1. PROGRAMMABLE
LOGIC
CONTROLLER
(PLC)
Prepared By:
ANSHUMAN MISHRA
MITALI SONI
RISHIKESH BAGWE
(BITS Pilani)

2. Preface
The following project is based on the study of Programmable Logic Controllers (PLC)
and their networking at NPCIL’s Tarapur Atomic Power Station, Units 3&4 (TAPS
3&4). It also gives information about the current profile of the nuclear energy
organization, Nuclear Corporation of India Limited (NPCIL) and its policies of
managing the 20 nuclear reactors in the country. It also explains the process of
conversion of nuclear energy into electricity. There are some other benefits of
nuclear energy shown in this report. The report emphasizes on the working of PLCs,
their various functions, the cards it consists, the microprocessor it is based on, Local
Area Network (LAN) type used for their interconnection and different errors faced at
TAPS 3&4. This report is made in the partial fulfilment of the course Practice School
-1(PS 1) of BITS Pilani on July 9, 2014. The data in the report was gathered from
various sources, the prominent being the manuals available at TAPS 3&4 and the
orientation session at TAPS 3&4. Our mentor at TAPS 3&4, Mr. Somnath Garad
(SO/E) guided us throughout the project and the members of Nuclear Training
Centre (NTC) at TAPS 3&4 helped and motivated in preparing this report.

3. Table of Contents
Page No.
Company Profile: NPCIL 5
Introduction to Nuclear Energy
Working Principle of a Nuclear Reactor
Types of Nuclear Reactors
Pressurised Water Reactor
Boiling Water Reactor
Pressurised Heavy Water Reactor
6
6
6
6
7
8
Details of NPCIL plants in India 10
Need for Nuclear Energy in India
Nuclear Energy – An Inevitable Option
11
11
India’s Three Stage Nuclear Program
Stage 1 - Pressurised Heavy Water Reactors
Stage 2 - Fast Breeder Reactors
Stage 3 - Thorium Based Reactors
12
13
13
13
Advantages and Disadvantages of Nuclear Energy
Advantages
Disadvantages
15
15
16
General Description of TAPS 3&4 17
The Power Plant Cycle and Main Systems Involved 18
Relay Logic System
Triplicated Logics
19
19
Programmable Logic Controller
Classification of PLCs
System Description
PLC Architecture
Input and Output Sub-systems
Digital Inputs
Digital Outputs
CPU Sub-system
VME I/O Board
Token Bus Controller (TBC) – VME Card
MODEM Card
Watch Dog Timer Card
Power Supply Scheme
Engineering Console (EC)
PLC Gateway
PLC System Grounding System
PLC System Functional Description
Application Programming
Application Program Execution
Monitoring and Maintenance
Stand-Alone Functions
Operating of CPU Sub-systems
PLC Startup
Failure of ‘Active’ CPU Sub-system
Failure of ‘Standby’ CPU Sub-system
Recovery from ‘Non-Redundant’ to ‘Redundant’ Mode
Response Time
PLC System Software
PLC Executive Software
20
20
20
23
23
23
23
24
25
25
25
25
27
28
30
31
31
31
31
31
31
32
32
32
32
32
32
33
33

4. Application Software
Programming Language for PLC
Security
Hardware Protection
Software Protection
User Command Logging with Personnel Identification
Administrator Privileges
User Privileges
Time Synchronisation with Central Clock
Manual Restart of PLC
Automatic Restart of PLC in case of Network Faults
33
33
34
34
34
34
34
35
35
35
36
LAN – Need for Interconnection of PLCs 37
OSI Model 38
IEEE 802 39
Token Ring
Token Frame
Token Priority
Token Ring Frame Format
Active and Standby Monitors
Token Ring Insertion Process
Token Passing
40
41
41
42
42
42
43
Ethernet
Shared Media
Repeaters and Hubs
Bridging and Switching
44
44
45
46
Token Ring vs Ethernet 48
Media Fault and System Replacement 49
References 50

5. Company Profile: NPCIL
Nuclear Power Industry has developed manifold since its inception in India. Studies
in nuclear science in a systematic basis began in India during the late forties with the
establishment of Tata Institute of Fundamental research (TIFR) at Mumbai.
Exploitation of Nuclear energy for generation of electricity has supplied the country
with nearly more of electricity so far.
Keeping in mind the increasing need of industry and global competitive challenges,
Nuclear Power Corporation India Limited (NPCIL) with its headquarter at Vikram
Sarabhai Bhavan, Mumbai was started. NPCIL is a Public Sector Enterprise under
the Department of Atomic Energy (DAE), Government of India. It was incorporated
on September 17, 1987 as a Public Limited Company under the Companies Act
1956, with the objective of operating the atomic power stations and implementing the
atomic power projects for the generation of electricity, in pursuance of the schemes
and programmes of Government of India under the Atomic Energy. The formation of
NPCIL was necessitated to give it operational flexibility and raise financial resources
from the capital market to finance the setting up of the projects.
Bhabha Atomic Research Center (BARC) at Mumbai is a premier institute aiming to
provide quality manpower for NPCIL’s nuclear power projects all over the country for
last 35 years. BARC encompasses fields like agriculture, medicine, computer,
electronics, R & D and other areas which are directly relevant to the development of
the nuclear resources of the country in a very efficient way.
The fundamental of the electricity generation at atomic power station is the
generation of heat by bombarding neutrons on the isotope of U-235. The heat, which
is thus being generated, is used to heat up the water to convert it into steam which is
used to rotate turbines, which further runs the turbo generator, and thus generates
electricity. It is estimate to have Nuclear Power Capacity of 20000 MW to make the
country self-sufficient in electricity production. Considering that Nuclear Power is a
safe and environmentally clean source of power generation and that India has vast
thorium reserves, NPCIL is going to play a leading role in future to meet energy
demands of the country.
With a total capacity of 1400 MW, Tarapur is the largest nuclear power station in
India. The facility is operated by the Nuclear Power Corporation of India Limited
(NPCIL). It was initially constructed with two boiling water reactor (BWR) units of 210
MWe. More recently, an additional two pressurised heavy water reactor (PHWR)
units of 540 MW each were added

6. Introduction to Nuclear Energy
Working Principle of a Nuclear Reactor
Nuclear Reactor is a source of heat, which is produced by self-sustained and
controlled chain reaction within the reactor core. The geometrical boundaries within
which the nuclear fuel, moderator, coolant and control rods are arranged to facilitate
production and control of the nuclear reaction to provide heat energy at desired rate
is called the reactor core.
The natural uranium is used as a fuel in our Pressured Heavy Water Reactors.
Uranium has a natural property to emanate radio-active particles. This element has 3
isotopes i.e. U-238, U-235 and U-234. Only the isotope U-235 which is around 0.7%
in the natural uranium is important for energy production. When thermal neutron
strikes the atom of U-235, fission of U-235 atom takes place breaking it up into two
or more fragments. During this process enormous heat energy is generated along
with production of two to three fast moving neutrons.
These fast moving neutrons are slowed down in the presence of moderator (heavy
water) and its probability to cause further fission with uranium atom increases. This
process continues and self-sustained chain reaction is maintained. This provides the
constant heat energy source. The energy produced in this process is proportional to
the neutron density in the reactor core. Thus the reactor power is regulated by
controlling the absorption of the excess neutrons in the core.
The heat produced in the reactor is used to generate light water steam at high
pressure, which drives the turbo-generator to produce electrical energy.
Types of Nuclear Reactors
1. Pressurised Water Reactor (PWR)
This is the most common type, with over 230 in use for power generation and
several hundred more employed for naval propulsion. The design of PWRs
originated as a submarine power plant. PWRs use ordinary water as both coolant
and moderator. The design is distinguished by having a primary cooling circuit which
flows through the core of the reactor under very high pressure, and a secondary
circuit in which steam is generated to drive the turbine. In Russia these are known
as VVER types - water-moderated and -cooled.
A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core,
and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of
uranium.
Water in the reactor core reaches about 325°C, hence it must be kept under about
150 times atmospheric pressure to prevent it boiling. Pressure is maintained by
steam in a pressuriser (see diagram). In the primary cooling circuit the water is also
the moderator, and if any of it turned to steam the fission reaction would slow down.
This negative feedback effect is one of the safety features of the type. The
secondary shutdown system involves adding boron to the primary circuit.

7. A Typical Pressurized Water Reactor (PWR)
(Source: http://www.world-nuclear.org/)
The secondary circuit is under less pressure and the water here boils in the heat
exchangers which are thus steam generators. The steam drives the turbine to
produce electricity, and is then condensed and returned to the heat exchangers in
contact with the primary circuit.
2. Boiling Water Reactor (BWR)
This design has many similarities to the PWR, except that there is only a single
circuit in which the water is at lower pressure (about 75 times atmospheric pressure)
so that it boils in the core at about 285°C. The reactor is designed to operate with 12-
15% of the water in the top part of the core as steam, and hence with less
moderating effect and thus efficiency there. BWR units can operate in load-following
mode more readily then PWRs.
The steam passes through drier plates (steam separators) above the core and then
directly to the turbines, which are thus part of the reactor circuit. Since the water
around the core of a reactor is always contaminated with traces of radionuclides, it
means that the turbine must be shielded and radiological protection provided during
maintenance. The cost of this tends to balance the savings due to the simpler
design. Most of the radioactivity in the water is very short-lived*, so the turbine hall
can be entered soon after the reactor is shut down.

