Scada primer

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SCADA Primer
This document discusses the basics of SCADA systems. It
serves as introduction for those who are not familiar with it,
and as a reviewer for those who are already
knowledgeable about the SCADA Systems.
What is SCADA?
What is Telemetry?
What is Data Acquisition?
Differences between SCADA and DCS?
Components of SCADA
Field Instrumentation
Remote Station
Communication Network
Central Monitoring System (CMS)
Typical System Configuration
Modes of Communication
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SCADA Primer
What is SCADA?
SCADA (Supervisory Control And Data Acquisition) system
refers to the combination of telemetry and data acquisition.
It consists of collecting information, transferring it back to a
central site, carrying out necessary analysis and control,
and then displaying this data on a number of operator
screens.The SCADA system is used to monitor and control
a plant or equipment. Control may be automatic or can be
initiated by operator commands.
SCADA Primer
What is telemetry?
Telemetry is usually associated with SCADA systems. It is
a technique used in transmitting and receiving information
or data over a medium. The information can be
measurements, such as voltage, speed or flow. These data
are transmitted to another location through a medium such
as cable, telephone or radio. Information may come from
multiple locations. A way of addressing these different sites
is incorporated in the system.
SCADA Primer
What is data acquisition?
Data acquisition refers to the method used to access and
control information or data from the equipment being
controlled and monitored. The data accessed are then

forwarded onto a telemetry system ready for transfer to the
different sites. They can be analog and digital information
gathered by sensors, such as flowmeter, ammeter, etc. It
can also be data to control equipment such as actuators,
relays, valves, motors, etc.
SCADA Primer
What are the differences between SCADA and DCS?
Similar to the SCADA systems are the Distributed Control
Systems (DCS). The DCS is usually used in factories and
located within a more confined area. It uses a high-speed
communications medium, such as local area network
(LAN). A significant amount of closed loop control is
present on the systemThe SCADA system covers
larger.geographical areas. It may rely on a variety of
communication links such as radio and telephone. Closed
loop control is not a high priority in this system.
SCADA Primer
Components of SCADA System
Components of a SCADA System A SCADA
system are composed of the following:
1. Field Instrumentation
2. Remote Stations
3. Communications Network
4. Central Monitoring Station
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Components of SCADA System
Field Instrumentation refers to the sensors and actuators that
are directly interfaced to the plant or equipment. They
generate the analog and digital signals that will be monitored
by the Remote Station. Signals are also conditioned to make
sure they are compatible with the inputs/outputs of the RTU
or PLC at the Remote Station.
The Remote Station is installed at the remote plant or
equipment being monitored and controlled by the central host
computer. This can be a Remote Terminal Unit (RTU) or a
Programmable Logic Controller (PLC).
The Communications Network is the medium for transferring
information from one location to another. This can be via
telephone line, radio or cable.
The Central Monitoring Station (CMS) refers to the location of
the master or host computer. Several workstation may be
configured on the CMS, if necessary. It uses a Man Machine
Interface (MMI) program to monitor various types data
needed for the operation. The following is a sample
configuration of a SCADA system for water distribution.

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SCADA System for Water Distribution
SCADA Primer
SCADA Component:
Field Instrumentation
Field Instrumentation refers to the devices that are
connected to the equipment or machines being controlled
and monitored by the SCADA system. These are sensors
for monitoring certain parameters; and actuators for
controlling certain modules of the system.

