Distributed Control System

Sirte Oil Company

Training Department

Prepared by A. Barka

Chapter 1

DCS general overview

1.1 Distributed Control System

DCS (Distributed Control System) is a computerized

control system used to control the production line in the

industry. The entire system of controllers is connected by

networks for communication and monitoring.

A distributed control system (DCS) refers to a control

system usually of a manufacturing system, process or any

kind of dynamic system, in which the controller elements

are not central in location (like the brain) but are

distributed throughout the system with each component

sub-system controlled by one or more controllers.

DCS is a very broad term used in a variety of industries,

to monitor and control distributed equipment.

Electrical power grids and electrical generation plants

Environmental control systems

Traffic signals

Radio signals

Water management systems

Oil refining plants

Metallurgical process plants

Chemical plants

Pharmaceutical manufacturing

Sensor networks

1.2 Elements of DCS

A DCS typically uses custom designed processors as

controllers and uses both proprietary interconnections and

communications protocol for communication. Input and

output modules form component parts of the DCS. The

processor receives information from input modules and

sends information to output modules. The input modules

receive information from input instruments in the process

(or field) and transmit instructions to the output

instruments in the field. Computer buses or electrical

buses connect the processor and modules through

multiplexer or demultiplexers. Buses also connect the

distributed controllers with the central controller and finally

to the Human–machine interface (HMI) or control

consoles. See Process automation system.

The elements of a DCS may connect directly to

physical equipment such as switches, pumps and valves

or they may work through an intermediate system such as

a SCADA system.

1.3 Applications

Distributed control systems (DCSs) are dedicated

systems used to control manufacturing processes that are

continuous or batch-oriented, such as oil refining,

petrochemicals, central station power generation,

fertilizers, pharmaceuticals, food and beverage

manufacturing, cement production, steelmaking, and

papermaking. DCSs are connected to sensors and

actuators and use setpoint control to control the flow of

material through the plant. The most common example is

a setpoint control loop consisting of a pressure sensor,

controller, and control valve. Pressure or flow

measurements are transmitted to the controller, usually

through the aid of a signal conditioning input/output (I/O)

device. When the measured variable reaches a certain

point, the controller instructs a valve or actuation device to

open or close until the fluidic flow process reaches the

desired setpoint. Large oil refineries have many thousands

of I/O points and employ very large DCSs. Processes are

not limited to fluidic flow through pipes, however, and can

also include things like paper machines and their

associated quality controls (see quality control system

QCS), variable speed drives and motor control centers,

cement kilns, mining operations, ore processing facilities,

and many others.

A typical DCS consists of functionally and/or

geographically distributed digital controllers capable of

executing from 1 to 256 or more regulatory control loops in

one control box. 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

sequential control. Modern DCSs also support neural

networks and fuzzy application.

DCS systems are usually designed with redundant

processors to enhance the reliability of the control system.

Most systems come with canned displays and

configuration software which enables the end user to set

up the control system without a lot of low level

programming. This allows the user to better focus on the

application rather than the equipment, although a lot of

system knowledge and skill is still required to support the

hardware and software as well as the applications. Many

plants have dedicated groups that focus on this task.

These groups are in many cases augumented by vendor

support personnel and/or maintenance support contracts.

DCSs may employ one or more workstations and can

be configured at the workstation or by an off-line personal

computer. Local communication is handled by a control

network with transmission over twisted pair, coaxial, or

fiber optic cable. A server and/or applications processor

may be included in the system for extra computational,

data collection, and reporting capability.

1.4 History of DCS

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 points) and analog signals to the digital domain.

The first industrial control computer system was built 1959 at the Texaco Port Arthur, Texas, refinery with an RW-300 of the Ramo-Wooldridge Company

The DCS was introduced in 1975. Both Honeywell and Japanese electrical engineering firm Yokogawa introduced their own independently produced DCSs at roughly the same time, with the TDC 2000 and CENTUM systems, respectively. US-based Bristol also introduced their UCS 3000 universal controller in 1975. In 1978 Metso(known as Valmet in 1978) introduced their own DCS system called Damatic (latest generation named Metso DNA). In 1980, Bailey (now part of ABB) introduced the NETWORK 90 system. Also in 1980, Fischer & Porter Company (now also part of ABB) introduced DCI-4000 (DCI stands for Distributed Control Instrumentation).