8. A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750
assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary
control system involves restricting water flow through the core so that more steam in
the top part reduces moderation.
A Typical Boiling Water Reactor (BWR)
(Source: http://www.world-nuclear.org/)
3. Pressurised Heavy Water Reactor (PHWR)
The PHWR reactor design has been developed since the 1950s in Canada as the
CANDU, and more recently also in India. PHWRs generally use natural uranium
(0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case
heavy water (D2O).** The PHWR produces more energy per kg of mined uranium
than other designs.
The moderator is in a large tank called a calandria, penetrated by several hundred
horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy
water under high pressure in the primary cooling circuit, reaching 290°C. As in the
PWR, the primary coolant generates steam in a secondary circuit to drive the
turbines. The pressure tube design means that the reactor can be refuelled
progressively without shutting down, by isolating individual pressure tubes from the
cooling circuit.

9. A Typical Pressurized Heavy Water Reactor (PHWR)
(Source: http://www.world-nuclear.org/)
A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods
(ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying
end to end in a fuel channel. Control rods penetrate the calandria vertically, and a
secondary shutdown system involves adding gadolinium to the moderator. The
heavy water moderator circulating through the body of the calandria vessel also
yields some heat (though this circuit is not shown on the diagram above).
Newer PHWR designs such as the Advanced CANDU Reactor (ACR) have light
water cooling and slightly-enriched fuel.
CANDU reactors can readily be run on recycled uranium from reprocessing LWR
used fuel, or a blend of this and depleted uranium left over from enrichment plants.
About 4000 MWe of PWR can then fuel 1000 MWe of CANDU capacity, with addition
of depleted uranium. Thorium may also be used in fuel.
(** with the CANDU system, the moderator is enriched (ie water) rather than the fuel,
- a cost trade-off.)

10. Details of NPCIL plants in India
At present 20 reactors are operating with an installed capacity of 4,780 MWe
(including RAPS-1 of 100 MWe owned by the Government) supplying quality
electricity to consumers in a cost effective manner.
Plant Unit and Location Type Capacity in MW
Existing
TAPS
Tarapur, Boisar,
Maharashtra
1BWR
2BWR
160
160
RAPS
Rawatbhata, Rajasthan
1 PHWR
2 PHWR
3 PHWR
4 PHWR
5 PHWR
6 PHWR
100
200
220
220
220
220
MAPS
Kalpakkam, Tamil Nadu
1 PHWR
2 PHWR
220
220
KGS
Kaiga, Karnataka
1 PHWR
2 PHWR
3 PHWR
4 PHWR
220
220
220
220
NAPS
Narora, Uttar Pradesh
1 PHWR
2 PHWR
220
220
KAPS
Kakrapar, Gujarat
1 PHWR
2 PHWR
220
220
Under Construction
Kudankulam, Tamil Nadu 1 LWR
2 LWR
1000
1000
Kakrapar, Gujarat 3 PHWR
4 PHWR
700
700
RAPS
Rawatbhata, Rajasthan
7 PHWR
8 PHWR
700
700

11. Need for Nuclear Energy in India
Nuclear Energy – An inevitable Option
Electricity is a basic input which is closely related to the economic development of a
country. In spite of the impressive strides in increasing overall installed capacity in
the country, we are still facing power shortages. Options available for commercial
electricity generation are hydro, thermal, nuclear and renewables. In the energy
planning of the country, a judicious mix of hydro, thermal, nuclear and renewable is
an important aspect. Diversified energy resource-base is essential to meet electricity
requirements and to ensure long-term energy security. With the limited resources of
coal and oil available in the country and with growing global concerns of greenhouse
gases generated by fossil fuel-fired stations, nuclear power is being called upon to
play a greater role in medium- and long term perspective.

12. India’s Three Stage Nuclear Program
India has been following a three-stage nuclear power programme, which aims at the
development of
1. Pressurized Heavy Water Reactors, (PHWR) based on natural uranium.
2. Fast breeder reactors utilizing plutonium-uranium fuel cycle, and
3. Breeder Reactors for utilization of thorium.
The ultimate focus of the programme is on enabling the thorium reserves of India to
be utilised in meeting the country's energy requirements. Thorium is particularly
attractive for India, as it has only around 1–2% of the global uranium reserves, but
one of the largest shares of global thorium reserves at about 25% of the world's
known thorium reserves.
THE THREE-STAGE PROGRAMME
(Source: www.conceptualphysicstoday.com )

13. Stage 1 – Pressurized Heavy Water Reactors
In the first stage of the programme, natural uranium fuelled pressurised heavy water
reactors (PHWR) produce electricity while generating plutonium-239 as by-product.
PHWRs was a natural choice for implementing the first stage because it had the
most efficient reactor design in terms of uranium utilisation, and the existing Indian
infrastructure in the 1960s allowed for quick adoption of the PHWR technology. India
correctly calculated that it would be easier to create heavy water production facilities
(required for PHWRs) than uranium enrichment facilities (required for LWRs).
Natural uranium contains only 0.7% of the fissile isotope uranium-235. Most of the
remaining 99.3% is uranium-238 which is not fissile but can be converted in a reactor
to the fissile isotope plutonium-239. Heavy water is used as moderator and coolant.
Indian uranium reserves are capable of generating a total power capacity of 420
GWe-years, but in order to ensure that existing plants get a lifetime supply of
uranium, it becomes necessary to limit the number of PHWRs fuelled exclusively by
indigenous uranium reserves. US analysts calculate this limit as being slightly over
13 GW in capacity. Several other sources estimate that the known reserves of
natural uranium in the country permit only about 10 GW of capacity to be built
through indigenously fuelled PHWRs. The three-stage programme explicitly
incorporates this limit as the upper cut off of the first stage, beyond which PHWRs
are not planned to be built.
Stage 2 – Fast Breeder Reactors
In the second stage, fast breeder reactors (FBRs) would use a mixed oxide (MOX)
fuel made from plutonium-239, recovered by reprocessing spent fuel from the first
stage, and natural uranium. In FBRs, plutonium-239 undergoes fission to produce
energy, while the uranium-238 present in the mixed oxide fuel transmutes to
additional plutonium-239. Thus, the Stage II FBRs are designed to "breed" more fuel
than they consume. Once the inventory of plutonium-239 is built up thorium can be
introduced as a blanket material in the reactor and transmuted to uranium-233 for
use in the third stage.
The surplus plutonium bred in each fast reactor can be used to set up more such
reactors, and thus grow the Indian civil nuclear power capacity till the point where the
third stage reactors using thorium as fuel can be brought online, which is forecasted
as being possible once 50 GW of nuclear power capacity has been achieved.
The uranium in the first stage PHWRs that yield 29 EJ of energy in the once-through
fuel cycle, can be made to yield between 65 and 128 times more energy through
multiple cycles in fast breeder reactor
Stage 3 – Thorium Based Reactors
Stage III Reactor or an Advanced Nuclear Power System involves a self-sustaining
series of Thorium-232 and Uranuim-233 fuelled reactors. This would be a thermal
breeder reactor, which in principle can be refuelled – after its initial fuel charge –

14. using only naturally occurring thorium. According to the three-stage programme,
Indian nuclear energy could grow to about 10 GW through PHWRs fuelled by
domestic uranium, and the growth above that would have to come from FBRs till
about 50GW , The third stage is to be deployed only after this capacity has been
achieved.

15. Advantages and Disadvantages of Nuclear Energy
Advantages
1 .Amount of Fuel Needed
With little fuel large amounts of energy are obtained. This saves on raw materials but
also in transport, handling extraction nuclear fuel. The cost of fuel is 20% of the cost
of energy generated.
2. Production of electric energy is continuous.
A nuclear power plant is generating electricity for almost 90% of the hours of the
year. This reduces the price volatility that exist in other fuels such as petrol. The fact
that is also conducive to continuous electrical planning as no such dependency in
natural aspects.
3. An alternative to fossil fuels
Thus, need not consume as much of carbon fuels like oil, so therefore the problem of
global warming, which is believed to have reduced one more important influence on
climate change on the planet. By reducing the consumption of fossil fuels we also
improve the quality of the air we breathe with all that this implies in the decline of
disease and quality of life. Interestingly, nuclear power plants these days carry a
zero carbon footprint policy!
4. Cheap electricity
The cost of uranium which is used as a fuel in generating electricity is quite low.
Also, set up costs of nuclear power plants is relatively high while running cost is low.
The average life of nuclear reactor range from 4.-60 years depending upon its
usage. These factors when combined make the cost of producing electricity very low.
Even if the cost of uranium rises, the increase in cost of electricity will be much
lower.
5. Low Fuel Cost
The main reason behind the low fuel cost is that it requires little amount of uranium
to produce energy. When a nuclear reaction happens, it releases million times more
energy as compared to traditional sources of energy
6. Nuclear power plants don't require a lot of space
They have to be built on the coast, but do not need a large plot like a wind farm