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These instruments convert physical parameters (i.e., fluid
flow, velocity, fluid level, etc.) to electrical signals (i.e.,
voltage or current) readable by the Remote Station
equipment. Outputs can either be in analog (continuous
range) or in digital (discrete values). Some of the industry
standard analog outputs of these sensors are 0 to 5 volts, 0
to 10 volts, 4 to 20 mA and 0 to 20 mA. The voltage
outputs are used when the sensors are installed near the
controllers (RTU or PLC). The current outputs are used
when the sensors are located far from the controllers.
Digital outputs are used to differentiate the discrete status
of the equipment. Usually, <1> is used to mean
EQUIPMENT ON and <0> for EQUIPMENT OFF status.
This may also mean <1> for FULL or <0> for EMPTY.
Actuators are used to turn on or turn off certain equipment.
Likewise, digital and analog inputs are used for control. For
example, digital inputs can be used to turn on and off
modules on equipment. While analog inputs are used to
control the speed of a motor or the position of a motorized
valve.

SCADA Primer
SCADA Component:
Remote Station
Field instrumentation connected to the plant or equipment
being monitored and controlled are interfaced to the
Remote Station to allow process manipulation at a remote
site. It is also used to gather data from the equipment and
transfer them to the central SCADA system. The Remote
Station may either be an RTU (Remote Terminal Unit) or a
PLC (Programmable Logic Controller). It may also be a
single board or modular unit.
RTU versus PLC
The RTU (Remote Terminal Unit) is a ruggedized computer
with very good radio interfacing. It is used in situations
where communications are more difficult. One
disadvantage of the RTU is its poor programmability.
However, modern RTUs are now offering good
programmability comparable to PLCs.
The PLC (Programmable Logic Controller) is a small
industrial computer usually found in factories. Its main use
is to replace the relay logic of a plant or process. Today,
the PLC is being used in SCADA systems to due its very
good programmability. Earlier PLC’s have no serial
communication ports for interfacing to radio for transferring
of data. Nowadays, PLC's have extensive communication
features and a wide support for popular radio units being
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used for SCADA system. In the near future we are seeing
the merging of the RTUs and the PLC’s.
Micrologic is offering an inexpensive RTU for SCADA
system wherein the PLC may be an overkill solution. It is a
microcontroller-based RTU and can be interfaced to radio
modems for transmitting of data to the CMS.
Single Board versus Modular Unit
The Remote Station is usually available in two types,
namely, the single board and the modular unit. The single
board provides a fixed number of input/output (I/O)
interfaces. It is cheaper, but does not offer easy
expandability to a more sophisticated system. The modular
type is an expandable remote station and more expensive
than the single board unit. Usually a back plane is used to
connect the modules. Any I/O or communication modules
needed for future expansion may be easily plugged in on
the backplane.
SCADA Primer
SCADA Component:
Communication Network
The Communication Network refers to the communication
equipment needed to transfer data to and from different
sites. The medium used can either be cable, telephone or
radio.
The use of cable is usually implemented in a factory. This
is not practical for systems covering large geographical
areas because of the high cost of the cables, conduits and
the extensive labor in installing them.

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The use of telephone lines (i.e., leased or dial-up) is a
cheaper solution for systems with large coverage. The
leased line is used for systems requiring on-line connection
with the remote stations. This is expensive since one
telephone line will be needed per site. Besides leased lines
are more expensive than ordinary telephone line. Dial-up
lines can be used on systems requiring updates at regular
intervals (e.g., hourly updates). Here ordinary telephone
lines can be used. The host can dial a particular number of
a remote site to get the readings and send commands.
Remote sites are usually not accessible by telephone lines.
The use of radio offers an economical solution. Radio
modems are used to connect the remote sites to the host.
An on-line operation can also be implemented on the radio
system. For locations wherein a direct radio link cannot be
established, a radio repeater is used to link these sites.
SCADA Primer
SCADA Component:
Central Monitoring Station (CMS)
The Central Monitoring Station (CMS) is the master unit of
the SCADA system. It is in charge of collecting information
gathered by the remote stations and of generating
necessary action for any event detected. The CMS can
have a single computer configuration or it can be
networked to workstations to allow sharing of information
from the SCADA system.
A Man-Machine Interface (MMI) program will be running on
the CMS computer. A mimic diagram of the whole plant or
process can be displayed onscreen for easier identification
with the real system. Each I/O point of the remote units can
be displayed with corresponding graphical representation