The DCS largely came about due to the increased availability of microcomputers and the proliferation of microprocessors in the world of process control. Computers had already been applied to process automation for some time in the form of both direct digital control (DDC) and set point control. In the early 1970s Taylor Instrument Company, (now part of ABB) developed the 1010 system, Foxboro the FOX1 system and Bailey Controls the 1055 systems. All of these were DDC applications implemented within minicomputers (DEC PDP-11, Varian Data Machines, MODCOMP etc.) and connected to proprietary Input/Output hardware. Sophisticated (for the time) continuous as well as batch control was implemented in this way. A more conservative approach was set point control, where process computers supervised clusters of analog process controllers. A CRT-based workstation provided visibility into the process using text and crude 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 were self-contained "blocks" of code that emulated analog hardware control components and performed tasks that were essential to process control, such as execution of PID algorithms. Function blocks continue to endure as the predominant method of control for DCS suppliers, and are supported by key technologies such as Foundation Fieldbus today.

Midac Systems, of Sydney, Australia, developed an objected-oriented distributed direct digital control system in 1982. The central system ran 11 microprocessors sharing tasks and common memory and connected to a serial communication network of distributed controllers each running two Z80s. The system was installed at the University of Melbourne.

Digital communication between distributed controllers, workstations and other computing elements (peer to peer access) was one of the primary advantages of the DCS. Attention was duly focused on the networks, which provided the all-important lines of communication that, for process applications, had to incorporate specific functions such as determinism and redundancy. As a result, many suppliers embraced the IEEE 802.4 networking standard. This decision set the stage for the wave of migrations necessary when information technology moved into process automation and IEEE 802.3 rather than IEEE 802.4 prevailed as the control LAN.

1.4.1 The Network Centric Era of the 1980s

The DCS brought distributed intelligence to the plant and established the presence of computers and microprocessors in process control, but it still did not provide the reach and openness necessary to unify plant resource requirements. In many cases, the DCS was merely a digital replacement of the same functionality provided by analog controllers and a panelboard display. This was embodied in The Purdue Reference Model (PRM) that was developed to define Manufacturing Operations Management relationships. PRM later formed the basis for ISA-95 standards activities today.

In the 1980s, users began to look at DCSs as more than just basic process control. A very early example of a Direct Digital Control DCS was completed by the Australian business Midac in 1981–82 using R-Tec Australian designed hardware. The system installed at the University of Melbourne used a serial communications network, connecting campus buildings back to a control room "front end". Each remote unit ran 2 Z80 microprocessors whilst the front end ran 11 in a Parallel Processing configuration with paged common memory to share tasks and could run up to 20,000 concurrent controls objects.

It was believed that if openness could be achieved and greater amounts of data could be shared throughout the enterprise that even greater things could be achieved. The first attempts to increase the openness of DCSs resulted in the adoption of the predominant operating system of the day: UNIX. UNIX and its companion networking technology TCP-IP were developed by the US Department of Defense for openness, which was precisely the issue the process industries were looking to resolve.

As a result suppliers also began to adopt Ethernet-based networks with their own proprietary protocol layers. The full TCP/IP standard was not implemented, but the use of Ethernet made it possible to implement the first instances of object management and global data access technology. The 1980s also witnessed the first PLCs integrated into the DCS infrastructure. Plant-wide historians also emerged to capitalize on the extended reach of automation systems. The first DCS supplier to adopt UNIX and Ethernet networking technologies was Foxboro, who introduced the I/A Series system in 1987.

1.5 (HMI) Computer

A computer is a general purpose device that can be programmed to carry out a finite set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem.

Conventionally, a computer consists of at least one processing element, typically a central processing unit (CPU) and some form of memory. The processing element carries out arithmetic and logic operations, and a sequencing and control unit that can change the order of operations based on stored information. Peripheral devices allow information to be retrieved from an external source, and the result of operations saved and retrieved.

The first electronic digital computers were developed between 1940 and 1945 in the United Kingdom and

United States. Originally they were the size of a large room, consuming as much power as several hundred modern personal computers (PCs). In this era mechanical analog computers were used for military applications.

