KEPServerEX: Limits on the Fastest Update Rate for Getting Data from a DNP Device
 

DNP is designed for utility industries that use update rates of 10 minutes or slower. As such, it is recommended that the Integrity Poll Interval not be set faster than 2 seconds. To locate the Integrity Poll Interval, open Device Properties and select the DNP Slave Configuration tab.

Common Methods of Reading from a DNP Device
Kepware's DNP drivers normally get data from the device from the event poll interval and integrity poll interval. They also get data when an unsolicited message is received from the device (if unsolicited messages have been enabled and are in use).

The DNP drivers maintain an internal cache with received data, and clients are supplied with data from this cache only. Therefore, doing device reads in the client (or setting the OPC update rate or non-OPC scan rate faster than the rate at which the driver receives data) will not cause updates to be more frequent.

For example, suppose the following:

  • The event poll interval is 60 seconds.
  • The integrity poll interval is 3600 seconds.
  • The device does not send unsolicited messages.

In this example, no matter how fast the OPC update rate or non-OPC scan rate is set, the client can only get updated data from the device every 60 seconds. If the client does a device read between polls of the device (such as 30 seconds after an event poll), the data returned to the client will be the same as received in the previous poll.

Explicit Reads from a DNP Device
In server version 4.280.435.0, an .Explicit subtype was added to the DNP drivers. Unlike with the common DNP read methods, the driver does read .Explicit points from the device at the OPC update rate or non-OPC scan rate (or as fast as the device and network will allow).

It is recommended that .Explicit points be used only if the device can't report the data using either of the DNP read methods. This is because .Explicit read points consume more bandwidth than a typical DNP session (especially if many are configured in a project).


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