DCS for Steam Turbine of a Thermal Power Plant

Abstract

In order To increase the performance and the flexibility of the recent
thermal power plant control, a DCS can be used.

The aim of this Project is to study the Distributed  Control System (DCS)
and its application on a thermal Power Plant. A thermal power plant and its
operation are described, and the DCS is implementedon the main element of the
thermal power plant—the Turbine.
Key words:  Thermal power Plant, Steam Turbine, synchronous Generator, DCS,
HMI/GUI, LAN, Ethernet, I/O Bus, Network Protocols,RTU, PLC, PID Controller

I- Introduction: 

Most modern thermal power plants generate from 125  MW to 1000 MW of power.
Any thermal power plant consists of a fuel handlingsystem, boiler, turbine, generator,
transformer, water handling, emission control system and other complementary accessories.
All these units are managed in such a way to generate, regulate electricity and ensure a good
product to the consumer.
A thermal power plant produces electricity startingfrom an energy source. This source
can be natural gas, oils, nuclear or even solar energy. This energy source transforms the fluid,
usually water, to a vapor. This vapor enters a turbine at high pressure and turns its shaft,
which, by the way, turns the shaft of an alternator. The turbine transforms thermal energy into
a mechanical one, and the generator transform this mechanical energy into electrical energy.
To make the turbine turn, the pressure of the vaporat the output has to be lower than
that of the input. This is done by condensing the vapor downstream in the turbine using a cold
source like water. The condensed water is always used as the source of the vapor; which
makes a closed thermodynamic cycle.
There are different types of thermal plants: flammable, nuclear, solar and geothermal.
In this chapter, we are going to have a look at oneof the thermal plants that is CAP-DJINET
thermal power plant.

II- CAP-DJINET Power Plant: 

CAP-DJINET power plant belongs to SONALGAZ Company.It is located at CAB-DJINET, in the front of the sea, in BOUMERDES. It has been constructed with the
association of the Austro-German companies: SIEMENS-KWU-SGP who were in charge of
the study and the supervision of the plant’s construction. Another company was Spanish
(DRAGARDOS) which has been in charge for the installation of the water pumping and
pipelines unit that pumps water from the sea. In addition, the Algerian companies: ENCC,
ETTERKIB, BATIMETAL, GENISIDER, INERGA, SNLB, PROSIDER, ENATUB, SNIC,
GTP, SONATRAM, and SOGEP were associated in the construction of CAP-DJINET.
CAP-DJINET power plant contains 4 thermal-vapor units with a capacity of about 176
MW each, and a total of 706 MW. 672MW are sent to the network and 32MW are used
internally in the plant. A boiler that serves the plant consumes 40,000 Nm3 gas per hour when
operating at full load. This gas is pipelined from HASSI R’MEL to CAP-DJINET.

III- Unit description: 

Fuel (gas) is fed into the boiler where it is burned in order to heat water and rise its
temperature to 540°C (1000F) to produce steam that  has a pressure of about 160 bar at the
boiler exits. As it enters the turbine, the steam is at a pressure between 138 to 140 bar. The
steam expands through the turbine, which consists of rows of blades attached to a shaft. The
steam turns the blades of the turbine, which then turn the shaft of a generator, made up of
magnets rotating between coils. The rotation of thegenerator induces alternating current in
the coils of the stator; this is the produced electricity. To ensure that the alternating current is
maintained constant at the standard frequency (50 Hertz), both turbine and generator rotate at
a constant speed (3000 rpm). The produced output voltage (15.5 kV) will be converted to 6

KV by step-down transformer for internal use, and to 220 kV by a step-up transformer, in
order to ensure efficient transmission over the entire network.
A simple schematic of CAP-DJINET thermal Power Plant is shown in Figure I-01

The production of electricity at the power plant ofCAP-DJINET starts from the
thermal energy and takes different transformations;these transformations occur at different
units that are mainly:
•  The Boiler
•  The steam turbine
•  The synchronous generator
•  The transformer
•  The condenser
III-1- The steam generator (Boiler):
The boiler (usually called a steam generator) is a closed vessel in which water flows in
transporting tubes. This water is heated under highpressure and transformed into steam that
turns the group-turbine-generator. The type of boiler is the drum one, which means a large
drum; it is used as a reservoir for fluid (no extracting pumps are used to feed back the water
into furnace chamber). At the boiler, the steam reaches a temperature of 540° and a pressure
of 160 bars.

III-2- Steam turbine: 

The steam turbine is a mechanical device that converts thermal energy from
pressurized steam into useful mechanical energy to rotate the shaft.
The turbine makes use of the fact that steam attains a high velocity, when passing
through a small opening. The attained velocity during expansion depends on the initial and
final heat content of the steam. This difference inheat content represents the heat energy
converted into kinetic energy during the process. The steam turbine is composed of: high
pressure (HP), medium pressure (MP), and low pressure (LP) cores. All these units turn at a
speed of 3000 TR/MIN.