Modern computers based on integrated circuits are millions to billions of times more capable than the early machines, and occupy a fraction of the space. Simple computers are small enough to fit into mobile devices, and mobile computers can be powered by small batteries. Personal computers in their various forms are icons of the Information Age and are what most people think of as "computers". However, the embedded computers found in many devices from mp3 players to fighter aircraft and from toys to industrial robots are the most numerous.

1.5.1 Components

A. Hardware

A general purpose computer has four main components: the arithmetic logic unit (ALU), the control unit, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by busses, often made of groups of wires.

Inside each of these parts are thousands to trillions of small electrical circuits which can be turned off or on by means of an electronic switch. Each circuit represents a bit (binary digit) of information so that when the circuit is on it represents a "1", and when off it represents a "0" (in positive logic representation). The circuits are arranged in logic gates so that one or more of the circuits may control the state of one or more of the other circuits.

The control unit, ALU, registers, and basic I/O (and often other hardware closely linked with these) are collectively known as a central processing unit (CPU). Early CPUs were composed of many separate components but since

the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.

B. Software

Computer software, or just software, is a collection of computer programs and related data that provides the instructions for telling a computer what to do and how to do it. Software refers to one or more computer programs and data held in the storage of the computer. In other words, software is a set of programs, procedures, algorithms and its documentation concerned with the operation of a data processing system. Program software performs the function of the program it implements, either by directly providing instructions to the digital electronics or by serving as input to another piece of software. The term was coined to contrast to the old term hardware (meaning physical devices). In contrast to hardware, software "cannot be touched". Software is also sometimes used in a more narrow sense, meaning application software only. Sometimes the term includes data that has not traditionally been associated with computers, such as film, tapes, and records. computer software is so called to distinguish it from computer hardware, which encompasses the physical interconnections and devices required to store and execute (or run) the software. At the lowest level, executable code consists of machine language instructions specific to an individual processor. A machine language consists of groups of binary values signifying processor instructions that change the state of the computer from its preceding state. Programs are an ordered sequence of instructions for changing the state of the computer in a particular sequence. It is usually written in high-level programming languages that are easier and more efficient for humans to use (closer to natural language) than machine language. High-level

languages are compiled or interpreted into machine language object code. Software may also be written in an assembly language, essentially, a mnemonic representation of a machine language using a natural language alphabet. Assembly language must be assembled into object code via an assembler.

Types of software

User

Application

Operating System

Hardeware

1.6 Binary Numeral System

The binary numeral system, or base-2 number system, represents numeric values using two symbols: 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2. Because of its straightforward implementation in digital electronic circuitry using logic gates, the binary system is used internally by almost all modern computers and computer-based devices such as mobile phones.

1.6.1 Counting

Counting in binary is similar to counting in any other number system. Beginning with a single digit, counting proceeds through each symbol, in increasing order. Decimal counting uses the symbols 0 through 9, while binary only uses the symbols 0 and 1.

When the symbols for the first digit are exhausted, the next-higher digit (to the left) is incremented, and counting starts over at 0. In decimal, counting proceeds like so:

000, 001, 002, ... 007, 008, 009, (rightmost digit starts over, and next digit is incremented)

010, 011, 012, ...

090, 091, 092, ... 097, 098, 099, (rightmost two digits start over, and next digit is incremented)

100, 101, 102, ...

After a digit reaches 9, an increment resets it to 0 but also causes an increment of the next digit to the left. In binary, counting is the same except that only the two symbols 0 and 1 are used. Thus after a digit reaches 1 in binary, an increment resets it to 0 but also causes an increment of the next digit to the left:

0000,0001, (rightmost digit starts over, and next digit is incremented) 0010, 0011, (rightmost two digits start over, and next digit is incremented) 0100, 0101, 0110, 0111, (rightmost three digits start over, and the next digit is incremented) 1000, 1001, ...

