User manual

IEC61131 User and Reference Manual
April 22, 2008
488
When communicating with those devices with insufficient memory it is necessary to limit the
maximum application layer fragment length to what the outstation can handle. In addition, limiting
the application layer fragment size beyond 249 also reduces the maximum Data Link layer frame
length.
Certain data radios may give better efficiency when transmitting data packets less than the maximum
data link fragment size of 249 bytes. With these radios, it is necessary to reduce the application
layer’s maximum fragment size below 249 bytes as required by the radio.
Other types of data radios, such as FreeWave’s 900 MHz Spread Spectrum radios, provide
configuration options to optimize efficiency by changing the maximum packet size. In this case, it is
not necessary to reduce the application layer maximum fragment size.
In addition, when the communication medium is noisy, it is typically more efficient to transmit
smaller packets than larger packets. In this case, setting small Application Layer fragments would
force smaller data link frames, which is a better strategy in a noisy environment.
Caution: Limiting the application layer fragment size reduces the rate at which event data is
retrieved from the buffers, thus increasing the possibility of event buffer overflows, if the event data
is not being retrieved in a timely fashion. Reducing the maximum application layer fragment size,
increases network traffic and also reduced data throughput as an Application Layer Confirmation is
required for each fragment of a multi-fragment message.
Why would I ever want to ‘Limit that maximum number of events in a read response?”
This is another strategy that can be used to limit the Application Layer fragment of an outstations’
response message. This strategy could be used under noisy environments.
Also, this could be used to prevent an outstation with a large collection of event data to hold the
communication media captive while transmitting all its events.
What behavior should we expect from a SCADAPack when the event logs are full?
When a new event is collected and the SCADAPack DNP event buffer is full, the oldest event is
deleted and the newest event added into the buffer.
What is the main difference between SCADAPack DNP driver configuration modes?
DNP Master
DNP Mimic Master
Address
Mapping
Router
Outstation
Define DNP I/O
points
Not necessary
Not necessary
Not necessary
Not
necessary
Yes
Enable Application
Layer Confirmation
No
No
No
No.
Not
necessary.
Should I Initiate
Time
synchronization?
Yes
No.
No.
No.
No.
Poll for Class DNP
static and Class
data?
Yes
Yes
Yes
No.
No
Initiates
Unsolicited
messages?
No
No
No
No
Yes
Router Messages
not destined to this
station?
No
Some
No
Yes
No