III-3- The Synchronous generator: 

Synchronous generators (alternators) which convert  mechanical energy to electrical
one with an alternating current (AC), consist of two essential elements: the stator and the
rotor. A direct current (DC) is applied to the rotor winding to produce a magnetic field. By the
rotation of the rotor, a three-phase voltage is induced in the stator windings by the rotation of
the magnetic field. The parameters of the generatedvoltage are: apparent power of 220 MVA,
power of 176 MW, voltage of 15500 V±10%, current of8165 A, and a frequency of 50 Hz.
The generator of CAP-DJINET is divided into three alternators shown below:

The function principle of the generator is described as follows: 

The turbine turns the exciting pilot which containsa natural magnet’s rotor. This 

alternator generates a current at the stator which  is converted to DC current by the Thyristor 

bridge. The voltage regulator controls the RMS of the DC current by changing the firing 

angle. After that, the DC current is transferred tothe stator of the principle exciter. The 

principle exciter generates an AC current that is converted to a DC current by a turning diode 

bridge. The DC current is transferred to the rotor  of the main alternator; this current is called 

the  exciting current. The main alternator generates a 3 phase current with a voltage that 

depends on the exciting current. 

III-4- The Condenser: 

The condenser is a heat-exchange system, mounted atthe outlet of the low pressure 

core of the steam turbine to condense steam. The condenser increases the efficiency of the 

turbines. From the bottom of the condenser, powerful pumps recycle the condensed steam 

(water) back to the feed water heaters for reuse. The heat absorbed by the circulation cooling 

water in the condenser tubes must also be removed to maintain the ability of the water to cool 

as it circulates. A proper operation of the condenser requires access to a large amount of 

water; therefore, the sea water is used. 

It has been proved that the efficiency can be improved, when feeding back heated 

water; the gas consumption is much lower than that for a cold distilled water. 

III-5- The heaters: 

Heaters are the main elements of the Rankin cycle.  Their role is to gradually heat up 

both the water coming from the condenser and the one extracted from high and intermediate 

pressure parts of the turbine. 

III-6- The transformer: 

There are two main transformers, the step down transformer that converts the 15.5 KV 

to 6 KV for internal uses, and the step up transformer that converts the voltage from 15500 V 

to 220 kV; this will decrease the current, reduce energy loss and ensure efficient transmission 

over the network. 

IV- Steam Plant (Rankin) cycle: 

The Rankin cycle is the most widely used cycle for  electric power generation. The 

steam cycle is based on dry saturated steam being supplied by a boiler to the turbine. The 

steam from the turbine exhausts to a condenser, from which the condensed steam is pumped 

back into the boiler passing through heaters. Higher plant efficiency is obtained if the steam is 

initially superheated, and this means that less steam and less fuel are required for a specific 

output. Heaters are often used in large utility plants because they give additional steam energy 

to the low-pressure portion of the turbine; thus, increasing the overall plant efficiency. 

By adding regenerative feed water heating, the Rankin cycle is improved significantly. 

This is done by extracting steam from different stages of the turbine (three extractions from 

high pressure core, two from medium pressure core,  and one from the condenser) to heat the 

fed-back water as it is pumped from the condenser to the boiler to complete the cycle. 

V- Overall plant presentation: 

The next page illustrates the overall thermal powerplant construction, and shows the 

main components with the auxiliary equipments.

II- distributed control system: 

I- Introduction: 

Early minicomputers were used in the control of industrial processes since the 

beginning of the 1960s. The IBM 1800, for example,  was an early computer that had 

input/output hardware to gather process signals in  a plant for conversion from field contact 

levels (for digital and analog signals) to the digital domain.

The DCS was introduced in 1975. Honeywell produced  a DCS called TDC 2000. At 

roughly the same time, a Japanese electrical engineering firm Yokogawa produced its own 

independently DCS called CENTUM systems. US-based Bristol also introduced their UCS 

3000 universal controller in 1975. In 1980, Bailey  (now part of ABB) introduced the 

NETWORK 90 system. Similarly in 1980, Fischer & Porter Company (now also part of ABB) 

introduced DCI-4000 (Distributed Control Instrumentation). 

The DCS largely came out due to the increased availability of microcomputers and the 

sudden increase of microprocessors’ use in the world of process control. Computers had 

already been applied to process automation for sometime in the form of both Direct Digital 

Control (DDC) and Set Point Control. The more conservative approach was Set Point 

Control, where process computers supervised clusters of analog process controllers. 