Since binary is a base-2 system, each digit represents an increasing power of 2, with the rightmost digit representing 2

0, the next representing 2

1, then 2

2, and so on. To

determine the decimal representation of a binary number simply take the sum of the products of the binary digits and the powers of 2 which they represent. For example, the binary number:

100101 is converted to decimal form by:

[(1) × 25] + [(0) × 2

4] + [(0) × 2

3] + [(1) × 2

2] + [(0) × 2

1] +

[(1) × 20] =[1 × 32] + [0 × 16] + [0 × 8] + [1 × 4] + [0 × 2] +

[1 × 1] = 37

To create higher numbers, additional digits are simply added to the left side of the binary representation.

1.6.2 Fractions in binary

Fractions in binary only terminate if the denominator has 2 as the only prime factor. As a result, 1/10 does not have a finite binary representation, and this causes 10 × 0.1 not to be precisely equal to 1 in floating point arithmetic. As an example, to interpret the binary expression for 1/3 = .010101..., this means: 1/3 = 0 × 2

−1 +

1 × 2−2

+ 0 × 2−3

+ 1 × 2−4

+ ... = 0.3125 + ... An exact value cannot be found with a sum of a finite number of inverse powers of two, and zeros and ones alternate forever.

1.6.3 Conversion to and from other numeral systems

A. Decimal

To convert from a base-10 integer numeral to its base-2 (binary) equivalent, the number is divided by two, and the remainder is the least-significant bit. The (integer) result is again divided by two, its remainder is the next least significant bit. This process repeats until the quotient becomes zero.

Conversion from base-2 to base-10 proceeds by applying the preceding algorithm, so to speak, in reverse. The bits of the binary number are used one by one, starting with the most significant (leftmost) bit. Beginning with the value 0, repeatedly double the prior value and add the next bit to produce the next value. This can be organized in a multi-column table. For example to convert 100101011012 to decimal:

Binary 1 0 0 1 0 1 0 1 1 0 1

Decimal 1×210

+

0×29

+

0×28

+

1×27

+

0×26

+

1×25

+

0×24

+

1×23

+

1×22

+

0×21

+

1×20

= 1197

B. Hexadecimal

0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

Binary may be converted to and from hexadecimal somewhat more easily. This is because the radix of the hexadecimal system (16) is a power of the radix of the binary system (2). More specifically, 16 = 2

4, so it takes

four digits of binary to represent one digit of hexadecimal, as shown in the table to the right.

To convert a hexadecimal number into its binary equivalent, simply substitute the corresponding binary digits:

3A16 = 0011 10102

E716 = 1110 01112

To convert a binary number into its hexadecimal equivalent, divide it into groups of four bits. If the number of bits isn't a multiple of four, simply insert extra 0 bits at the left (called padding). For example:

10100102 = 0101 0010 grouped with padding = 5216

110111012 = 1101 1101 grouped = DD16

To convert a hexadecimal number into its decimal equivalent, multiply the decimal equivalent of each hexadecimal digit by the corresponding power of 16 and add the resulting values:

C0E716 = (12 × 163) + (0 × 16

2) + (14 × 16

1) + (7 ×

160) = (12 × 4096) + (0 × 256) + (14 × 16) + (7 × 1) =

49,38310

1.6.4 Binary Coded Decimal BCD

In computing and electronic systems, binary-coded decimal (BCD) is a class of binary encodings of decimal numbers where each decimal digit is represented by a fixed number of bits, usually four or eight, although other sizes (such as six bits) have been used historically. Special bit patterns are sometimes used for a sign or for other indications (e.g., error or overflow).

In byte-oriented systems (i.e. most modern computers), the term uncompressed BCD usually implies a full byte for each digit (often including a sign), whereas packed BCD typically encodes two decimal digits within a single byte by taking advantage of the fact that four bits are enough to represent the range 0 to 9. The precise 4-bit encoding

may vary however, for technical reasons, see Excess-3 for instance.

BCD's main virtue is a more accurate representation and rounding of decimal quantities as well as an ease of conversion into human-readable representations. As compared to binary positional systems, BCD's principal drawbacks are a small increase in the complexity of the circuits needed to implement basic arithmetics and a slightly less dense storage.

BCD was used in many early decimal computers. Although BCD is not as widely used as in the past, decimal fixed-point and floating-point formats are still important and continue to be used in financial, commercial, and industrial computing, where subtle conversion and rounding errors that are inherent to floating point binary representations cannot be tolerated.