16. Disadvantages
1. Efficiency
The use of nuclear energy for conversion into mechanical energy is very low.
2. Security in their use remains the responsibility of individuals.
Although there are many automated safety systems at nuclear power plants, people
can make wrong or irresponsible decisions. A series of bad decisions led the worst
nuclear accident in Chernobyl. Once an accident has occurred, the way how it is
managed is also dependent on the decisions made by people who are in office.
3. Use that can be given to nuclear power in the defence industry.
Interestingly, nuclear debuted in front of the world as two bombs dropped on Japan
at end World War II.
4. Generation of nuclear waste
The difficulty to manage the waste and it takes many years to lose its radioactivity
and danger.
5. Nuclear reactors, once constructed, have an expiration date.
After this date must be dismantled, so that in the main countries producing nuclear
energy to maintain constant the number of operating reactors should be built about
80 new nuclear reactors the next ten years. The investment for the construction of a
nuclear plant is very high and must be recovered in no time, so this raises the cost of
electricity generated. In other words, the energy generated is cheap compared to the
cost of fuel, but having to repay the construction of the nuclear plants significantly
more expensive.
6. Nuclear power plants are targets for terrorist organizations
7. Generation of external dependence.
Shortly countries have uranium mines and not all countries have nuclear technology,
so both have to be hired abroad
8. Current nuclear reactors work by fission nuclear reactions.
These chain reactions occur so that if the control systems should fail every time
more and more reactions would occur to cause a radioactive explosion that would be
virtually impossible to contain

17. General Description of TAPS-3&4
Tarapur Atomic Power Station (T.A.P.S.) is located in Tarapur, Maharashtra (India).
It was initially constructed with two boiling water reactor (BWR) units of 210 MWe
each initially by Bechtel and GE under the 1963 123 Agreement between India,
the United States, and the International Atomic Energy Agency. The capacity of units
1 and 2 was reduced to 160 MWe later on due to technical difficulties. Units 1 and 2
were brought online for commercial operation on October 28, 1969. These were the
first of their kind in Asia. More recently, an additional two pressurised heavy water
reactor (PHWR) units of 540 MW each were constructed by L &T and Gammon
India, seven months ahead of schedule and well within the original cost estimates.
Unit 3 was brought online for commercial operation on August 18, 2006, and Unit 4
on September 12, 2005.
With a total capacity of 1400 MW, Tarapur is the largest nuclear power station in
India. The facility is operated by the Nuclear Power Corporation of India Limited
(NPCIL) Tarapur nuclear plant has received the highest safety awards given to any
electricity producing plants in India.

18. The Power Plant Cycle and Main Systems Involved
The conversion to electrical energy takes place indirectly, as in conventional thermal
power plants. The heat is produced by fission in a nuclear reactor (a light water
reactor). Directly or indirectly, water vapour (steam) is produced. The pressurized
steam is then usually fed to a multi-stage steam turbine. After the steam turbine has
expanded and partially condensed the steam, the remaining vapour is condensed in
a condenser. The condenser is a heat exchanger which is connected to a secondary
side such as a river or a cooling tower. The water is then pumped back into the
nuclear reactor and the cycle begins again.
Nuclear Power Plant Cycle
(Source: www.ems.psu.edu )
(The power plant cycle showing the reactor vessel, control rods, reactor, steam generator, pumps,
generator, turbine, and condenser and cooling tower.)

19. Relay Logic System
Triplicated Logics
Station logic control for all safety critical systems and some of the safety related
systems are based on triplicated logics. In this philosophy of control all sensors and
control circuits are triplicated, including power supply. A two-out-of-three voting is
taken and the output drives the final control element which is single and designed to
operate in control safe manner. The two-out-of-three scheme is a universally
accepted control scheme for critical applications. It provides the best compromise
between availability and reliable operation (from safety point of view) of system
compared to other schemes such as one-out-of-two or one-out-of-two-taken twice,
etc.
There are three categories of triplicated logics in the plant:
a) Reactor regulating system designated as Channel A/Y, channel B/Z, and channel C
b) Reactor protective system-1 designated as channel D, E and F
c) Reactor protective system-2 and engineered safety critical systems designated as
Channel G, Channel H and Channel J
Even though PLCs are used for the vast majority of safety related systems, in limited
cases due to safety reasons relays are used. Relay panels pertaining to item a)
above belongs to this class.
The relays in these panels are used for the following applications:
a) Multiplication of triplicated relay contacts of Channel G, H & J for automatic closure
on sensing containment isolation logic for active drainage, fuelling machine vault
leakage & compressor system valves provided on reactor containment & V1/V2
boundaries.
b) Triplicated logic requirement for Automatic Liquid Poison injection system.
c) Multiplication of large number of circuit breaker auxiliary relay contacts and to
generate 2/3 logic for process water system pump control under reactor shutdown.
d) Multiplication of PDCS contacts for various process parameters like BCD pressure
high, BCD level high, pressurizer isolated status, etc for feeding to Dual Processor
Hot Standby Process Control System (DPHS_PCS)
e) Multiplication of parameters like pressurizing pump suction pressure low/very low for
redundant pressurizing pump trip and feed control valve logics through DPHS_PCS
f) Multiplication of triplicated PDCS contacts for giving contacts to redundant PLCs in
Liquid Zone Control System (LZC) system

20. Programmable Logic Controller
A Programmable Logic Controller is an industrially hardened computer based unit
that performs discrete or continuous control functions. Originally they were intended
as relay replacement equipments. In TAPS 3 &4 they are mainly used for digital
inputs only.
The station logic system has been divided into 2 parts- Relay Logic System and
Programmable Logic Controller (PLC) system. The relay based system is used for
critical applications (safety). All other logics are controlled by PLC system.
The function of the station logic system is the ON-OFF automatic control of various
pumps, valves, fans, etc. Also it communicates and logs failure annunciations,
status information of hand switches & pumps in the control room through
Computerized Operator Information System (COIS). It also gives alarm through
dynamic window annunciation system and drives various indicating lamps to indicate
the status of pumps, fans, valves, etc.
PLCs are interconnected by dual redundant Local Area Network and exchange Data
& Control information via this medium. Each PLC id configured as Common Input
Output System. The operator interfaces with the system via Engineer’s console
located in the main control room. A PLC to COIS gateway has been provided for
transmission of hand switch status information to COIS for data logging. PLC system
has been configured as distributed system.
Classification of PLCs
There are total of 18 PLCs distributed geographically. There are 3 independent PLCs
network, one network of 8 PLCS for safety related groups (SR) and other 2 networks
having 4 and 6 PLCs in each for Non Safety Related Groups (NSR) i.e. NSR-1 and
NSR-2 respectively. The gateway is a node on the PLC LAN as well as CSMA/ CD
based LAN for COIS.
Safety related groups are further divided into 2 groups:
1. SR-CH_A/Y located in Control Equipment Room (CER) in Channel A room
2. SR-CH_B/Z located in Control Equipment Room (CER) in Channel B room
Non Safety Related Groups (NSR) are also divided as:
1. NSR CH-N located in Control Equipment Room (CER) in Channel N panel area
2. NSR CH-X located in Control Equipment Room (CER) in Channel X panel area
System Description
In TAPS 3 & 4 to avoid mixing signal of SR & NSR group loads, PLCs are divided
into SR-CH_A/Y, SR-CH_B/Z, NSR CH-N and NSR CH-X. SR has 8 PLCs and
NSR-1 & NSR-2 have 4 and 6 PLCs respectively. Both SR & NSR networks are
physically separated. Each group is interconnected using dual redundant LAN and
exchange data & information for alarm and annunciation via this medium using TBC

21. (Token Bus Controller) as the access protocol. The exchange of Global Data from
one PLC network to other is through hard wired connections.

22. Each PLC network is configured as dual redundant CPU and Common Input Output
System. The operation interface with the system is via Engineer’s Consoles,
individually provided for SR, NSR-1 & NSR-2 group of PLCs. The PLC COIS
gateway is provided for communication and logging PLC failure/ PLC power supply
failure annunciation for transmission of status of hand switches and pumps to COIS.