Workstations provided visibility into the process using text and character graphics. 

Availability of a fully functional graphical user interface was a way away. 

Central to the DCS model was the inclusion of control function blocks. Function 

blocks evolved from early, more primitive DDC concepts of "Table Driven" software; one of 

the first embodiments of object-oriented software. Function blocks remain as the predominant 

method of control for DCS suppliers, and supported by key technologies. 

Digital communication between distributed controllers, workstations and other 

computing elements (peer to peer access) was one ofthe primary advantages of the DCS. 

Attention was duly focused on the networks, which provided all the important lines of 

communication that, for process applications, had to incorporate specific functions such as 

determinism and redundancy. 

II- Definition of the DCS: 

Distributed Control Systems (DCS) use decentralized elements or processors, and use 

communication systems to control distributed processes or complete manufacturing systems. 

They do not require user intervention for routine operation, but may permit operator 

interaction via a Supervisory Control and Data Acquisition (SCADA) interface, or Human 

Machine Interface (HMI). Distributed Control Systems (DCS) consist of a remote control 

panel, communications medium, and a central controlpanel. They use process-control 

software and input/output (I/O) database. Some suppliers refer to their controllers as remote 

transmission units (RTU) or Digital Communication Units (DCU). Regardless of their name, 

distributed controllers contain terminal blocks, I/O modules, a processor, internal memories 

and a communication interface; the input/output devices (I/O) can be integral with the 

controller or located remotely via a field network.Today’s controllers have extensive 

computational capabilities and, in addition to proportional, integral, and derivative (PID) 

control, can generally perform logic and sequentialcontrol. The communications system is a 

wired or wireless link with transmission over twisted pair, coaxial, or fiber optic cable that 

connect the distributed controllers and the centralcontrol panel. The central control panel is a 

SCADA or Human Machine Interface (HMI) based system; it employs one or several 

workstations that can be configured. A server and/or applications processor may be included 

in the system for extra computational, data collection, and reporting capability. 

Figure II-01 represents the different levels of theDCS. The supervisory level contains 

the control panel which is a Human Machine Interface (HMI) based system. The control level 

contains the RTUs (PLCs). The device level or the process level contains the controlled 

elements that control the process. The control network represents the communication system. 

In a Distributed control system, the operator sendssetting values to the distributed 

controllers and acquires data from them through thecommunication network; these data and 

setting values are stored in the memory of the controllers. The controllers receive information 

through input modules and send information through  output modules. The input modules 

receive information from input instruments in the process (e.g. switches, sensors, 

transducers). The controller’s processors execute the instructions of the process. The output 

modules update instructions of output instruments in the field (e.g. electro vanes, pumps). The 

controllers typically use communication network to  communicate with the actuators, sensors 

and human machine interface eliminating the need for point-to-point wiring to each device. 

III- Applications of DCS: 

Distributed Control Systems are dedicated systems used to control manufacturing 

processes that are continuous or batch-oriented, such as oil refining, petrochemicals, central 

station power generation, pharmaceuticals, food & beverage manufacturing, cement 

production and papermaking. DCSs are connected to sensors and actuators and use set point 

control to control the flow of material through theplant. The most common example is a set 

point control loop consisting of a pressure sensor,controller, and control valve. Pressure or 

flow measurements are transmitted to the controller. When the measured variable reaches a 

certain point, the controller instructs a valve or  an actuation device to open or close until the 

fluidic flow process reaches the desired set point.Large oil refineries have many thousands of 

I/O points and employ very large DCSs. Processes can also include things like variable speed 

drives, motor control centers and many others. 

IV- Elements of DCS: 

IV -1- FIELD DEVICES: 

Field devices form the "eyes and ears" of a DCS. Devices such as reservoir level 

meters, water flow meters, valve position transmitters, temperature transmitters, power 

consumption meters, and pressure meters all provideinformation that can tell an experienced 

operator how well a system is performing. All thesedevices transmit the information in a 

form of an electric signal that can be analog or digital. 

In addition, equipments such as electric valve actuators, motor control switchboards, 

and electronic chemical dosing facilities can be used to form the "hands" of the DCS and 

assist in automating the process of a system. The distributed controllers control the process by 

sending an electric signal to the actuators that will convert the signal into the appropriate form 

(pressure, position, speed or other form). Typical field instrumentations are: 

– Switches and Pushbuttons  – Flow Meters 

– Level Transducers  – Temperature Transmitters 

– Pressure Switches  – Gas Detector 

– Control Relays  – Actuators and Drives

IV -2- DISTRIBUTED CONTROLLERS: 

IV -2-1- REMOTE TERMINAL UNITS (RTUS): 

An RTU is a standalone data acquisition and controlunit, generally microprocessor 

based, which monitors and controls equipment at some remote location from the central 

panel. 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 control panel. It generally also has the 

facility for having its configuration and controls  dynamically downloaded programs from 

central panel. There is also a facility to be configured locally by some RTU programming 

unit. Although traditionally the RTU communicates back to some central panels, it is also 

possible to communicate on a peer-to-peer basis with other RTUs. 

