User manual

IEC61131 User and Reference Manual
April 22, 2008
486
If either the Application or Data Link Layer Confirmation is enabled, retries should be configured to
a low non-zero value. Typical retry values lie between 1 and 3. Lengthy retries may instead burden
the communication medium
Recommendation:
Application and Data Link Layer
confirmations in an outstation can be set
according to the following table:
Communication Medium Reliability
High
Low
Data Acquisition
Configuration
master polls outstation
frequently for event data
(also limits number of
events in read response)
Enable Application Layer
Confirmation
Disable Data Link Layer
Confirmation
Disable Application
Layer Confirmation
Enable Data Link
Layer Confirmation
master does not poll
frequently enough and
outstation generates lot of
unsolicited messages
Enable Application Layer
Confirmation
Disable Data Link Layer
Confirmation
Disable Application
Layer Confirmation
Enable Data Link
Layer Confirmation
Regardless of the data
acquisition strategy, if the
Max Application Layer
fragment is set to a values
less than 249
Enable Application Layer
Confirmation
Disable Data Link Layer
Confirmation
Enable Application
Layer Confirmation
Disable Data Link
Layer Confirmation
Note: It is never required to enable BOTH the Application and Data Link Layer
Confirmations.
Setting relatively large Application Layer timeouts
On a high speed link, such as Ethernet, configuring a high Application Layer timeout does not
increase network reliability. Instead this reduces system performance, as there will be a significant
portion of time within the timeout period, after which the IP transaction may have been terminated.
Typically, an Ethernet transaction is completed in the order of a millisecond and a DNP master
SCADAPack controller, by default, closes its DNP TCP port within 10 seconds of no activity. A
DNP SCADAPack controller acting as an outstation closes its port by default in about 4 minutes.
Under these default conditions, if the application layer timeout on a SCADAPack DNP master is set
for 15 seconds, for instance, the port may have closed 10 seconds after last activity, but the
application may still be waiting for a timeout.
If a message is somehow lost, and the timeout is set for 5 seconds, for instance, the application will
still be waiting for a response even though the IP transaction has terminated. This results to wasted
bandwidth.
Recommendation: When operating over high speed links, make Application Layer timeouts as
small as possible.