User manual

IEC61131 User and Reference Manual
April 22, 2008
483
2. System with multiple outstations, each containing numerous Class 1 events, configured
with a Class 1 Hold Count of 1.
3. Relying on unsolicited messaging to get event data to master. System not designed
around master polling for events.
4. Multiple masters with poor communication link.
5. Insufficient use of Deadband and debounce to curb event generation.
6. Master RTU has Application Layer confirmation enabled.
7. Enabling both the Application and Data Link Layer confirmations.
8. Setting very high Application Layer timeout values over high speed networks.
9. DNP Address mapping contains multiple analog and digital output points that change
rapidly.
The aforementioned statements and recommendations are provided below. Note that these
recommendations are to ensure, consistent performance under worse case situations, and are based
on the special considerations provided in the previous section.
Multiple High Priority Unsolicited Messages
A common configuration malpractice is to enable numerous high priority events objects within an
outstation, and configure the outstation to trigger an unsolicited message to the master each time a
new event occurs. In a SCADAPack controller, this is accomplished by configuring numerous Class
1 event objects, and enabling Class 1 Unsolicited Responses (Messaging) with a Hold Count or Hold
Time of 1.
A Hold Count of 1 and Hold Time of 60 seconds specified for Class 1 events, imply that the
controller will immediately trigger an unsolicited event as one occurs. If this outstation and others
have a multitude of Class 1 event objects, visualize worse case scenario as a burst of messages being
transmitted to the master at the same time. Given that a SCADAPack serial port buffer can only
handle three DNP Data Link frames at any given time, some messages might get lost, especially if
the master is required to immediately retransmit this message to some other node in return.
Such a system is designed around unsolicited messaging and is, therefore, far more susceptible to
network collisions if proper management of bandwidth it not exercised. Given that a SCADAPack
controller can only process one DNP transaction at a time, there is also a good chance that the serial
port receive buffer will overflow, adding to the cost of lost messages.
Recommendations:
In general, bandwidth is used more efficiently in a large DNP system if the master is designed to poll
for event data more frequently and static data less regularly.
Recommended practice is also to reserve unsolicited messaging for a small number of critical data.
If possible, it may be best to ensure that no more than 3 messages are sent to the master at exactly the
same time, under worse case scenario, as some event data may be lost if the master is currently busy
processing another transaction, unless random retry intervals are put in place.
If unsolicited messaging is the predominant data transfer method, an approach to manage network
usage, could be to configure a group of three or less outstations with a Hold Time that is unique
within the group.
The table below shows an example configuration for Hold Time and Hold Counts for Class 1 events
across six outstations.