There are two basic types of RTU - the "single board RTU" which is compact, and 

contains fixed number of I/O on a single board, andthe "modular RTU" which has a separate 

CPU module, and can have other linked modules like  analog modules and other specialized 

modules, normally by plugging into a common "backplane" (see figure II-02).

Typical RTU hardware modules include: 

• Control processor and associated memory 

• Analog inputs & analogue outputs 

• Digital inputs 

• Digital outputs 

• Communication interface(s) 

• Power supply 

IV -2-2- PROGRAMMABLE LOGIC CONTROLLERS (PLCs): 

A PLC (programmable logic controller) is a small industrial computer which 

originally replaces relay logic with a combination  of ladder–logic software and solid state 

electronic input and output module. It has inputs and outputs similar to those an RTU has. It 

contains a program which executes a loop, scanning  the inputs and taking actions based on 

these inputs. Originally the PLC had no communications capability, but they began to be used 

in situations where communication was a desirable feature. Hence communication modules 

were developed for PLC's, supporting Ethernet (for  use in distributed control systems) and 

MODBUS communications protocol for use over dedicated (wire) links(see Figure II-03). 

They are often used in the implementation of a DCS’RTU as they offer a standard hardware 

solution, which is very economically priced.

IV -3- CENTRAL CONTROL PANEL: 

The central control panel contains computers with larger monitors running system 

software. These computers are known as application  work stations (AW), and work station 

processors (WP). The control station contains localPrinters and shared printers connected to 

the network to print reports, files and alarms failures records. 

1-  Workstation Processors (WP): 

They are Operator terminals that do the following functionalities:

•  Provide Human-machine Interface (HMI) for operatorsand provide a real time 

interface between the operators and the DCS. 

•  Accept, process and store data from RTU/RPU 

•  Process regulation using the HMI 

•  Accept, process and store data from RTU 

•  Distribute data to controllers 

2-  Application Processors (AP): 

The common functions of the APs are: 

• Files server 

• back up files and DCS controller programs Storage 

• Files and data base storage 

• System supervision and warning when failures occur in the system 

• Execution of application software and programs (e.g. save history files, 

making reports and statistics) 

3-  Application Workstation (AW): 

This station can be used for AP and WP applications. 

AP and WP can also replace the system server for files and data base storage. 

IV-4- COMMUNICATION INTERFACES STANDARDS:

Regardless of the type of peripheral used, communication interfaces must be used to 

achieve correct communication. Typical peripherals  communicate in serial form at speeds 

ranging from 110 to 19,200 bit/sec (baud), with parity and non parity, and use various 

communication interface standards. An interface standard defines the electrical and 

mechanical details that allow communication equipments from different manufacturers to be 

connected together and to function efficiently. 

IV-4-1- Serial Communication:

It occurs in serial form through simple, twisted-pair cables. Serial data transmission is 

used for most peripheral communication devices. Serial communication allows peripheral 

equipment, such as terminals, operator interface panels, and printers, to receive ASCII 

characters. The most popular standards for serial communication is the RS-232. Other PLC 

standards are the RS-422 and RS-485, which improve performance and give greater flexibility 

in data communication interfaces. 

The data communication links used with peripheral equipment can be unidirectional or 

bidirectional. Unidirectional  serial signal line is used where data transmission  occurs in only 

one direction (input or output devices). Devices that serve as both input and output devices 

require bidirectional links. There are two ways to  achieve this bidirectional communication. 

First, a single data line can be used as a shared communication line. The data can be sent in 

either direction, but only in one direction at a time. This operation is known as half duplex. If 

simultaneous bidirectional communication is required, two lines can connect the PLC to the 

peripheral. One line would be assigned permanently  as an input, while the other would be a 

permanent output. This mode is known as full duplex. This is illustrated in figure II-04. 

IV-4-2- EIA RS-232 Standard:

The EIA RS-232 is a proclaimed standard set by the Electronic Industries Association 

is a proclaimed standard set by the Electronic Industries Association (EIA). It defines the interfacing between data equipment and communication equipment that employs serial binary data interchange

employs serial binary data interchange(see figure II-05). 

