User manual

IEC61131 User and Reference Manual
April 22, 2008
481
Note: For proper operation of the router, there must be two routing entries in the routing
table for each outstation; An entry specifying how the communication path from this
router to the outstation and another communication path from the router to the
SCADA DNP Master.
Design Considerations
The strength of DNP lies in its ability to offer time-stamped data, scheduled polling of data from
multiple outstations and time synchronization, event data buffering and reporting by exception.
DNP was originally design to be used over a serial point-to-point (RS-232) link. As such, the
protocol implements certain measures against data corruption and data loss in its Application and
Data Link layers. Such measures include timeouts, retries, and checksums.
These data recovery mechanisms provided by the protocol, can be counter productive when not
properly configured over an underlying communication medium, such as Ethernet, that already
provides robust measures. In almost all of such cases, the recovery mechanisms offered by DNP
need to be turned off. Such considerations together with good engineering judgment, therefore, must
be practiced before one embarks on the design of a large DNP network.
This chapter outlines special considerations of the DNP protocol and implications within the
SCADAPack DNP driver that should be considered when designing large networks. We also list
common malpractices and a list of Frequently Asked Questions (FAQs) that arise during the course
of network design.
Considerations of DNP3 Protocol and SCADAPack DNP Driver
To ensure consistent network performance, even under worse case scenarios, the following DNP
specification rules should be considered when designing a DNP network using SCADAPack as the
main nodes.
Unsolicited Messages always request for a Confirmation
An outstation will always request for an Application Layer confirmation when it sends an unsolicited
message, even if the Application Layer confirmation field is not enabled. If no response is received
within an Application Layer timeout, the outstation will retry the message a number of times as
determined by the Application Layer Retry parameter.
Master shall never request for Application Layer Confirmation
A Master request is always accompanied by a response message from an outstation. Hence, the
Application Layer confirmation on the master RTU should never be enabled.