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Supervisory Control
In the ‘SC’ portion of SCADA one is uploading
commands which either control the processes right away, or at some later time.
Initiation of this output can be manually triggered, scheduled, or a response to
some other system event. In all cases the master controller must retrieve the
relevant data from the database, generate the appropriate set of messages using
communications protocols understood by the target devices, and send them via the
appropriate communications channels. The digital data must then be transported
via appropriate communications media, which may or may not include repeaters, to
the target devices which must decode the messages, execute commands embedded in
them, and possibly send back replies.
Data Acquisition
In the ‘DA’ portion of SCADA one is
downloading data from remote devices. Downloads may be master initiated
(polling, command confirmations), or remote device initiated (quiescent, i.e.
event triggered reports,
or scheduled reports). The data may originate in analog form in which case it
must first be digitized. The digital data must then be transported via
appropriate communications media, which may or may not include repeaters, to the
master. The latter must be ready to receive each message, correctly identify its
origin, properly convert it to some format uniformly used throughout the system,
time stamp it, perform any necessary run time calculations, check and/or enforce
any data range limits, store it in the database, and make it available for
display to any clients connected to the database server at the time, and to
other higher level systems.
Data
Availability
Data in databases must be made available to those interested in them. The latter
can be either human operators, or other systems dependent on that data for their
own functioning (such as, for example, load management systems). Operators tie
in to the system through user interfaces, while other systems utilize other
interfaces. Both are clients being served by the server hosting the databases
(Fig. 1). Clients may or may not be on the same computer as the server.
Location
of the Master
The physical system monitored and controlled by the master may be so large that
it may be impractical to have the master (and associated databases) hosted by
only one computer. The master must therefore run on several machines, possibly
at different geographic locations, networked together as shown in Fig. 2. It is
desirable that any client can monitor and/or control any part of the
system if so desired.
More
on how NTMC implements above can be found
here.
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