The RS-232 standard defines 25 electrical connections. Theelectrical connections are

four groups: 

• Data lines

• Control lines

• Timing lines

• Special secondary functions

IV-4-3- EIA RS-422 Standard: 

The RS-422 standard overcomes some of the RS-232 shortcomings. Like the RS-232, 

the RS-422 standard still deals with the traditional serial/binary switch signals of two voltage 

levels across the interface. The RS-232 is an unbalanced link communication method 

(master/slave relationship). The RS-422, however, is a balanced link (master/master or 

master/slave). The RS-422 specifies electrically balanced receivers and generators that 

tolerate and produce less noise. These provide superior performance up to 10,000 Kbit/sec, 

Distances of up to 1200 m, and can be used in a network communication of up 10 devices. . 

The EIA selected a 37-pin and 9-pin connectors for  the RS-422 standard, because it satisfies 

interface channel requirements.

IV-4-4- EIA RS-485 Standard:

The RS-485 standard, like the RS-422, has dual transmitting and receiving lines 

(differential signals). This type of interface is best suited for industrial applications, because it 

provides better electrical isolation from the PLC or host than the RS-422 standard. It is also 

capable of being used in a network for multi-point communication (e.g., multiple transmitters 

and receivers operated on a common media, up to 32 devices can be connected). Distances of 

up to 1200 meter and Data rates of up to 10 Mbit/sec can be attained with this standard. 

IV-4-5- Interface Converters:

Communications standards are used extensively in applications with a PLC or with a 

computer in a network where one or more interfaces are used. Sometimes a PLC with an RS-232C or RS-422 communication interface must communicate with an RS-485 device. In this 

case, an RS-232C–to–RS-485 converter (or an RS-422–to–RS-485 converter) can provide this 

communication. These converters provide electrical isolation, in addition to longer distance.

IV-5- Communication Network: 

The communications network is intended to provide the means by which data can be 

transferred between the central control panel and the field-based controllers. The 

Communication Network refers to the equipment and protocols needed to transfer data to and 

from different sites. The medium used can either becable, telephone or radio. 

Historically, DCS networks have been dedicated networks; however, with the 

increased deployment of office LANs and WANs as a solution for interoffice computer 

networking, there exists the possibility to integrate DCS LANs into office computer networks. 

The technology came out with another network which  is the I/O bus network, this 

network is used by the intelligent field devices ofthe DCS, and this network is getting 

popular. The most common types of I/O bus network are: FIELDBUS and PROFIBUS. 

IV-5-1- LOCAL AREA NETWORK (LAN): 

As control systems become more complex, they require more effective 

communication schemes between the system components. Some machine and process control 

systems require programmable controllers to be interconnected, so that data can be transferred 

rapidly and easily to accomplish the control task. 

A local area network is a high-speed, medium-distance communication system. For 

most LANs, the maximum distance between two nodes in the network is at least one mile, and 

the transmission speed ranges from 1 to 20 megabit/second. Also, most local networks 

support at least 100 stations, or nodes. A special  type of local area network, the industrial 

network, is one which meets the following criteria:

• Capable of supporting real-time control 

• High data integrity (error detection) 

• High noise immunity 

• High reliability in harsh environments 

• Suitable for large installations 

IV-5-1-1- ADVANTAGES OF LANS: 

Before local area networks came into use, PLCs communicated through their 

programming ports via a central computer (RS-232, RS-422 and RS-485). The disadvantages 

of this method were that it limited the data rate of the PLC’s programming port. Also the 

network would fail if the central computer failed due to the system’s star topology. 

The local area network offers distinct advantages because it greatly reduces the cost of 

wiring for large installations. It also uses a dedicated communication link to efficiently 

exchange large amounts of usable data among PLCs and other hosts; Moreover, PLCs in the 

network can communicate independently with each others. 

IV-5-1-2- THE USE OF LAN: 

Centralized data acquisition and distributed control are the most common applications 

of local area networks. The use of LAN can provide the following functions: 

• Communication between programmable controllers 

• upload and download capability of a host computerto or from any PLC 

• Reading/writing of I/O values and registers to any PLC 

• monitoring of PLC status and control of PLC operation 

IV-5-1-3- NETWORK TOPOLOGIES: 

The topology  of a local area network is the geometry of the network. A network’s 

topology greatly affects its throughput rate, implementation cost, and reliability. The basic 

network topologies used today are star,  common bus,  and ring (see figure II-07);however, a 

large network may consist of multiple topologies. 

A- Star topology: 

The first PLC networks consisted of a multiport host controller with each port 

connected to the programming port of a PLC or another intelligent device. Most 

commercial computer installations are star networks. The main advantage of this 

topology is that it can be implemented with a simple point-to-point protocol. Star 

topology, however, has some disadvantages: 

• It does not lend itself to distributed processingdue to the central node. 

• The wiring costs are high for large installations. 

• Messages must pass through the central node, resulting in low throughput. 

• Failure of the central node will crash the network. 

