User manual

IEC61131 User and Reference Manual
April 22, 2008
482
DNP Write Messages always request for a Confirmation
As implemented in the SCADAPack DNP driver, a DNP Write request (FC 02) requires an
Application Layer response from the outstation. If an acknowledgement is not received within the
configured Application Layer timeout interval, the message is retried a number of times as
determined by the Application Layer retry parameter.
Only one DNP3 transaction can be pending at a time
A SCADAPack DNP station will not initiate or process another DNP transaction, as long as one is
outstanding. Thus, once a SCADAPack has initiated a DNP transaction, all subsequent DNP3
messages received but not related to the original transaction are buffered.
SCADAPack controllers buffer 3 DNP messages
A SCADAPack serial port receive buffer can hold a maximum of 3 DNP messages or Data Link
frames. If an additional DNP message is received when the buffer is full, the oldest message in the
buffer is replaced with the newest one.
Output points in DNP Address Mapping issue DNP Write
Digital and analog output points contained within the DNP Address Mapping of a SCADAPack
controller automatically issue DNP Write messages when their value or state changes.
Typical Configuration Malpractices and Recommendations
DNP is a capable protocol that effectively transfers some of the system engineering effort from
designing a sophisticated logic program, to configuring and tuning the system using parameters.
However, DNP does not eliminate the need to properly evaluate and engineer the communication
media to support the performance expectations of the system, especially under worse case scenarios.
DNP networks can be designed around polling or report-by-exception. In a polling environment,
each master request can be viewed as an invitation for an outstation device to transmit data on the
shared communications medium. The master controls which device can transmit, thereby preventing
collisions from occurring, as the timing of responses is predictable under all situations. In addition,
masters can ask again if a response is not received, thus providing an opportunity for the outstation
to re-send lost data. Using this strategy, a master effectively manages media access thereby
preventing contention with those outstations unexpectedly transmitting on their own.
DNP networks can also be designed around unsolicited communications. In this case, the outstations
transmit events to the master as they occur. When using this strategy, the communications media
must be evaluated carefully in regards to the need for collision detection and prevention, if consistent
network performance is to be expected.
Given that typical systems are designed using a combination of both strategies, is a good idea to start
by configuring the network for poll mode, as it can be easily tuned to cater for unsolicited
messaging, when system characteristics under worst case conditions become known. As with any
communications system, the designer should pay careful attention to bandwidth allocation and
management for a successful system implementation.
Below are several requirements of DNP system architecture that require careful engineering
judgment.
1. Multiple high priority unsolicited messages configured in outstation.