User manual

IEC61131 User and Reference Manual
April 22, 2008
485
Master Confirmation and Retries
Application or Data Link Layer confirmations should never be enabled on a master as:
1. Master requests typically will fit within a single Application Layer fragment hence there is need
for Data Link Layer confirmations.
2. Master request typically require a response, hence no need for Application Layer confirmations.
Thus, enabling the Application Layer Confirmation on a DNP master is obsolete practice and may
instead degrade system performance.
Recommendation: Disable the Application Layer Confirmation in a master SCADAPack
controller. Typical retry values for Application Layer retries lie between 1 and 3. Lengthy retries
may instead burden the communication medium
Outstation Confirmations and Retries
Confirmations on an outstation serve two useful purposes:
1. Ensure that a master received unsolicited responses from the outstation.
2. To ensure that a master correctly received responses to its request
Unsolicited messages will always request for an Application Layer confirmation, whether or not the
Application Layer Confirmation is enabled on the outstation. If network traffic is predominantly
unsolicited messaging, the Application Layer confirmation does not need to be enabled.
When the master is configured, as recommended, to frequently poll the outstation for event data
using read request, while imposing a limit on the number of events the outstation should include in
its response, the outstation still needs to know if the master received its replies so that it can:
 Remove these events from its buffer
 Know what to transmit next.
To cater for confirmations to read responses, Application Layer Confirmation in the outstation
typically needs to be enabled.
The Data Link Layer breaks down Application Layer fragments into smaller frames. Smaller packet
sizes reduce bit error in noisy environments. While it is better to accept the overhead of confirming
each Data Link Layer frame of a multi-frame message, and re-transmit corrupted frames, than to re-
send an entire Application Layer fragment, a viable alternative is to reduce the Application Layer
fragment size and use only Application Layer confirmations. When fragments are reduced to the size
of a Data Link Layer frame, the overhead of Application Layer confirmations, and the probability of
noise corrupting those confirmation messages, is nearly the same as for Data Link Layer
confirmations.
Enabling the Data Link layer confirmation on the outstation, therefore, is not required when the
communication medium is not reliable. For example, certain data radios, e.g. FreeWave 9000 MHz
spread spectrum radios, implement a robust mechanism to ensure that a data packet make it to their
desired destination; TCP/IP incorporates robust mechanisms to prevent data loss; a local serial link
between stations is also very reliable. In these cases, it is not necessary to enable the Data Link
Layer confirmations.
If, however, physical medium quality if below par, such as in the case of noisy radio networks, or a
shaky PSTN connections, then one should enable the Data Link Layer confirmation only, or as
mentioned earlier, reduce the Application Layer maximum fragment length below 249 bytes.