B- Bus topology: 

The bus topology has a main trunk-line to which individual PLC nodes are 

connected in a multi-drop fashion without the use of a network controller. 

Common bus topologies are very useful in distributed control applications, 

since each station has equal independent control capability. Moreover, common bus 

topology is scalable (add, remove devices easily).  The main disadvantage of this 

topology is the dependency on a common bus which can be overcome with the use of 

redundant bus. And it’s commonly known as dual node-bus in DCS. 

Another configuration of the bus topology is the master/slave bus topology, 

consisting of several slave controllers and one master network controller. The master 

sends data to the slaves; if the master needs data from a slave, it will poll (address) the 

slave and wait for a response. No communication takes place without the master. 

C- Ring topology: 

In this topology the data flow in one direction, with each node passing data on to the 

next node and so on. A failure of any node (not just the master) will crash the network, 

unless the failed node is bypassed. Some LAN manufacturers have overcome the 

problem of node failure in a ring topology by using a wire center. The wire center 

automatically bypasses failed nodes in the ring. 

IV-5-1-4- NETWORK ACCESS METHODS: 

An access method is the manner in which a PLC accesses the network to transmit 

information. As mentioned in the previous section, a bus topology requires that the nodes take 

turns transmitting on the medium. This process requires that each node is able to shut down 

its transmitter without interfering with the network’s operation. The most commonly used 

methods are: polling, collision detection, and token passing. 

A- Polling 

The access method most often used in master/slave protocols is polling. In 

polling, the master interrogates each station (slave) in sequence to see if it has data to 

transmit. The master sends a message to a specific  slave and waits a fixed amount of 

time for the slave to respond. The slave should respond by sending either data or a 

short message indicating that it has no data to send. If the slave does not respond 

within the allotted time, the master assumes that the slave is dead and continues 

polling the other slaves. Polling is often referredto as the master/slave access method, 

and the physical topology used is typically a bus type network. 

B- Collision Detection 

Collision detection  is generally referred to as CSMA/CD (carrier sense 

multiple access/collision detection). In this access method, each node with a message 

to send waits until there is no traffic on the network and then transmits. While the 

node is transmitting, it checks for the presence ofanother transmitter. If the circuit 

detects a collision (two nodes transmitting at a time), the node will disable its 

transmitter and wait a random amount of time beforeretransmission. 

CSMA/CD uses a bus topology and operates over a half-duplex connection. 

The most common example of CSMA/CD is Ethernet (or  the IEEE 802.3). Can bus 

and  IEEE802.11 wireless networking technologies uses a similar version called 

CSMA/CA (carrier sense multiple access/collision avoidance).  In CSMA/CA the 

device examines the media for the presence of a data signal. If the media is free, the 

device sends a notification across the media of itsintent to use it. The device then 

sends the data. 

C- Token Passing 

Token passing means that all participants on the network have a list of all the 

participants on the network including itself, usually in the form of an address or node 

number in ascending order. At any time, one of the participants has the token for an 

amount of time. During this time it may send data to or request data from any other 

node. When it is finished, or its maximum time has elapsed, it will 'pass-on' the token 

to the next node in the list of participants and listen, as though it were a slave until it 

receives the token again. The token-passing access method is preferred in distributed 

control applications that have many nodes or stringent response time requirements. It 

can be used also in star or ring topology. 

IV-5-2- I/O BUS NETWORKS: 

Advances in large-scale electronic integration and surface-mount technology, coupled 

with trends towards decentralized control and distributed intelligence to field devices, have 

created the need for a more powerful type of network—the I/O bus network. This new 

network lets controllers better communicate with I/O field devices, to take advantage of their 

growing intelligence.I/O bus networks  allow PLCs to communicate with I/O devices in a 

manner similar to how local area networks let supervisory PLCs communicate with individual 

PLCs. This configuration decentralizes control in the PLC system, yielding larger and faster 

control systems. The topology, or physical architecture, of an I/O bus network follows the bus 

or extended bus (tree) configuration, which lets field devices (e.g., limit, photoelectric, and 

proximity switches) connect directly to either a PLC or to a local area network bus. 

The basic function of an I/O bus network is to communicate information with, as well 

as supply power to, the field devices that are connected to the bus without the use of I/O 

modules; therefore, the PLC connects to and communicates with each field I/O device 

according to the bus’s protocol. 

IV-5-2-1- TYPES OF I/O BUS NETWORKS: 

I/O bus networks can be separated according to the following diagram: 

A- Device Bus Networks 

Interface with low-level information devices (e.g.,push buttons, limit switches, 

etc.), which transmit data relating to the state ofthe device (ON/OFF) and its 

operational status. These networks generally process only a few bits to several bytes of 

data at a time. 