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and the present I/O reading. The flow reading can be
displayed on a graphical representation of a flowmeter. A
reservoir can be displayed with the corresponding fluid
contents depending on the actual tank level.
Set-up parameters such as trip values, limits, etc. are
entered on this program and downloaded to the
corresponding remote units for updating of their operating
parameters.
The MMI program can also create a separate window for
alarms. The alarm window can display the alarm tag name,
description, value, trip point value, time, date and other
pertinent information. All alarms will be saved on a
separate file for later review.
A trending of required points can be programmed on the
system. Trending graphs can be viewed or printed at a
later time. Generation of management reports can also be
scheduled on for a specific time of day, on a periodic basis,
upon operator request, or event initiated alarms.
Access to the program is permitted only to qualified
operators. Each user is given a password and a privilege
level to access only particular areas of the program.. All
actions taken by the users are logged on a file for later
review.

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MMI Screen Showing Pipe System Diagram and Repair
Areas
SCADA Primer
Typical System Configurations
There are two typical network configurations for the wireless
telemetry radio-based SCADA systems. They are the point-
to-point and the point-to-multipoint configurations.
Point-to-Point Configuration
The Point-to-Point configuration is the simplest set-up for a
telemetry system. Here data is exchanged between two
stations. One station can be set up as the master and the

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other as the slave. An example is a set-up of two RTUs: one
for a reservoir or tank and the other for a water pump at a
different location. Whenever the tank is nearly empty, the
RTU at the tank will send an EMPTY command to the other
RTU. Upon receiving this command, the RTU at the water
pump will start pumping water to the tank. When the tank is
full, the tank’s RTU will send a FULL command to the
pump’s RTU to stop the motor.
Point-to-Point Configuration
Point-to-Multipoint Configuration
The Point-to-Multipoint configuration is where one device is
designated as the master unit to several slave units. The
master is usually the main host and is located at the control
room. While the slaves are the remote units at the remote
sites. Each slave is assigned a unique address or
identification number.

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SCADA Primer
Modes of Communication
There are two modes of communication available, namely,
the polled system and the interrupt system.

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Polled System
In the Polled or Master/Slave system, the master is in total
control of communications. The master makes a regular
polling of data (i.e., sends and receives data) to each slave
in sequence. The slave unit responds to the master only
when it receivers a request. This is called the half-duplex
method. Each slave unit will have its own unique address
to allow correct identification. If a slave does not respond
for a predetermined period of time, the master retries to
poll it for a number of times before continuing to poll the
next slave unit.
Advantages:
• Process of data gathering is fairly simple
• No collision can occur on the network
• Link failure can easily be detected
Disadvantages:
• Interrupt type request from a slave requesting
immediate action cannot be handled immediately
• Waiting time increases with the number of slaves
• All communication between slaves have to pass
through the master with added complexity
Interrupt System
The interrupt system is also referred to as Report by
Exception (RBE) configured system. Here the slave
monitors its inputs. When it detects a significant change or
when it exceeds a limit, the slave initiates communication
to the master and transfers data. The system is designed

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with error detection and recovery process to cope with
collisions. Before any unit transmits, it must first check if
any other unit is transmitting. This can be done by first
detecting the carrier of the transmission medium. If another
unit is transmitting, some form of random delay time is
required before it tries again. Excessive collisions result to
erratic system operation and possible system failure. To
cope with this, if after several attempts, the slave still fails
to transmit a message to the master, it waits until polled by
the master.
Advantages:
• System reduces unnecessary transfer of data as in
polled systems
• Quick detection of urgent status information
• Allows slave-to-slave communication
Disadvantages:
• Master may only detect a link failure after a period of
time, that is, when system is polled
• Operator action is needed to have the latest values
Collision of data may occur and may cause delay in
the communication

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Scada primer

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Scada primer