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MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: ADDITIONAL FACILITIES

LESSON 3: COMMUNICATION PROTOCOLS FOR

NETWORKING PLCs AND COMPUTERS

PLC - 7 - 3

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

In the final lesson of this topic we examine the means whereby PLCs and

microprocessors are connected together, i.e. how they are networked and the

protocols which define what sort of signals pass along the network, the

connectors and the mediums used for the networks. Traditionally, manufacturers

devised they own methods of connecting their equipment which led to wholesale

disparity and incompatibility of equipment between different manufacturers with

the attendant inefficiencies and costs to industry. Protocols are an attempt to

rationalise the differences between manufacturers’ equipment, such that a piece

of equipment from any manufacturer will perform to the same communication

and control standards and characteristics. Even now there are many protocols

covering many aspects of PLC and microprocessor communication and

networking and it will be some time before a totally rationalised and relatively

simple system of process control equipment emerges.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

On complettion of this lesson you should be able to:

• understand what is meant by the term ‘networking’

• be aware that there are different types of network

• be aware of different types of medium for the interconnection of

networks

• be aware of the ‘Open Systems Interconnection Basic Reference

Model

• be aware of some common Networking Standards and Protocols.

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________________________________________________________________________________________

NETWORKS ________________________________________________________________________________________

The interconnection of PLCs and computers for the purpose of communication

between devices generally comes under the term networking.

With the rapid growth of new technology and equipment in industrial process

control, computing and digital communications manufacturers adopted their

own forms of data transfer, network configurations, interconnections, data

recognition, etc. This led to a plethora of different manufacturer-orientated

operating systems and equipment, usually to build a captive customer base.

Early on the need for equipment and operating system compatibility was

recognized but the formation and implementation of such standards and

protocols was a much slower process, still at present developing. Different

standards have been established some of which are discussed below. Some

standards are used where a PLC only ‘talks’ to a second PLC and others where

PLCs interact with each another and other equipment (PLC to printer, VDU,

etc). Each standard has its own operating characteristics and protocol

requirements. It follows that equipment not sharing the same standard with

other equipment with which it wants to interact, will be unlikely to be able to

do so.

Even with many manufacturers now conforming to agreed international

standards, there is a multitude of protocols and standards which are classified

into several ‘layers’ (as we shall see a little later) for different pieces of

equipment and different methods of interconnection over different mediums.

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ORGANISATION OF NETWORKS

Master Slave Networks

In computer networking, master/slave is a model for a communication protocol

in which one device or process, known as the master, controls one or more

other devices or processes known as slaves. Once the master/slave relationship

is established, the direction of control is always from the master to the slave(s).

Other communication protocol models include the client/server model, in

which a server program responds to requests from a client program, and the

peer-to-peer model (see below), in which either of the two devices involved

can initiate a communication session.

Peer-to-Peer Networks

Often referred to simply as peer-to-peer, or abbreviated P2P, peer-to-peer

architecture is a type of network in which each workstation has equivalent

capabilities and responsibilities. Peer-to-peer networks are generally simpler

than Master/Slave networks but they usually do not offer the same

performance under heavy loads. The P2P network itself relies on computing

power at the ends of a connection rather than from within the network itself.

The peer way consists of twin axial cables providing two highways for full

redundancy. The Highway Interface Adaptor (HIA) permits the connection of

multiple peer ways locally. Message routing is automatic such that path usage

is optimised.

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The Rosemount Peer Way, for example, operates on a transmission cycle that

occurs four times per second, during which every device is granted access to

the peer way for the transmission and reception of data and messages. These

messages are of three types:

• broadcast messages with no specific address

• point to point messages

• no-data messages.

FIGURE 1 is a peer way control network used on a Rosemount process control

system (known as a distributed control system) that uses microprocessors

distributed throughout an industrial process plant to control the process.

