shut down the OPC Server. Refer to the documentation which came with the OPC server to see whether they address this issue. If not, try shutting down the server, or, if Windows is understood and the programs running on that machine also are understood quite well, use Task Manager to kill the thread. If in doubt, reboot the machine. This is not a problem which can be resolved by a change in the interface: once the connection is broken, the interface has no way to tell the OPC server that it needs to clean up its act.