23. PLC Architecture
Input Output Sub-system
The PLC system comprises of 18 PLC units, each having 512 Input- Outputs (I/O).
The I/O Sub-system (Refer fig 4) is the PLC interface to plant process. It comprises
of the following:
 Field Input interface card
 Logic Output Card
 Field Output Interface Card
 I/O Bus Interface Card
 Logic input Card
 I/O Bus
Digital Inputs
Each PLC is having 22 field Input cards & each field Input card caters to 16 number
of inputs. Therefore the PLC System (consisting of 18 PLCs) capacity is a total of
6336 contact inputs. Number of inputs used in PLC system is about 3200 and the
remaining are spares.
Output of 2 field input cards are connected to one logic input card. Therefore one
logic input card caters to 32 digital inputs. The following are the types of input
contacts, which are wired to PLC:
 Handswitch contact
 Programmable Digital Comparator System (PDCS) contacts
 Relay contacts
 Motor Control Centre (MCC) contacts (48 & 42)
 Circuit Breaker (CB) contacts (86, 52 ax&bx)
 Contacts of level switch (LS), Pressure switch (PS), Temperature switch (TS) &
Limit switch (GS)
 Emergency Transfer (EMTR) System contacts
 Output contacts from other PLCs
Field isolation of 1500V DC is provided for each input by use of opto-isolators.
Channel to channel isolation is provided by proper spacing of signal lines and
connectors. A separate 24DC field input power supply is provided in the PLC cabinet
which is also used for Finite Impulse Test (FIT). In each logic Input card first 4 inputs
are assigned for Global Communications (for annunciation information) and are
transmitted to other PLCs over LAN.
Digital Outputs
Each PLC node is having 10 field Output cards & each field output card caters to 16
number of relay outputs. Therefore the PLC system capacity is total of 2880 relay
outputs. Number of outputs used in PLC system is about 1500 and the remaining are
spares.
Each output is a card mounted Omron make G2R-2 electromagnetic relay having
two change over contacts. This relay is energized by the PLC system. One change

24. over contact of all output relays is brought to terminal block through interface module
for wiring. The other change over contact for wiring is used for self-diagnostics by the
PLC.
The isolation of PLC outputs from field is provided by this electromagnetic relay. The
supply for output relay mounted on output cards is drawn from internal power supply
of 24V, 5A (in redundant mode). The output contact rating is 2 Amp inductive load of
15m sec at 24V.
Another finder make electromechanical relay mounted on the terminal block (TB)
with indication is provided to further isolate the field output card from the faults in the
field. This relay is energized by the NO contact of the relay mounted on the field
output card. The supply for this relay also is drawn from internal power supply of
24V, 5A(in redundant mode). This relay is having one change over contact having
contact current rating of 6A. This relay on TB can be easily replaced if required
without affecting the other circuits.
The following are the devices driven by the output contacts:
 Motor Starters
 Interposing relays for CB closing (3C) & opening circuits (3T)
 Indicating lamps
 Solenoid valves
 Window & COIS annunciation
CPU Sub-System
The CPU Subsytem (Refer fig 4) of the PLC is designed to operate in dual redundant
Hot Standby Mode. Each CPU subsystem in a PLC comprises of the following:
a) CPU card with on board memory
b) VME I/O interface card
c) Watch Dog Timer Card (WDT)
d) TBC- VME card
e) Modem card
f) VME Bus
The CPU system is based on VME bus.
The CPU card is based on 32 bit microprocessor, Motorola 68020 operating at a
clock rate of 16MHz.
This card comprises of the following types of memory:
· 256Kbytes of UVEPROM memory for storing application software and PLC
executive software.
· 256Kbytes of CMOS RAM as scratch pad/Data memory

25. · 512 Kbytes of dual ported memory for communicating with CPU compatible
devices
During commissioning stages, CMOS RAM with battery backup is used under
administrative and password control. All changes are logged in a file. For final
running of plant, UVEPROM is used. UVEPROM is used for storing the system
logics and associated data. Two serial ports (RS 232C standard) and one parallel
port are provided. One serial port and one parallel port (provided for functional
diversity) are used for the exchange of control information between the redundant
CPUs. Another Serial Port is a spare port, which is used for monitoring the CPU for
diagnostic purposes.
VME I/O Board
The VME I/O interface card connects the I/O subsystem with control subsystem. This
card provides 44TTL I/O lines for CPU to interface with I/O bus signals in I/O
subsystem. This card sits on VME bus Motherboard. He data is transmitted/ received
on 50 core flat cable.
Token Bus Controller (TBC) - VME Card
The PLC interface to LAN is via the VME bus based TBC card, which occupies one
slot in CPU bin. The card is an 80186 microprocessor based intelligent network
controller card connected to the dual redundant LAN media through a MODEM Card.
The network controller used is M68824 Token Bus Controller. This card has a dual
ported RAM having capacity of 512Kbytes containing the buffers for data exchange
between host CPU (PLC) and CPU on TBC card. It implements IEEE 802.4
communication protocol for the network with bus topology and token passing access.
The wiring media used is coaxial cable.
MODEM Card
MODEM Card is an IEEE 802.4 compliant token bus MODEM card with dual
redundant media support, which can tolerate a single point failure on the physical
layer. MODEM CARD Comprises of two numbers of RELCOM piggy cards for two
media and one EPLD. MODEM card occupies a slot in VME back plane and support
a data rate of 10 Mbps. It receives a signal from TBC- VME card through a flat cable
and communicates to a dual redundant LAN media.
MODEM card functions as an intelligent auto line selector monitoring both the media,
concurrently detecting occurrence of fault in both of them, selecting the media to
receive data, passing on the received data to TBC VME card and presenting the
status information of both the media on its facia. In addition physical management
commands can be issued from TBC VME card like configuring for single media
operation, putting either of the channels in internal loop back for diagnostics and
disabling the transmitters of either of the channel.
Watch Dog Timer Card
The Watch Dog Timer is Double Euro VME Compatible Board. The Watch Dog
Timer is used for failure detection of CPU. It switches off the I/O subsystem output

26. field relay supply during PLC fault condition leading to de-energisation of output
relays.
During PLC operation the CPU generates pulse trains at the input of retriggerable
monoshots present in the WDT. The monoshots output remains high only if the time
period of the generated pulse is less than the time constant of monoshots. Otherwise
the monoshots will change into low state and keeping relay contacts closed thereby
grounding the remote shutdown points of I/O system power supply.
Each CPU subsystem has got one WDT. Each WDT has got 4 relays. Each relay
gas got one NO contact (During normal operation of CPU, relays are energized so
the contacts remain closed).
Out of 4 relays, 2 are redundant and are used for monitoring functioning of CPU
through monoshot and thereby remote shutdown of I/O system power supply (both
relay contacts are connected in series. Similarly both relay contacts of WDT of
redundant CPU are also connected in parallel).
One relay is used to monitor whether CPU is gone out of network and its contact in
series with the similar relay contact of redundant CPU is connected to the coil of an
1minute OFF delay timer (RC relay). Under PLC working condition these contacts
will be closed and RC relay remain energized. Whenever any one of the CPU
system goes out of network for continuously for more than one min RC relay de-
energises. First NO contact of this relay in series with the first output of PLC (which
represents PLC healthiness) is used to energize another relay RA. First NO contact
of RA is used to provide ‘PLC XX* TROUBLE* alarm in WAN/COIS. Second NO
contact of RC is used to provide ‘PLX XX*TROUBLE’ alarm in COIS.
Fourth relay of WDT is spare.
*XX represents PLC Number

27. Power Supply Scheme
Refer fig5 for typical power supply arrangement for PLCs. Each PLC cabinet is fed
through two 380V DC buses from intermediate DC output of separate Uninterrupted
Power Supplies (UPS). 380 V DC battery bank of the UPS provides backup for four
hours in case of class 3 fails.
The low voltage DC Power supply is needed for the operation of the various cards
/modules within the PLC is derived from the above DC buses (which are dual
redundant) using in built power supply modules. These power supply modules are
basically DC to DC converters. Independent power supply modules have been
provided for the CPU subsystem and I/O subsystem of each PLC. The I/O power
supply comprises of Dual redundant power supplies of 5V, 25V Amps (with Auxiliary
+/- 14V, 0.5 Amps) and 24V, 5Amps operating in Hot Standby Mode.

28. Engineering Console (EC)
The operator’s interface to the PLC system is through the EC. The block diagram of
EC is shown in fig 6.
It comprises of the following hardware:
1. Motorola 68020 microprocessor card with 256KB EPROM and 512KB RAM with
battery backup.
2. VME Bus
3. TBC VME card
4. MODEM Card
5. Dell make Industrial PC having 19’’ color monitor & CD R/W drive without floppy disk
drive
6. 300 CPS dot matrix printer
The processor card, TBC card and MODEM are identical to that used in PLCs. Three
ECs are provided. The ECs for SR network, NSR Network-1 & NSR network-2 are
mounted on OIC-1,2& 3 in control room respectively.
All functions from EC are done through a menu driven structure of screens having a
uniform screen layout. There are 3 exclusive areas on the screen layout: the
message area, Ladder Graphic Language (LGL) statement area and logic area.
System prompts for use actions, user commands and system messages are
displayed in the message area. The LGL statement area displays the LGL
statements which define the application programs. The application program in the
form of LGL is displayed in the logic area.
Some of the important functions of ECs are given below:
 Configuring PLC
 Configuring plant I/O
 Offline program generation of user logic
 I/O status monitoring
 I/O forcing
 Documentation
 Back up of user logic
 Display of PLC diagnostic messages
 Network nodes & media status display
 Online retrieval of user logic from PLC nodes for editing/ monitoring/ maintaining
 Hot version
 Version Display

30. PLC Gateway
The diagram of PLC gateway is shown in fig 7. The PLC gateways are provided for
the purpose of transferring handswitch & pump status information to COIS for
logging. PLC alarms are also logged in COIS. There are 3 PLC gateways, one for
each network. There are TBC VME card and MODEM card for interfacing to PLC
LAN and two redundant LAN Controller for Ethernet (LANCE) cards for interfacing to
redundant switches of COIS LAN. This unit serves only as a protocol translator
between the two LANs.
The status changes are reported within a period from one second from their
occurrence. The timing of the events however is maintained as per real time clock.
The system has been designed for a maximum load of 20 contact status changes in
one second.

