Civil Engineering Reference
In-Depth Information
A typical basic plant SCADA system will include:
Central SCADA system consisting of operator terminal(s), data archiving,
SCADA server(s), communications interfaces, and printer(s). A simple system
may consist of one PC-based terminal that combines the functions of operator
terminal, SCADA server, and data archiving. Larger systems may consist of a
number of PC-based terminals with dedicated terminals for SCADA server and
data archiving functions and multiple operator terminals, all interconnected on a
local network such as Ethernet.
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Communications between central SCADA and the field system, which can be
radio, wireline, telephone, or fiber-optic.
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Field system, which will typically consist of intelligent devices such as Program-
mable Logic Controllers (PLCs) or specialized Remote Terminal Units (RTUs).
Both will communicate with the central SCADA and receive data (input-output)
from field devices. Most will have capabilities to be programmed to perform field
analog control strategies and digital (on-off) control.
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Field wiring is needed between field devices and the input-output of the PLC or
RTU.
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Field devices and instruments consist of a variety of signals and devices, such as
run contacts in motor control center starters, motor ready contacts, motor in auto
or manual contacts, pressure switch contacts, float switch contacts, analog pres-
sure transmitters, analog level transmitters, and similar devices that provide con-
tact closures (digital) or analog signals to a PLC or RTU.
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Some field devices now have enhanced communications capabilities and intelli-
gence that allows them to communicate directly with a SCADA central or directly
with a PLC or RTU through a communications port rather than through an input-
output module.
A small, isolated facility such as a pumping station may have only a local PLC for
data collection and control, with a local panel-mounted operator interface such as an
LCD indicator and touch pad for communication with the PLC. The interface to the
PLC can also be a laptop computer plugged into a communications port on the PLC.
Many isolated facilities may have no direct communication to a central SCADA ter-
minal.
A more extensive water facility—such as a water treatment plant, even a small
plant—can benefit from a plantwide SCADA system. Typically, this system will consist
of one or more PLCs or RTUs out in the plant for local process control and monitoring
that will communicate with a central SCADA terminal. Typical communications within
a plant site will be wireline or fiber-optic and sometimes radio. Facilities off the plant
site, such as pumping stations and reservoirs, can also be a part of the same SCADA
system by communicating over telephone or radio.
A SCADA system can be tailored to the needs of the facility or facilities. Typically,
it will have a number of in-plant process control centers within a larger facility con-
sisting of one or more PLCs or RTUs. The central control area can consist of multiple
SCADA operator terminals at various locations on a local network such as Ethernet.
Terminals can be in the main control room, laboratory, programming room, and other
areas as desired. It is also possible to place complete operator terminals out in plant
process control centers as needed. The possibilities are almost unlimited.
