SCADA SYSTEM
A system consisting of a number of
remote terminal units (or RTUs)
collecting field data connected back to
a master station via a communications
system.
The master station displays the
On a more complex SCADA system there
are essentially five levels or hierarchies:
• Field level instrumentation and control
devices
• Marshalling terminals and RTUs
• Communications system
• The master station(s)
DISTRIBUTED CONTROL SYSTEM (DCS)
The data acquisition and control functions are performed by a
PROGRAMMABLE LOGIC CONTROLLER (PLC)
PLCs have replaced hardwired relays with a combination of
SMART INSTRUMENT
Typically means an intelligent (microprocessor based) digital
measuring sensor (such as a flow meter) with digital data communications provided to some diagnostic panel or
REMOTE TERMINAL UNITS
a standalone data acquisition and control unit, generally
microprocessor based, which monitors and controls equipment at some remote location from the central station.
Its primary task is to control and acquire data from process
equipment at the remote location and to transfer this data back to a central station
The RTU can also act as a relay station (sometimes referred to
as a store and forward station) to another RTU, which may not be accessible from the central station.
Small sized RTUs generally have less than 10 to 20 analog and
Typical RTU hardware modules include:
Control processor (or CPU)
This is generally microprocessor based (16 or 32 bit)
e.g. 68302 or 80386. Total memory capacity of 256
kByte (expandable to 4 Mbytes)
Analog input modules
There are five main components making up an analog
input module. They are:
• The input multiplexer
• The input signal amplifier • The sample and hold circuit • The A/D converter
DIGITAL INPUTS
These are used to indicate items such
Digital output module
A digital output module drives an output
voltage at each of the appropriate output
channels with three approaches possible:
• Triac switching
• Reed relay switching
COMMUNICATION INTERFACES
The modern RTU should be flexible
enough to handle multiple
communication media such as:
• RS-232/RS-442/RS-485
• Dialup telephone lines/dedicated landlines
• Microwave/MUX
• Satellite
• X.25 packet protocols
PLCS USED AS RTUS
The advantage of a PLC over the RTU offerings from
various manufacturers is that it can be used in a
general-purpose role and can easily be set up for a
variety of different functions.
PLCs are popular for the following reasons:
Economic solution
Versatility and flexibility Ease of design and insta More reliable
Sophisticated control
Physically compact
THE MASTER STATION
The central site/master station can be pictured as having one
or more operator stations connected to a communication system consisting of modem and radio receiver/transmitter.
The features that should be available are:
• Operator interface to display status of the RTUs and enable operator
control
• Logging of the data from the RTUs
• Alarming of data from the RTU
master station has two main functions:
• Obtain field data periodically from RTUs and submaster stations
The master station has the following typical functions:
Establishment of communications
• Configure each RTU
• Initialize each RTU with input/output parameters
• Download control and data acquisition programs to the RTU
Operation of the communications link
• If a master slave arrangement, poll each RTU for data and write to
RTU
• Log alarms and events to hard disk (and operator display if necessary)
• Link inputs and outputs at different RTUs automatically
Diagnostics
• Provide accurate diagnostic information on failure of RTU and possible
problems
It may also be necessary to set up a submaster
station. This is to control sites within a specific
region.
The submaster station has the following
functions:
• Acquire data from RTUs within the region
• Log and display this data on a local operator
station
• Pass data back to the master station
• Pass on control requests from the master station
COMMUNICATION ARCHITECTURES
Point-to-point (two stations)
Communication philosophies
There are two main communication
philosophies possible. These are; polled (or
masterslave) and carrier sense multiple