31. PLC System Grounding Scheme
All the safety ground points including PLC chassis are connected to the grounding
bus bar provided in the PLC cabinets and finally connected to station ground. Shield
of LAN cable is connected to copper bus bars provided on top of all PLC cabinets in
each row using 80/0.2 sq.mm wires and finally connected to station ground using 1C,
16 sq.mm cable. The signal ground in PLC system is floating.
PLC System Functional Description
Application Programming
The PLC system provides various features to meet the application requirements.
These include facilities to create, edit and debug programs. Facilities to load and
retrieve application program to/from PLC are provided. All actions required for
application programming are to be carried out from the EC. Back-up of user
programs can be kept on a secondary storage device such as a hard disk or CD-
R/W drive provided on the EC. Hard copy of the user programs in the form of ladder
diagrams, the list of inputs and outputs and corresponding LGL statements can be
obtained on the printer. Editing of application programs can be performed offline and
online.
Application Program Execution
The functions under this category are internal to the PLC and are through application
programs created using Ladder Graphic Language (LGL). The functions that are
supported include: On-delay timer, Off-delay timer, Pulse delay timer, Up/Down
counter. Of the above features, only the On-delay and Off-delay timers have actually
been used in our application. The On-delay and Off-delay timers are functionally
equivalent to the slow to operate and slow to release type of relays respectively. The
timers have a preset delay range of 0.1s to 999.9s with a delay setting resolution of
0.1s. The accuracy is ±1% of the preset delay or 100ms, whichever is more. For the
counter, the maximum preset count value is 9999.
Monitoring and Maintenance
The PLC system provides facilities to monitor application programs, inputs and
outputs on the EC. The display can be chosen to be either the ladder rung being
monitored or the I/O table showing the statues of inputs and outputs. The PLC
system also allows the user to force inputs/outputs/memory coils from the EC
through menu driven procedures. The above tools are useful for debugging and
maintenance of application programs. The forcing facility can also be used as a
manual override.
Stand-Alone Functions
The stand-alone functions performed by the EC of PLC system comprise of
UVEPROM programming, taking user backups of application programs and printing
of application program and configuration of data (inputs, outputs ad memory only).

32. Normally EC is in online monitoring mode. For these functions, the EC has to be
taken offline (it however continues to remain on the network and ready to receive all
fault messages).
Operating of CPU Sub-Systems
The 2 CPU sub-systems of a PLC operate in the Dual Redundant, Hot Standby
mode of operation. The ‘Active’ and ‘Standby’ CPUs perform identical tasks in their
respective scan cycles except that the ‘Standby’ CPU does not actuate the physical
outputs. It however, does generate the output image in memory by solving the logic.
The 2 CPU sub-systems coordinate their activities by exchanging information over
the Inter CPU Communication (ICC) lines.
PLC Startup
When the power is switched on, the CPU sub-system with the lower node address
gets control by hardware means and starts executing in ‘Non-Redundant’ mode. The
CPU sub-system with the higher node address starts after a few seconds and
initiates recovery action from the other CPU. Once the recovery is complete the CPU
enters into ‘Standby’ mode and the other CPU switches over to the ‘Active’ mode.
The PLC now operates in ‘Redundant’ mode.
Failure of ‘Active’ CPU Sub-System
When the PLC system is operating in ‘Redundant’ mode, any failure in the ‘Active’
CPU sub-system transfers the controls to the ‘Standby’ CPU sub-system
automatically without operator intervention. The switchover is ‘bumpless’ and does
not affect the outputs. Following the switchover, the ‘Standby’ CPU operates in ‘Non-
Redundant’ mode and a fault message is conveyed to the EC and the Printer.
Failure of ‘Standby’ CPU Sub-System
This results in a switch-over of the ‘Active’ CPU sub-system into the ‘Non-
Redundant’ mode of operation without operator intervention. Fault message will
appear on EC monitor and printer.
Recovery from ‘Non-Redundant’ to ‘Redundant’ Mode
The state is initiated just after the second CPU of PLC starts up after manual reset.
The complete recovery process takes about 3-5 minutes. After recovery is complete,
the CPU sub-system which was on ‘Non-Redundant’ mode switches to ‘Active’ mode
and the recovered CPU enters in ‘Standby’ mode. Message for ‘Non-Redundant’ to
‘Redundant’ operation will appear on EC monitor and printer.
Response Time
Response time is the time taken for an output to change status following a change in
any one of the inputs controlling the output. It includes the time taken to scan inputs,
process application logic and set output (include output relay operate time).
Two response times are specified. The local response time has a worst case value
of 100ms and is within limits as requires by the process. The average value is

33. around 80ms. The global response time which involves inter-PLC communication
has a worst case value of 500ms.
PLC System Software
PLC Executive Software
The PLC system (PLC and EC) uses the real time operating system pSOS. The
services used are those for process management, memory management,
communication and synchronisation between processes, time management and
exception management. Process management services are used to span, activate,
suspend, resume, signal and delete processes. Memory management services are
used to acquire and return memory buffers. Exception management services are
used to affect process scheduling after occurrence of exceptions. pHILE, a
companion software of pSOS, is used in the EC for file management functions.
Application Software
At the highest level the PLC software is divided into the PLC sub-system software,
EC sub-system software and the LAN sub-system software based on the type of
node on which it runs. The PLC sub-system software based on the type of node on
which it runs. The PLC sub-system will run on all PC nodes (i.e. on the host CPU of
the PLC), EC sub-system software on the EC node and LAN sub-system software on
the network controller card of PLC (i.e. on the CPU of the TBC-VME card).
The PLC sub-system module is further divided into the STARTUP module, the SCAN
CYCLE module and the INTERRUPTS AND EXCEPTIONS module. The startup
module is designed to cater to software initialization. The interrupts and exceptions
module caters to all interrupts and exceptions, which need to be processed
immediately. The scan cycle module is the main software block that is designed to
accomplish the chief function of reading inputs, solving logic and wiring outputs. This
module is continuously running unless interrupted. The activities are designed by
half a cycle for the ‘Active’ and ‘Standby’ CPU.
Programming Language for PLC
The programming language developed for programming the PLC is known as
LADDER GRAPHIC LANGUAGE (LGL). LGL is ideally suited for expressing logic
involving contacts. All statements have a graphical representation, which is similar in
form and appearance to the logic expressed using relay contacts. The statements
are organised into RUNGS. Each rung sets either an input coil or a memory coil. A
memory coil is a temporary storage area for storing the intermediate result of some
logic. The output energizes the physical output of the PLC. Every rung has a name
associated with it by which it is referred/recalled. The name is the number assigned
to the output coil or memory coil, which is energized by that rung. There is also an
upper limit of 20 statements per rung and 384 rungs per PLC. The following are the
valid LGL statements:
The entire logic is expressed and evaluated in ‘Reverse Polish’ notation, i.e. the two
operands are fetched first and then the logical operator is fetched and applied on the
operands. It is possible to print the ladder rungs with the printer.

34. Security
To prevent any inadvertent use of PLC system the following protection is provided:
Hardware Protection
Key and lock switch is provided on the EC as a hardware protection. This has the
highest priority for access.
Software Protection
Multilevel passwords are provided to meet various operation and maintenance
requirement.
At the highest level a master password is provided to permit change of lower level
passwords.
At the second level two passwords are provided. One password is for operation
engineers for checking the output read back error and for forcing of Input and
Output. The other password is for maintenance engineers for editing rung, entry of
rung, module isolation/insertion (Hot Repair) and for taking back up etc. There is one
more password for the maintenance personnel for enabling loading of application
software.
Monitoring function is EC is permitted without any password.
User Command Logging with Personnel Identification
In EC, during power on sequence, it validates data in the CPU memory and reads
data file from hard disk. If it fails in getting valid data then it prompts the user to enter
Administrator password and ‘LOAD PC’ password. It registers administrator by giving
‘ADMIN’ as Administrator User ID and category as ‘ADMN’. Hereinafter the user is
called the System Administrator.
Here Administrator can login with ‘ADMIN’ as User ID and with a password. He can
issue all commands as per his privileges.
An administrator can add a maximum of 20 users. These 20 users can have their
user name description of maximum 30 characters, user ID of maximum 6 characters,
Category (Main/Oper) and password.
Administrator Privileges
Administrator can
 Add new user
 Delete an existing user
 Change his own password and also of other users irrespective of their passwords
 Display user list
 Save user data to the hard disk
 Execute all EC commands