B- Process Bus Networks 

A process bus network is a high-level, open, digital communication network 

used to connect analog field devices to a control system. The size of the information 

packets delivered to and from these analog field devices is large, due to the nature of 

the information being collected at the process level. The most commonly used process 

bus networks are FIELDBUS and PROFIBUS. These networks can transmit data at a 

speed of 1 to 2 megabits/sec. Process bus networks  will eventually replace the 

commonly used analog networks, which are based on the 4–20 mA standard for analog 

devices. A PLC or computer communicates with a process bus network through a host 

controller interface module using either FIELDBUS or PROFIBUS protocol format.

•  FIELDBUS Process Bus Network 

The FIELDBUS process bus network from the FIELDBUS  Foundation 

(FF) is a digital, serial, multiport, two-way communication system that 

connects field equipment, such as intelligent sensors and actuators, with 

controllers, such as PLCs. This process bus networkoffers the desirable 

features inherent in 4–20 mA analog systems, such as: 

 A standard physical wiring interface 

 Bus-powered devices on a single pair of wires 

 Intrinsic safety options 

The FIELDBUS network technology offers the following additional 

advantages: 

 reduced wiring due to multi-drop devices 

 Compatibility among FIELDBUS equipment 

 Digital communication reliability 

•  PROFIBUS Process Bus Network 

PROFIBUS is a digital process bus network capable of communicating 

information between a master controller and an intelligent, slave process field 

device, as well as from one host to another. It is  based on a token bus/floating 

master system. There are three different types of PROFIBUS: PROFIBUS-FMS, -DP and –PA: 

 The PROFIBUS-FMS network is the universal solution  for 

communicating between the field device level and the 

supervisory systems. 

 The PROFIBUS-DP network is used when fast communications 

are needed. The PROFIBUS-DP is a suitable replacement for 

24-Volt parallel and 4–20 mA wiring interfaces. 

 The PROFIBUS-PA network is the process automation version 

of the PROFIBUS network. It provides bus-powered stations 

and intrinsic safety. 

IV- 5- 2-2- I/O BUS NETWORK ADDRESSING: 

Addressing of the I/O devices in an I/O bus networkoccurs during the configuration, 

or programming, of the devices in the system. Depending on the PLC, this addressing can be 

done either directly on the bus network via a PC orthrough a PC connected directly to the bus 

network interface. It can also be done through the PLC’s RS-232 port.

IV -6- NETWORK PROTOCOLS 

A protocol is a set of rules that two or more devices must follow if they are to 

communicate with each other. Protocol includes everything from the meaning of data to the 

voltage levels on connection wires. A network protocol defines how a network will handle the 

following problems and tasks: 

• Communication line errors 

• flow control (to keep buffers from overflowing) 

• Access by multiple devices 

• Failure detection 

• Data translation 

• Interpretation of messages 

IV -6-1- OSI REFERENCE MODEL 

Networks follow a protocol to implement the transmission and reception of data over 

the network medium. In 1979, the International Standards Organization (ISO) published the 

Open Systems Interconnection (OSI) reference model  to provide guidelines for network 

protocols. This model divides the functions that protocols must perform into seven 

hierarchical layers (see Figure II-9). Each layer interfaces only with its adjacent layers and is 

unaware of the existence of the other layers. Figure II-9 describes the seven layers of the OSI. 

The OSI model further subdivides the second layer into two sub-layers, 2A and 2B, called 

medium access control  (MAC) and  logical link control  (LLC), respectively. In network 

protocols, the physical layer (layer 1) and the medium access control sub-layer (layer 2A) are 

usually implemented with hardware, while the remaining layers are implemented using 

software. The hardware components of layers 1 and 2A are generally referred to as modems 

(or transceivers) and drivers (or controllers), respectively. 

IV -6-2- IEEE STANDARDS 

The Institute of Electrical and Electronic Engineers (IEEE) computer society 

established the Standards Project 802 in 1980 for the purpose of developing a local area 

network standard that would allow equipment from different manufacturers to communicate 

through a local area network. After studying all the users’ and manufacturers’ requirements, 

the committee developed standards that define several types of local networks.  The open 

system interconnection, OSI or seven layers model,  is undoubtedly becoming the standard 

model for communications definition. 

Whereas RS232 and IEEE 802.3 for instance are interface standards, the OSI model is 

an attempt to set up standards for the whole communications structure. In this model, the 

interface standards become part of Layer 1, the physical layer. Figure II-10 illustrates the 

different parts of the IEEE 802 standard and its relationship to the OSI model. 

