User manual

IEC61131 User and Reference Manual
April 22, 2008
468
 Direct operate messages from SCADA Host
The following DNP messages cannot be mimicked (Mimic does not respond on behalf of target DNP
station), and are routed directly to the target outstation by the Mimic:
 Select and Operate messages
 Data Link Layer messages (e.g. get link status, reset link status, etc)
 Enable/Disable Unsolicited Message commands (FC 20 and 21)
 Other control messages
Routing for those messages that cannot be mimicked is subjected to the following rule:
if (a message is received which needs to be retransmitted to someone else)
if (the message target is configured in our routing table)
if (the destination port is different from the incoming port)
or (routing is enabled on the incoming port)
then retransmit the message
In order to provide current outstation data to the SCADA Host, the SCADAPack mimicking master
independently communicates with each outstation to update a local copy of its database with data
from the outstations. This communication may be initiated by the SCADAPack mimicking master,
either by polling each outstation in turn using solicited messages; or the outstations could initiate
unsolicited messages back to the mimicking master. There could also be a combination of solicited
and unsolicited messages between the mimicking master and the outstations.
In the Mimic mode diagram above the SCADAPack mimic master polls each outstation, A and B,
for data and holds images of this data in its memory. When the SCADA Host poll outstations A and
B for data, the mimic master replies from its own images of the outstations. The SCADA Host can
also poll the SCADAPack master for its own local data.
Typically the messaging strategy chosen will depend on the relative importance of the data, and the
required maximum end-to-end delays for data being transferred through the network. If the
requirement is for a reasonably short end-to-end delay for all data points, a round-robin polling
scheme is best, without any unsolicited messages. If there are some data points, which are higher
priority and must be transferred as fast as possible, unsolicited messages should be used.
The advantage of having the SCADA system communicating with the SCADAPack 32 mimic,
instead of direct communication to the outstations is that communication delays and high error rates
are effectively removed. The physical connection between the SCADA system and mimic master
SCADAPack is typically a direct high-speed reliable connection and all message transactions are
fast. Outstations may often be connected via slow PSTN or radio links, and therefore message
transactions are subject to substantial delays. They may also be unreliable communication links
subject to high error rates.
By having a multiple-level network the communication between the SCADAPack master and
outstations is separated from communication between SCADA system and the SCADAPack master.
The delays and error rates, which may be inherent in the outstation communication paths, can be
isolated from communications with the SCADA system, thereby increasing overall system
performance.
One particular advantage of Mimic Mode is that the master SCADAPack does not need to
know, or be configured with, any details of the DNP points configured in the outstations. This
makes it relatively simple to insert such a SCADAPack master into any existing DNP network. The
SCADAPack master in Mimic Mode behaves transparently to the higher-level SCADA system, and