1.6.5 ASCII

The American Standard Code for Information Interchange (ASCII, pronunciation: /ˈæski/ ASS-kee;) is a character-encoding scheme originally based on the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many additional characters.

Binary Oct Dec Hex Glyph

010 0000 040 32 20 ␠

010 0001 041 33 21 !

010 0010 042 34 22 "

010 0011 043 35 23 #

010 0100 044 36 24 $

010 0101 045 37 25 %

010 0110 046 38 26 &

010 0111 047 39 27 '

010 1000 050 40 28 (

010 1001 051 41 29 )

010 1010 052 42 2A *

010 1011 053 43 2B +

010 1100 054 44 2C ,

010 1101 055 45 2D -

010 1110 056 46 2E .

010 1111 057 47 2F /

011 0000 060 48 30 0

Binary Oct Dec Hex Glyph

100 0000 100 64 40 @

100 0001 101 65 41 A

100 0010 102 66 42 B

100 0011 103 67 43 C

100 0100 104 68 44 D

100 0101 105 69 45 E

100 0110 106 70 46 F

100 0111 107 71 47 G

100 1000 110 72 48 H

100 1001 111 73 49 I

100 1010 112 74 4A J

100 1011 113 75 4B K

100 1100 114 76 4C L

100 1101 115 77 4D M

100 1110 116 78 4E N

100 1111 117 79 4F O

101 0000 120 80 50 P

101 0001 121 81 51 Q

Binary Oct Dec Hex Glyph

110 0000 140 96 60 ̀

110 0001 141 97 61 a

110 0010 142 98 62 b

110 0011 143 99 63 c

110 0100 144 100 64 d

110 0101 145 101 65 e

110 0110 146 102 66 f

110 0111 147 103 67 g

110 1000 150 104 68 h

110 1001 151 105 69 i

110 1010 152 106 6A j

110 1011 153 107 6B k

110 1100 154 108 6C l

110 1101 155 109 6D m

110 1110 156 110 6E n

110 1111 157 111 6F o

111 0000 160 112 70 p

111 0001 161 113 71 q

011 0001 061 49 31 1

011 0010 062 50 32 2

011 0011 063 51 33 3

011 0100 064 52 34 4

011 0101 065 53 35 5

011 0110 066 54 36 6

011 0111 067 55 37 7

011 1000 070 56 38 8

011 1001 071 57 39 9

011 1010 072 58 3A :

011 1011 073 59 3B ;

011 1100 074 60 3C <

011 1101 075 61 3D =

011 1110 076 62 3E >

011 1111 077 63 3F ?

101 0010 122 82 52 R

101 0011 123 83 53 S

101 0100 124 84 54 T

101 0101 125 85 55 U

101 0110 126 86 56 V

101 0111 127 87 57 W

101 1000 130 88 58 X

101 1001 131 89 59 Y

101 1010 132 90 5A Z

101 1011 133 91 5B [

101 1100 134 92 5C \

101 1101 135 93 5D ]

101 1110 136 94 5E ̂

101 1111 137 95 5F _

111 0010 162 114 72 r

111 0011 163 115 73 s

111 0100 164 116 74 t

111 0101 165 117 75 u

111 0110 166 118 76 v

111 0111 167 119 77 w

111 1000 170 120 78 x

111 1001 171 121 79 y

111 1010 172 122 7A z

111 1011 173 123 7B {

111 1100 174 124 7C |

111 1101 175 125 7D }

111 1110 176 126 7E ~

1.7 Logic Gates

A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device (see Ideal and real op-amps for comparison).

Logic gates are primarily implemented using diodes or transistors acting as electronic switches, but can also be constructed using electromagnetic relays (relay logic), fluidic logic, pneumatic logic, optics, molecules, or even mechanical elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic.

Symbols

Type Distinctive shape Rectangular shape Boolean algebra

between A & B Truth table

AND

INPUT OUTPUT

A B A AND B

0 0 0

0 1 0

1 0 0

1 1 1

OR

INPUT OUTPUT

A B A OR B

0 0 0

0 1 1

1 0 1

1 1 1

NOT

INPUT OUTPUT

A NOT A

0 1

1 0

In electronics a NOT gate is more commonly called an inverter.