35. User Privileges
Maintenance User Privileges:
 Change his password
 Login for EC commands
 Display command logs
 Print log
 Execute online/offline commands such as Ladder:Entry, Insert, Delete, Hot
Repair, Load_PC, Back-Up, Force, Media Selection, CHK I/P TST 0/TST 1.
Operator User Privileges:
 Change his password
 Login for EC commands
 Display command logs
 Print log
 Execute online commands such as Ladder Recall, I/O Xreff, Force, O/P
Reedback etc.
When these registered users execute the privileged commands, it is logged into the
command log queue with the following information
 Date
 Time
 User ID
 Command executed
Time Synchronisation with Central Clock
One PLC input in each network is configured for accepting timing signal from plant
master clock and one output in the same PLC is identified for the communicating to
EC. Timing signal is provided as potential free contact from the central clock, which
can close for a duration of 500ms at half past hour. The trailing edge of the pulse will
coincide with HH:30:00 hours. When a sync pulse is received for a period of 500ms
the trailing edge of the pulse is detected, the output contact is closed and a message
is sent to the EC for setting the time to HH:30:00. The central clock will be showing
the Indian Standard Time (IST). The accuracy of the internal real time clock of the
PLC is better than 1 second per hour. The real time clock input and EC is connected
to PLC as follows.
When a sync pulse is not received for a period of 1 hour and 2 seconds the output
contact is opened and a message is sent to EC that sync pulse is not received. The
same information is also logged in the COIS.
During system start up, the output contact is closed, a message is sent to the EC
that the sync pulse is not received and timer is initialized.
Manual Restart of PLC
Manual reset facility for PLC is available through push buttons PB2 of CPU in ‘Non
Redundant’ mode. The PLC will identify the status of push button PB2 ad issue the

36. PLC restart command in ‘Non Redundant’ PLC. During this process, all outputs will
remain in (n-1) state.
Automatic Restart of PLC in case of Network Faults
The PLC is able to identify the following types of network faults occurring on the
LAN. These faults shall be collectively known as network faults.
· PLC node out of network
· SME errors
When the PLC is in ‘Redundant’ mode, and a network fault occurs, the
corresponding CPU shall halt and let the other CPU operate so that the PLC
functions in ‘Non Redundant’ (NONR) mode.
When the PLC is in NONR mode and a network fault occurs, the PLC PCU shall
initiate a process automatically to restart the PLC without affecting the current
outputs.
Initially upon Power On, the PLC is operating in ‘S0’ state. If any of the faults occur,
then the PLC annunciates the fault. Simultaneously, it issues a soft reset to itself and
enters the ‘S1’ state.
While in ‘S1’ state if no fault occurs in this state for a maximum of T1 period, the PLC
declares itself as healthy and enters the ‘S0’ state. If any error is detected, it again
annunciates the fault and enters the ‘S2’ state.
The PLC shall again verify the status of the network. If no fault is detected for a
maximum of T2 period, then the PLC enters back into the ‘S1’ state. If any error is
detected, it again issues a soft reset to it and enters the ‘S3’ state.
The PLC shall again verify the status of the network. If no fault is detected for a
maximum of T3 period, the PLC enters the ‘S0’ state. If any error is detected, it again
issues a soft reset to it and enters the ‘S4’ state.
While in the ‘S4’ state, if no fault occurs for a maximum of T4 period, the PLC
declares itself as healthy and enters the ‘S3’ state. If any error is detected, it again
annunciates the fault and enters the isolation state ‘S5’. If no fault is detected for a
maximum of T5 period, the PLC enters the S0 state. Otherwise, the PLC gets
isolated from the network and does not receive or transmit any data over the LAN.
Only a hard reset will bring the PLC into normal state ‘S0’.
The core functions shall be performed in all the states. The transition of the PLC
from one state to another shall not affect the PLC outputs.

37. LAN - Need for interconnection of PLCs
In an industry many PLCs are used due to large number of inputs and outputs. An
input coming into one PLC may be logically responsible for the output connected to
some other PLC. In such cases these two PLCs should be connected for successful
application of the logic.
In TAPS 3&4, there are a total of 18 PLCs interlinked with each other. These PLCs
collect inputs from various level switches, pressure switches etc, and through a logic
programmed in PLC it gives the desired output. Ladder Graphic Language(LGL) is
used for programming a PLC
For example, In Calendria Vault Cooling System (CVCS), there are three pumps.
These three pumps maintain a particular of flow of Moderator(D2O) through the
calendria vault. In normal conditions only two pumps are required to maintain the
flow. But if the flow decreases below a set point valve, the flow sensors give an input
to PDCS which gives a contact input to PLC-3. Sensing the input and by following
the programmed logic, the PLC-3 gives a contact signal to PLC-4, where the control
for the third pump is connected. Here at PLC-4 again the programmed logic is
followed and an output to start the third pump is given.

38. OSI Model
The Open Systems Interconnection model (OSI) is a conceptual model that
characterizes and standardizes the internalfunctions of a communication system by
partitioning it into abstraction layers. The model is a product of the Open
SystemsInterconnection project at the International Organization for Standardization
(ISO), maintained by the identification ISO/IEC7498-1.
The model groups communication functions into seven logical layers. A layer serves
the layer above it and is served by the layerbelow it. For example, a layer that
provides error-free communications across a network provides the path needed by
applicationsabove it, while it calls the next lower layer to send and receive packets
that make up the contents of that path. Two instances atone layer are connected by
a horizontal connection on that layer.
OSI Model
Data Unit Layer Function
Host
Layers
Data
7.Application Network process to application
6.Presentation
Data representation, encryption and
encryption, convert machine dependent
data tomachine independent data
5.Session
Inter-host communication, managing
sessions between applications
Segments 4.Transport
Reliable delivery of packets between
points on a network.
Media
Layers
Packet/Datagram 3.Network
Addressing, routing and (not necessarily
reliable) delivery of datagrams between
pointson a network.
Bit/Frame 2.Data Link
A reliable direct point-to-point data
connection.
Bit 1.Physical
A (not necessarily reliable) direct point-
to-point data connection.

39. IEEE 802
IEEE 802 refers to a family of IEEE standards dealing with local area networks and
metropolitan areanetworks.
More specifically, the IEEE 802 standards are restricted to networks carrying
variable-size packets. (Bycontrast, in cell relay networks data is transmitted in short,
uniformly sized units called cells. Isochronousnetworks, where data is transmitted as
a steady stream of octets, or groups of octets, at regular time intervals,are also out of
the scope of this standard.) The number 802 was simply the next free number IEEE
couldassign,[1] though “802” is sometimes associated with the date the first meeting
was held — February 1980.
The services and protocols specified in IEEE 802 map to the lower two layers (Data
Link and Physical) of theseven-layer OSI networking reference model. In fact, IEEE
802 splits the OSI Data Link Layer into two sub layersnamed Logical Link Control
(LLC) and Media Access Control (MAC), so that the layers can be listedlike this:
 Data link layer
o LLC Sub layer
o MAC Sub layer
 Physical layer
The IEEE 802 family of standards is maintained by the IEEE 802 LAN/MAN
Standards Committee (LMSC).
The most widely used standards are for the Ethernet family, Token Ring, Wireless
LAN, Bridging and VirtualBridged LANs. An individual Working Group provides the
focus for each area.
IEEE 802.3 details the Ethernet standards, IEEE 802.4 details the Token Bus
standards while IEEE 802.5 defines the MAC layer for a Token Ring.

40. Token Ring
Token ring local area network (LAN) technology is a protocolwhich resides at the
data link layer (DLL) of the OSI model. It used aspecial three-byte frame called a
token that travels around the ring.Token-possession grants the possessor
permission to transmit on themedium. Token ring frames travel completely around
the loop.Initially used only in IBM computers, it was eventually standardizedwith
protocol IEEE 802.5.
The data transmission process goes as follows:
 Empty information frames are continuously circulated on the ring.
 When a computer has a message to send, it seizes the token. The computer
will then be able to send the frame.
 The frame is then examined by each successive workstation. The workstation
that identifies itself to be the destination for the message copies it from the
frame and changes the token back to 0.
 When the frame gets back to the originator, it sees that the token has been
changed to 0 and that the message has been copied and received. It removes
the message from the frame.
 The frame continues to circulate as an "empty" frame, ready to be taken by a
workstation when it has a message to send.
The token scheme can also be used with bus topology LANs.
Stations on a token ring LAN are logically organized in a ringtopology with data being
transmitted sequentially from one ring stationto the next with a control token
circulating around the ring controllingaccess. This token passing mechanism is
shared by ARCNET, tokenbus, 100VG-AnyLAN (802.12) and FDDI, and has
theoreticaladvantages over the stochastic CSMA/CD of Ethernet.
Physically, a token ring network is wired as a star, with 'MAUs' and arms out to each
station and the loop goingout-and-back through each.
Cabling is generally IBM "Type-1" shielded twisted pair, with unique hermaphroditic
connectors, commonlyreferred to as IBM data connectors in formal writing or
colloquially as Boy George connectors. Theconnectors have the disadvantage of
being quite bulky, requiring at least 3 x 3 cm panel space, and beingrelatively fragile.
Connectors at the computer were usually DE-9 female.
Initially (in 1985) token ring ran at 4 Mbit/s, but in 1989 IBM introduced the first 16
Mbit/s token ring productsand the 802.5 standard was extended to support this. In
1981, Apollo Computer introduced their proprietary12 Mbit/s Apollo token ring (ATR)
and Proteon introduced their 10 Mbit/s ProNet-10 token ring network in1984.
However, IBM token ring was not compatible with ATR or ProNet-10.
Each station passes or repeats the special token frame around the ring to its nearest
downstream neighbour.This token-passing process is used to arbitrate access to the
shared ring media. Stations that have data framesto transmit must first acquire the
token before they can transmit them. Token ring LANs normally use differential
Manchester encoding of bits on the LAN media.