IV -6-3- TCP/IP PROTOCOL: 

Most manufacturers who offer Ethernet compatibilityto implement supervisory 

functions over equipment controlling plant floor functions use a TCP/IP protocol for layers 3 

and 4 of the OSI model. The transmission control protocol/internet protocol (TCP/IP) was 

initially developed for Arpanet, a computer networkcreated in the early 1970s in the United 

States. The U.S. Department of Defense established this protocol to communicate information 

in a reliable manner from one computer to another over the Arpanet network. 

Nowadays, the TCP/IP protocol is used in the Internet data network. In the TCP/IP 

protocol, the TCP guarantees control of end-to-end  connections. The TCP makes several 

services available to the user, such as the establishment of network connections and 

disconnections, guaranteed data sequencing, protection against loss of sequence, connection 

time control, and transparent multiplexing and transport of data. The IP (internet protocol) 

performs complementary functions such as addressingnetwork data, distributing data 

packages, and routing data in multi-network systems. Some PLC manufacturers offer 

programmable controllers with TCP/IP over-Ethernet  protocol built into the PLC processor 

(see Figure II-11). This allows the PLC to connect directly to a supervisory Ethernet network. 

Note that the PLC in Figure II-11 can also have a control network with other PLCs. These 

PLCs can communicate with other intelligent devicesand systems. Figute-11 shows the 

TCP/IP model and the encapsulation process of data to be sent over the network bus. 

IV-6-4- FIELD-BUS NETWORK PROTOCOLS:

The FIELDBUS network protocol is based on three layers of the ISO’s model. These 

three layers are layer 1 (physical interface), layer 2 (data link), and layer 7 (application). In 

addition to the ISO’s model, FIELDBUS adds an extralayer on top of the application layer 

called the  user layer. This user layer provides several key functions, which are function 

blocks, device description services, and system management.

IV-6-5- PROFI-BUS NETWORK PROTOCOLS: 

A-Profibus Network Protocol:

The PROFIBUS network follows the ISO model; however, each type of 

PROFIBUS network contains slight variations in the model’s layers. 

 B-4The physical layer: 

The physical layer specifies the type of PROFIBUS transmission medium. The 

RS-485 voltage standard is defined for the FMS and DP versions of PROFIBUS. The 

IEC 1158-2 standard is used in the PA version. For  FMS and DP a maximum number 

of 255 stations are possible. 

• FMS (RS-485): 187.5 kbps General use. 

• DP (RS-485): 500 kbps /1.5 Mbps/12 Mbps Fast devices. 

• PA (IEC 1158-2): 31.25 kbps intrinsically safe. 

C-Data link layer 

The data link layer is defined by PROFIBUS as the FIELDBUS data link Layer 

(FDL). The medium access control (MAC) part of the  FDL defines when a station 

may transmit data. The MAC ensures that only one station transmits data at any given 

time. PROFIBUS communication is termed hybrid medium access. Through this 

hybrid medium access protocol, a PROFIBUS network can function as a master-slave 

system, a master-master system (token passing), or a combination of both systems. 

D-The application layer: 

This consists of two sections: 

• FIELDBUS message specification (FMS) which provides powerful 

network communication services and user interfaces 

• Lower layer interface (LLI) conducts the data flow control and connection 

monitoring. 

V - DCS software components: 

Many DCS systems employ commercial proprietary software upon which the  DCS 

system is developed. The proprietary software is often configured for a specific hardware 

platform and may not interface with the software orhardware produced by competing 

vendors. A wide range of commercial off-the-shelf (COTS) software products also are 

available, some of which may suit the required application. COTS software usually is more 

flexible, and will interface with different types of hardware and software.  Software products 

typically used within a DCS system are as follows: 

• Computers operating system: Software used to control the central host computer 

hardware. The software can be based on UNIX, Windows or other operating systems. 

• Operator terminal operating system: Software usedto control the central host 

computer hardware. The software is usually the sameas the central host computer 

operating system. This software, along with that for the central host computer, 

usually contributes to the networking of the central host and the operator terminals. 

• Computer application: Software that handles the transmittal and reception of data to 

and from the RTUs and the central host. The software also provides the graphical 

user interface which offers site mimic screens, alarm pages,, and control functions. 

• Operator terminal application: Application that enables users to access information 

available on the central host computer application.It is usually a subset of the 

software used on the central host computers. 

• Communications protocol drivers: Software that isusually based within the central 

host and the RTUs, and is required to control the translation and interpretation of the 

data between ends of the communications links in the system. The protocol drivers 

prepare the data for use either at the field devices or the central host end of the 

system. 

• Communications network management software: Software required to control the 

communications network and to allow them to be monitored for performance and 

failures. 

• RTU automation software: Software that allows engineering staff to configure and 

maintain the application housed within the RTUs (orPLCs). Most often this includes 

the local automation application and any data processing tasks that are performed 

within the RTU.