The circle on the symbol is called a bubble, and is used in logic

diagrams to indicate a logic negation between the external

logic state and the internal logic state (1 to 0 or vice versa). On

a circuit diagram it must be accompanied by a statement

asserting that the positive logic convention or negative logic

convention is being used (high voltage level = 1 or high voltage

level = 0, respectively). The wedge is used in circuit diagrams to

directly indicate an active-low (high voltage level = 0) input or

output without requiring a uniform convention throughout the

circuit diagram. This is called Direct Polarity Indication. See IEEE

Std 91/91A and IEC 60617-12. Both the bubble and the wedge

can be used on distinctive-shape and rectangular-shape

symbols on circuit diagrams, depending on the logic convention

used. On pure logic diagrams, only the bubble is meaningful.

NAND

INPUT OUTPUT

A B A NAND B

0 0 1

0 1 1

1 0 1

1 1 0

NOR

INPUT OUTPUT

A B A NOR B

0 0 1

0 1 0

1 0 0

1 1 0

XOR

INPUT OUTPUT

A B A XOR B

0 0 0

0 1 1

1 0 1

1 1 0

XNOR

or

INPUT OUTPUT

A B A XNOR B

0 0 1

0 1 0

1 0 0

1 1 1

1.8 Local Rea Network

A local area network (LAN) is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building using network media.

[1] The defining characteristics of LANs, in

contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

ARCNET, Token Ring and other technology standards have been used in the past, but Ethernet over twisted pair cabling, and Wi-Fi are the two most common technologies currently used to build LANs.

Network topology describes the layout pattern of interconnections between devices and network segments. Switched Ethernet has been for some time the most common Data Link Layer and Physical Layer implementation for local area networks. At the higher layers, the Internet Protocol (TCP/IP) has become the standard. Smaller LANs generally consist of one or more switches linked to each other, often at least one is connected to a router, cable modem, or ADSL modem for Internet access.

Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic with VLANs. Larger LANs also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, and sensors.

LANs may have connections with other LANs via leased lines, leased services, or by tunneling across the Internet using virtual private network technologies. Depending on how the connections are established and secured in a LAN, and the distance involved, a LAN may also be

classified as a metropolitan area network (MAN) or a wide area network (WAN).

1.8.1 Ethernet

Ethernet /ˈiːθərnɛt/ is a family of computer networking technologies for local area networks (LANs). Ethernet was commercially introduced in 1980 and standardized in 1985 as IEEE 802.3. Ethernet has largely replaced competing wired LAN technologies.

The Ethernet standards comprise several wiring and signaling variants of the OSI physical layer in use with Ethernet. The original 10BASE5 Ethernet used coaxial cable as a shared medium. Later the coaxial cables were replaced by twisted pair and fiber optic links in conjunction with hubs or switches. Data rates were periodically increased from the original 10 megabits per second, to 100 gigabits per second.

Systems communicating over Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses and error-checking data so that damaged data can be detected and re-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-bit MAC address and Ethernet frame format have influenced other networking protocols.

RJ45 cable commonly used on Ethernet networks

1.8.2 Repeater and hubs

For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size. Somewhat larger networks could be built by using an Ethernet repeater. Early repeaters had only two ports, allowing, at most, a doubling of network size. Once repeaters with more than two ports became available, it was possible to wire the network 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 its bus topology was in conflict with the star topology cable plans designed into buildings for telephony. Modifying Ethernet to conform to twisted pair telephone wiring already installed in commercial buildings provided another opportunity to lower costs, expand the installed 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 1987 SynOptics introduced the first twisted-pair Ethernet at 10 Mbit/s in a star-wired cabling topology with a central hub, later called LattisNet. These evolved into 10BASE-T,

which was designed for point-to-point links only, and all termination was built into the device. This changed repeaters from a specialist device used at the center of large networks to a device that every twisted pair-based network with more than two machines had to use. The tree structure that resulted from this made Ethernet networks easier to maintain by preventing most 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 pair and fiber media, repeater based Ethernet networks still use half-duplex and CSMA/CD, with only minimal activity by the repeater, primarily the Collision Enforcement signal, in dealing with packet collisions. Every packet is sent to every port on the repeater, so bandwidth and security problems are not addressed. The total throughput of the repeater is limited to that of a single link, and all links must operate at the same speed.