41. IBM popularized the use of token ring LANs in the mid-1980s when it released its
IBM token ring architecturebased on active MAUs (Media Access Unit, not to be
confused with Medium Attachment Unit) and the IBMStructured Cabling System. The
Institute of Electrical and Electronics Engineers (IEEE) later standardized atoken ring
LAN system as IEEE 802.5. Although Token Ring runs on LLC, it includes Source
Routing [3] toforward packets beyond the local network.
Token ring LAN speeds of 4 Mbit/s and 16 Mbit/s were standardized by the IEEE
802.5 working group. Anincrease to 100 Mbit/s was standardized and marketed
during the wane of token ring's existence while a 1000Mbit/s speed was actually
approved in 2001, but no products were ever brought to market.
When token ring LANs were first introduced at 4 Mbit/s, there were widely circulated
claims that they weresuperior to Ethernet, but these claims were fiercely debated.
With the development of switched Ethernet and faster variants of Ethernet, token
ring architectures laggedbehind Ethernet, and the higher sales of Ethernet allowed
economies of scale which drove down prices further,and added a compelling price
advantage. Token Ring MAC hardware was more complex than Ethernet,requiring a
specialized processor and licensed MAC/LLC firmware for each interface. The
Ethernet MACincluded both the (simpler) firmware and the lower licensing cost in the
MAC chip. Token Ring interface partscost (using a Texas Instruments TMS380C16
MAC and PHY) was approximately 3x the cost of an Ethernetinterface using the Intel
82586 MAC and PHY. The lower cost of unshielded twisted pair (CAT3 cable)
wasalso significant, as the 10-BASE-T and 100-BASE-T signalling waveforms were
optimized for this media, whilethe Token Ring waveform with its sharp edges and
short risetimes caused EMI issues when used on unshieldedcables.
Token ring networks have since declined in usage and the standards activity has
since come to a standstill as100Mbit/s switched Ethernet has dominated the
LAN/layer 2 networking market.
Token Frame
When no station is transmitting a data frame, a special token frame circles the loop.
This special token frame isrepeated from station to station until arriving at a station
that needs to transmit data. When a station needs totransmit data, it converts the
token frame into a data frame for transmission. Once the receiving station receivesits
own data frame, it converts the frame back into a token. If a transmission error
occurs and no token frame,or more than one, is present, a special station referred to
as the active monitor detects the problem and removesand/or reinserts tokens as
necessary. On 4 Mbit/s token ring, only one token may circulate; on 16 Mbit/s
tokenring, there may be multiple tokens.
The special token frame consists of three bytes as described below (J and K are
special non-data characters,referred to as code violations).
Token priority
Token ring specifies an optional medium access scheme allowing a station with a
high-priority transmission torequest priority access to the token.8 priority levels, 0–7,
are used. When the station wishing to transmit receives a token or data frame with
apriority less than or equal to the station's requested priority, it sets the priority bits to
its desired priority. Thestation does not immediately transmit; the token circulates

42. around the medium until it returns to the station. Uponsending and receiving its own
data frame, the station downgrades the token priority back to the original priority.
Token ring frame format
A data token ring frame is an expanded version of the token frame that is used by
stations to transmit mediaaccess control (MAC) management frames or data frames
from upper layer protocols and applications.
Token Ring and IEEE 802.5 support two basic frame types: tokens and
data/command frames. Tokens are 3bytes in length and consist of a start delimiter,
an access control byte, and an end delimiter. Data/commandframes vary in size,
depending on the size of the Information field. Data frames carry information for
upper-layerprotocols, while command frames contain control information and have
no data for upper-layer protocols.
Active and standby monitors
Every station in a token ring network is either an active monitor (AM) or standby
monitor (SM) station.
However, there can be only one active monitor on a ring at a time. The active
monitor is chosen through anelection or monitor contention process.
The monitor contention process is initiated when
 a loss of signal on the ring is detected.
 an active monitor station is not detected by other stations on the ring.
 a particular timer on an end station expires such as the case when a station
hasn't seen a token frame inthe past 7 seconds.
When any of the above conditions take place and a station decides that a new
monitor is needed, it will transmita "claim token" frame, announcing that it wants to
become the new monitor. If that token returns to the sender, itis OK for it to become
the monitor. If some other station tries to become the monitor at the same time then
the station with the highest MAC address will win the election process. Every other
station becomes a standbymonitor. All stations must be capable of becoming an
active monitor station if necessary.
The active monitor performs a number of ring administration functions. The first
function is to operate as themaster clock for the ring in order to provide
synchronization of the signal for stations on the wire. Anotherfunction of the AM is to
insert a 24-bit delay into the ring, to ensure that there is always sufficient buffering in
the ring for the token to circulate. A third function for the AM is to ensure that exactly
one token circulateswhenever there is no frame being transmitted, and to detect a
broken ring. Lastly, the AM is responsible forremoving circulating frames from the
ring.
Token ring insertion process
Token ring stations must go through a 5-phase ring insertion process before being
allowed to participate in thering network. If any of these phases fail, the token ring
station will not insert into the ring and the token ringdriver may report an error.

43.  Phase 0 (Lobe Check) — A station first performs a lobe media check. A
station is wrapped at the MSAU and is able to send 2000 test frames down its
transmit pair which will loop back to its receive pair. The station checks to
ensure it can receive these frames without error.
 Phase 1 (Physical Insertion) — A station then sends a 5 volt signal to the
MSAU to open the relay.
 Phase 2 (Address Verification) — A station then transmits MAC frames with
its own MAC address in the destination address field of a token ring frame.
When the frame returns and if the Address Recognized (AR) and Frame
Copied (FC) bits in the frame-status are set to 0 (indicating that no other
station currently on the ring uses that address), the station must participate in
the periodic (every 7 seconds) ring poll process. This is where stations identify
themselves on the network as part of the MAC management functions.
 Phase 3 (Participation in ring poll) — A station learns the address of its
Nearest Active Upstream Neighbour (NAUN) and makes its address known to
its nearest downstream neighbour, leading to the creation of the ring map.
Station waits until it receives an AMP or SMP frame with the AR and FC bits
set to 0. When it does, the station flips both bits (AR and FC) to 1, if enough
resources are available, and queues an SMP frame for transmission. If no
such frames are received within 18 seconds, then the station reports a failure
to open and de-inserts from the ring. If the station successfully participates in
a ring poll, it proceeds into the final phase of insertion, request initialization.
 Phase 4 (Request Initialization) — finally a station sends out a special request
to a parameter server to obtain configuration information. This frame is sent to
a special functional address, typically a token ring bridge, which may hold
timer and ring number information the new station needs to know.
Token passing
In telecommunication, token passing is a channel access method where a signal
called a token is passedbetween nodes that authorizes the node to communicate.
The most well-known examples are token ring andARCNET.
Token passing schemes provide round-robin scheduling, and if the packets are
equally sized, the scheduling ismax-min fair. The advantage over contention based
channel access is that collisions are eliminated, and that thechannel bandwidth can
be fully utilized without idle time when demand is heavy. The disadvantage is that
evenwhen demand is light, a station wishing to transmit must wait for the token,
increasing latency.
Some types of token passing schemes do not need to explicitly send a token
between systems because theprocess of "passing the token" is implicit. An example
is the channel access method used during "ContentionFree Time Slots" in the ITU-T
G.hn standard for high-speed local area networking using existing home wires(power
lines, phone lines and coaxial cable).

44. Ethernet
Ethernet is a family of computer networking technologiesfor local area networks
(LANs). Ethernet was commerciallyintroduced in 1980 and standardized in 1983 as
IEEE 802.3.
Ethernet has largely replaced competing wired LAN technologiessuch as token ring,
FDDI, and ARCNET.
The Ethernet standards comprise several wiring and signalling variantsof the OSI
physical layer in use with Ethernet. The original 10BASE5Ethernet used coaxial
cable as a shared medium. Later the coaxialcables were replaced with twisted pair
and fiber optic links inconjunction with hubs or switches. Data rates were
periodicallyincreased from the original 10 megabits per second to 100 gigabitsper
second.
Systems communicating over Ethernet divide a stream of data into shorter pieces
called frames. Each framecontains source and destination addresses and error-
checking data so that damaged data can be detected andre-transmitted. As per the
OSI model, Ethernet provides services up to and including the data link layer.
Since its commercial release, Ethernet has retained a good degree of compatibility.
Features such as the 48-bitMAC address and Ethernet frame format have influenced
other networking protocols.
Shared media
Ethernet was originally based on the idea of computerscommunicating over a shared
coaxial cable acting as a broadcasttransmission medium. The methods used were
similar to those used inradio systems, with the common cable providing the
communicationchannel likened to the Luminiferous aether in 19th century
physics,and it was from this reference that the name "Ethernet" wasderived.
Original Ethernet's shared coaxial cable (the shared medium)traversed a building or
campus to every attached machine. A schemeknown as carrier sense multiple
access with collision detection(CSMA/CD) governed the way the computers shared
the channel.
This scheme was simpler than the competing token ring or token bustechnologies.[d]
Computers were connected to an Attachment UnitInterface (AUI) transceiver, which
was in turn connected to the cable(later with thin Ethernet the transceiver was
integrated into thenetwork adapter). While a simple passive wire was highly reliable
forsmall networks, it was not reliable for large extended networks,where damage to
the wire in a single place, or a single bad connector,could make the whole Ethernet
segment unusable.
Through the first half of the 1980s, Ethernet's 10BASE5implementation used a
coaxial cable 0.375 inches (9.5 mm) indiameter, later called "thick Ethernet" or
"thicknet". Its successor,10BASE2, called "thin Ethernet" or "thinnet", used a cable
similar tocable television cable of the era. The emphasis was on makinginstallation
of the cable easier and less costly.
Since all communications happen on the same wire, any informationsent by one
computer is received by all, even if that information isintended for just one

45. destination. The network interface cardinterrupts the CPU only when applicable
packets are received: Thecard ignores information not addressed to it. Use of a
single cablealso means that the bandwidth is shared, such that, for example,
available bandwidth to each device is halvedwhen two stations are simultaneously
active.
Collisions happen when two stations attempt to transmit at the same time. They
corrupt transmitted data and require stations to retransmit. The lost data and
retransmissions reduce throughput. In the worst case where multiple active hosts
connected with maximum allowed cable length attempt to transmit many short
frames, excessive collisions can reduce throughput dramatically. However, a Xerox
report in 1980 studied performance of an existing Ethernet installation under both
normal and artificially generated heavy load. The report claims that 98% throughput
on the LAN was observed. This is in contrast with token passing LANs (token ring,
token bus), all of which suffer throughput degradation as each new node comes into
the LAN, due to token waits. This report was controversial, as modeling showed that
collision-based networks theoretically became unstable under loads as low as 37%
of nominal capacity. Many early researchers failed to understand these results.
Performance on real networks is significantly better.
In a modern Ethernet, the stations do not all share one channel through a shared
cable or a simple repeater hub; instead, each station communicates with a switch,
which in turn forwards that traffic to the destination station. In this topology, collisions
are only possible if station and switch attempt to communicate with each other at the
same time, and collisions are limited to this link. Furthermore, the 10BASE-T
standard introduced a full duplexmode of operation which has become extremely
common. In full duplex, switch and station can communicatewith each other
simultaneously, and therefore modern Ethernets are completely collision-free.
Repeaters and hubs
For signal degradation and timing reasons, coaxial Ethernet segmentshad a
restricted size. Somewhat larger networks could be built byusing an Ethernet
repeater. Early repeaters had only two ports,allowing, at most, a doubling of network
size. Once repeaters withmore than two ports became available, it was possible to
wire thenetwork in a star topology. Early experiments with star topologies(called
"Fibernet") using optical fiber were published by 1978.
Shared cable Ethernet was always hard to install in offices because itsbus topology
was in conflict with the star topology cable plansdesigned into buildings for
telephony. Modifying Ethernet to conformto twisted pair telephone wiring already
installed in commercialbuildings provided another opportunity to lower costs, expand
theinstalled base, and leverage building design, and, thus, twisted-pair.
Ethernet was the next logical development in the mid-1980s.
Ethernet on unshielded twisted-pair cables (UTP) began with StarLAN at 1 Mbit/s in
the mid-1980s. In 1987SynOptics introduced the first twisted-pair Ethernet at 10
Mbit/s in a star-wired cabling topology with a centralhub, later called LattisNet. These
evolved into 10BASE-T, which was designed for point-to-pointlinks only, and all
termination was built into the device. This changed repeatersfrom a specialist device
used atthe center of large networks to a device that every twisted pair-based network
with more than two machineshad to use. The tree structure that resulted from this

46. made Ethernet networks easier to maintain by preventingmost faults with one peer or
its associated cable from affecting other devices on the network.
Despite the physical star topology and the presence of separate transmit and receive
channels in the twisted pairand fiber media, repeater based Ethernet networks still
use half-duplex and CSMA/CD, with only minimalactivity by the repeater, primarily
the Collision Enforcement signal, in dealing with packet collisions. Everypacket is
sent to every port on the repeater, so bandwidth and security problems are not
addressed. The totalthroughput of the repeater is limited to that of a single link, and
all links must operate at the same speed.
Bridging and switching
While repeaters could isolate some aspects of Ethernet segments, such as cable
breakages, they still forwardedall traffic to all Ethernet devices. This created practical
limits on how many machines could communicate on anEthernet network. The entire
network was one collision domain, and all hosts had to be able to detect
collisionsanywhere on the network. This limited the number of repeaters between the
farthest nodes. Segments joined byrepeaters had to all operate at the same speed,
making phased-in upgrades impossible.
To alleviate these problems, bridging was created to communicate at the data link
layer while isolating thephysical layer. With bridging, only well-formed Ethernet
packets are forwarded from one Ethernet segment toanother; collisions and packet
errors are isolated. At initial startup, Ethernet bridges (and switches) worksomewhat
like Ethernet repeaters, passing all traffic between segments. By observing the
source addresses ofincoming frames, the bridge then builds an address table
associating addresses to segments. Once an address islearned, the bridge forwards
network traffic destined for that addressonly to the associated segment, improving
overall performance.Broadcast traffic is still forwarded to all network segments.
Bridgesalso overcame the limits on total segments between two hosts andallowed
the mixing of speeds, both of which are critical to deploymentof Fast Ethernet.
In 1989, the networking company Kalpana introduced theirEtherSwitch, the first
Ethernet switch.[h] This worked somewhatdifferently from an Ethernet bridge, where
only the header of theincoming packet would be examined before it was either
dropped orforwarded to another segment. This greatly reduced the forwardinglatency
and the processing load on the network device. Onedrawback of this cut-through
switching method was that packets that had been corrupted would still bepropagated
through the network, so a jabbering station could continue to disrupt the entire
network. Theeventual remedy for this was a return to the original store and forward
approach of bridging, where the packetwould be read into a buffer on the switch in its
entirety, verified against its checksum and then forwarded, butusing more powerful
application-specific integrated circuits. Hence, the bridging is then done in
hardware,allowing packets to be forwarded at full wire speed.
When a twisted pair or fiber link segment is used and neither end is connected to a
repeater, full-duplex Ethernetbecomes possible over that segment. In full-duplex
mode, both devices can transmit and receive to and fromeach other at the same
time, and there is no collision domain. This doubles the aggregate bandwidth of the
linkand is sometimes advertised as double the link speed (for example, 200 Mbit/s).
The elimination of thecollision domain for these connections also means that all the

47. link’s bandwidth can be used by the two deviceson that segment and that segment
length is not limited by the need for correct collision detection.
Since packets are typically delivered only to the port they are intended for, traffic on
a switched Ethernet is lesspublic than on shared-medium Ethernet. Despite this,
switched Ethernet should still be regarded as an insecurenetwork technology,
because it is easy to subvert switched Ethernet systems by means such as ARP
spoofingand MAC flooding.
The bandwidth advantages, the improved isolation of devices from each other, the
ability to easily mix differentspeeds of devices and the elimination of the chaining
limits inherent in non-switched Ethernet have madeswitched Ethernet the dominant
network technology.

48. Token Ring vs Ethernet
Ethernet networks provide high speed data transfer at low cost as the network can
be set up using Twisted Cables instead of Coaxial cables without sacrificing the
basic signal strength.
The cost and availability of materials required in establishing the network is far lesser
than those required in Token Ring systems.
The fall back of Ethernet systems is that the system performance decreases
substantially when the load is increased. This is tackled well by the Token Ring
system which can handle network expansion without drop in performance of the
system.
The new systems designed with the Ethernet networking are tested at the ECIL
facilities, where the prototypes are tested and properly certified before they are
approved to be used in the field.

49. Media Fault and System Replacement
The Token Ring system transmits 1 clock cycle of 5Mhz for a logic 0 and 2 clock
cycles of 10Mhz for logic 1. The transmitter can send the signals at a strength of
maximum 70dBmV according to the IEEE-802.4(IEC-8802.4) standard.
The signals get attenuated at the terminators by around 20dBmV each resulting in
the signal strength at the receiver end to be reduced to 30dBmV. If the signal
strength drops below 16dBmV, the system annunciates a media fault.
To correct the loss of strength in transmission, several measures were taken
including:
 Checking connections
 Cleaning terminator contacts
 Replacing terminator contacts
 The strength of the signals were checked by placing a network analyser at
various nodes of the network.
The network analyser did not detect any discrepancies in the network. Yet, the
system displayed a media fault on the annunciation system.
After having tried various remedies, it has been decided to replace the existing
Token Ring system with an Ethernet system.

50. References
Websites
www.wikipedia.org -
Nuclear Power Plant
Tarapur Atomic Power Station
India’s three-stage nuclear power programme
www.google.com
www.cyberphysics.co.uk
www.conserve-energy-future.com
www.npcil.nic.in
www.ofnuclearenergy.com
Literature
CMD, NPCIL Statement 2013
Corporate Profile NPCIL 2012
NPCIL 26th Annual Report 2012-13