Network interface card

1.8.3 Bridging and switching

While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forwarded all traffic to all Ethernet devices. This created practical limits on how many machines could communicate

on an Ethernet network. The entire network was one collision domain, and all hosts had to be able to detect collisions anywhere on the network. This limited the number of repeaters between the farthest nodes. Segments joined by repeaters 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 the physical layer. With bridging, only well-formed Ethernet packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. Prior to learning of network devices on the different segments, Ethernet bridges (and switches) work somewhat like Ethernet repeaters, passing all traffic between segments. After the bridge learns the addresses associated with each port, it forwards network traffic only to the necessary segments, improving overall performance. Broadcast traffic is still forwarded to all network segments. Bridges also overcame the limits on total segments between two hosts and allowed the mixing of speeds, both of which are critical to deployment of Fast Ethernet.

In 1989, the networking company Kalpana introduced their EtherSwitch, the first Ethernet switch. This worked somewhat differently from an Ethernet bridge, where only the header of the incoming packet would be examined before it was either dropped or forwarded to another segment. This greatly reduced the forwarding latency and the processing load on the network device. One drawback of this cut-through switching method was that packets that had been corrupted would still be propagated through the network, so a jabbering station could continue to disrupt the entire network. The eventual remedy for this was a return to the original store and forward approach of bridging, where the packet would be read into a buffer on the switch in its entirety, verified against its checksum and then forwarded, but using 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 Ethernet becomes possible over that segment. In full-duplex mode, both devices can transmit and receive to and from each other at the same time, and there is no collision domain. This doubles the aggregate bandwidth of the link and is sometimes advertised as double the link speed (e.g., 200 Mbit/s). The elimination of the collision domain for these connections also means that all the link's bandwidth can be used by the two devices on 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 less public than on shared-medium Ethernet. Despite this, switched Ethernet should still be regarded as an insecure network technology, because it is easy to subvert switched Ethernet systems by means such as ARP spoofing and MAC flooding.

The bandwidth advantages, the slightly better isolation of devices from each other, the ability to easily mix different speeds of devices and the elimination of the chaining limits inherent in non-switched Ethernet have made switched Ethernet the dominant network technology.

1.8.4 Network Address IP Address = Internet protocol address of Ethernet node, in accordance with TCP/IP protocol. Ex. 172.16.1.32 Subnet mask The bits set in the subnet mask decides the part of the IP address that contains the address of the subnet/network. In general: ● The network address is obtained by an AND operation on the IP address and subnet mask. IP.AND. subnet mask ● The node address is obtained by an AND NOT operation on the IP address and subnet mask. IP.AND. NOT subnet mask Subnetwork All the devices interconnected by switches are nodes of the same network or subnet. All the devices in a subnet can communicate directly with each other. All devices in the same subnet have the same subnet mask. A subnet is physically restricted by a router

Switch Industrial Ethernet is made up of point-to-point links: Each communication node is connected directly to one other communication node. Multiple communication nodes are interconnected at the port of an active network component, that is, at the switch. Other communications nodes (including switches) can then be connected to the other ports of the switch. The connection between a communication node and the switch remains a point-to-point link. The task of a switch is therefore to regenerate and distribute received signals. The switch

"learns" the Ethernet address(es) of a connected PROFINET device or other switches and forwards only the signals intended for the connected PROFINET device or connected switch. A switch has a certain number of ports. At each port, connect a maximum of one PROFINET device or a further switch. Router A router interconnects two subnets. A router works in a similar way to a switch. With a router, however, you can also specify which communication nodes may communicate via the router and which may not. The communication nodes on various sides of a router can only communicate with one another if you have explicitly enabled communication between these nodes via the router. Real-time data cannot be exchanged beyond subnet boundaries.

1.9 Serial Links

Serial links take many forms, from 2400 bps dial-up modems (and their faster cousins), to dedicated T3 leased lines. All share two common traits - they interconnect two computers and transmit a single bit at a time in each direction. The stream of bits is assembled into bytes and then packets. The speed of a serial line is rated in bits per second (bps). They can be broken into two broad categories - synchronous and asynchronous.

Synchronous Serial Links

Synchronous links use some form of clocking, by which a

clock signal is transmitted along with the data. This can

take many forms. A seperate signal may carry the clock:

In this example, the sender transmits each bit on the rising edge

of the clock. The receiver latches the data signal on the trailing

edge of the clock. In this manner, race conditions between the

two are avoided. Every clock cycle, a single bit is transferred.

It is also possible to combine the clock and data signals together

into one:

In this example, every zero bit is encoded as a full clock cycle,

while one bits are encoded by skipping a transition in the middle

of the cycle. The decoding circuitry is more complex, but fewer

wires are required in this scheme.

In any event, such serial links are termed synchronous because

the data signal is synchronized with a clock signal. Most serial

links are synchronous; low speed modems and printers form the

exception.

Typically, one side of the link provides clocking (gives clock)

for data traveling in both directions. The other side of the link

takes clock. If a telephone company (telco) leased line is

involved, the telco will give clock, since the data must be

carefully synchronized as it moves through the telco's network.

Asynchronous Serial Links

Asynchronous links lack any form of clock signal. Rather, a

start bit is used to signal the beginning of a transmission. Once

the receiver has seen the start bit, it begins counting bit times

according to the pre-configured line speed:

Without an explicit clock signal, the receiver risks gradually

losing synchronization with the sender. For this reason, almost

all asynchronous links transmit only a single byte at a time. The

next byte requires a new start bit to re-synchronize the sender

and receiver.

Asynchronous links, since they lack clocking, require the sender

and receiver to agree on the bit speed of the link. If they do not

agree, for example if the sender transmits at 14.4 kbps but the

receiver is configured for 9.6 kbps, gibberish will result, as

almost anyone with an external modem has experienced from

time to time.

Framing

No matter what kind of serial lines are in use, the bits and bytes

transferred over them must be grouped together into packets.

The beginning and end of each packet must be clearly

delineated, usually a checksum will be included to ensure the

packet is undamaged, and often a type field is required to

differentiate between, say, an IP packet and one for IPX.

Most serial links use HDLC or some varient of it. Dialup

modem lines first used SLIP, but PPP is now preferred. Both are

HDLC-based, but PPP is more elaborate, supporting dynamic

address assignment, on-the-fly data compression, and multiple

network-layer protocols. In addition to data compression, header

compression is also supported.

Baud rate

Unit of measuring transmission speed in bit/s

Modbus

Protocol for the data transmission on serial interfaces, this protocol allows data to be transmitted in either master or slave mode. Registers (Word) or coils (Bool) can be transmitted with Modbus protocol.

For the transmission mode, it is possible to choose between RTU (Remote Terminal Unit) or standard ASCII character set.

Hardware

RS 232, RS422, RS 485

Configuration Example

Baud rate: 9600, 19200 baud, Data bits: 7, 8 Stop bits: 1,2 parity bit: No, Even, Odd

Mode: RTU, ASCII

1.10 Analogue to Digital Converter ADC

An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts the input continuous physical quantity to a digital number that represents the quantity's amplitude. The conversion involves quantization of the input, so it introduces a small amount of error. The inverse operation is performed by a digital-to-analog converter (DAC). Instead of doing a single conversion, an ADC often performs the conversions ("samples" the input) periodically. The result is a sequence of digital values that have converted a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signal.

An ADC may also provide an isolated measurement such as an electronic device that converts an input analog

voltage or current to a digital number proportional to the magnitude of the voltage or current.

1.10.1 12 bit unsigned integer

In computer architecture, 12-bit integers, memory addresses, or other data units are those that are at most 12 bits (1.5 octets) wide. Also, 12-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. Possibly the best-known 12-bit CPU is the PDP-8 and its relatives, such as Intersil 6100 microprocessor produced in various incarnations from August 1963 to mid-1990. Many ADCs (analog to digital converters) have a 12-bit resolution. Some PIC microcontrollers use a 12-bit word size.

12 binary digits have 4096 (10000 octal, 1000 hexadecimal) unique combinations.

(4 to 20 MA current, which is analogue signal is converted to (809 to 4095) digital signal by ADC with 12 bit resolution.