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FIG. 1 (Courtesy of Rosemount Ltd)

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NETWORK LINKS

Digital signals are easier to transmit over distance because of the two threshold

limit nature of them. Analogue signals can be lost in noise picked up. It is in

order to minimise the effects of noise and interference from extraneous

magnetic fields, when transmitting analogue data signals over distance, that

special communication links or cables are used.

Unshielded Twisted Pair Cable (UTP Cable)

Twisted pair cables were first used in telephone systems by Bell (Laboratories)

in 1881 and by 1900 the entire American network was twisted pair, or else

open wire with similar arrangements to guard against interference. Twisting

pairs of cores together reduces what is referred to in communications as

‘crosstalk’, i.e. external field interference.

In telephone applications, UTP is often grouped into sets of 25 pairs according

to a standard 25-pair colour code originally developed by the American

Telephone and Telegraph Co (AT&T). Twisted pair cabling is often used in

data networks for short and medium length connections because of its

relatively lower costs compared to fibre optic and coaxial cabling. An

‘Ethernet’ network (see below) often uses UTP cable as the interconnecting

medium between equipments. FIGURE 2(a) shows a UTP cable and

FIGURE 2(b) a common telecommunications termination.

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FIG. 2(a)

(Copyright Qnet Group, www.click2qnet.com)

FIG. 2(b)

Currently there are 8 catagories of UTP cable for various applications (cat 1 –

cat 7 with two cat 5, cat 5 and cat 5e, catagories 1 and 2 not being suitable for

Ethernet applications).

Screened twisted pair cable is also available which further reduces the

susceptibility to interference from external fields.

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Coaxial cable

FIGURE 3 shows the construction of coaxial cable. It consists of a central

conducting wire separated from an outer conducting cylinder, a grounded

shield of braided wire, by an insulator, often ptfe.

The shield minimises electrical and radio frequency interference. The central

conductor is +ve with respect to the outer conductor. These cables do not

produce external electric and magnetic fields and are therefore not affected by

them.

Coaxial cable is the primary type of cabling used by the cable television

industry. It is also used for computer networks. It is often used as the data

highway in a ‘Local Area Network’ (LAN, see below). It is more expensive

than standard telephone wire but can carry more data and is less susceptible to

interference

FIG. 3

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Optical Fibre

Two ways of transmitting information are by cables or radio wave (usually

microwave) transmission. A third way is by fibre optics which sends

information coded in a beam of light down a glass or plastic pipe. It was

originally developed in the 1950s f or medical pr ognostic camer a

investigations. In the 1960s, engineers found a way of using the same

technology to transmit telephone calls and today a single optical fibre can carry

in excess of tens of thousands of telephone conversations.

Optical fibre is known for its reliability and immunity to sources of

interference. It has an extremely large bandwidth which is required if large

amounts of information are to be transmitted, ranging from 600 MHz to

1000 MHz. It is therefore suited to multimedia applications. Optical fibre

cables do not conduct electricity or support ground loops, so will be unaffected

by lightning strikes.

Optical fibres are filaments of transparent dielectric material, usually glass or

plastic, and usually circular in cross section. These constitute wave guides but

instead of the transmission of microwave frequency and above electromagnetic

waves, light waveguides condition the transmission of light. An optical fibre,

shown in FIGURE 4, usually has a cylindrical core surrounded by, and in

intimate contact with, a cladding of similar geometry. The refractive index of

the core must be slightly higher than that of the cladding for the light to be

guided by the fibre. FIGURE 4 shows a schematic of an optical fibre.

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FIG. 4

The simplest type of optical fibre is called single-mode. It has a very thin core

about 5 – 10 microns (millionths of a metre) in diameter. In a single-mode

fibre, all signals travel straight down the middle without bouncing off the

edges.

Another type of optical fibre cable is called multi-mode cable. Each optical

fibre in a multi-mode cable is about 10 times bigger than one in a single-mode

cable. This means light beams can travel through the core by following a

variety of different paths.

Multi-mode cables can send information only over relatively short distances

and, among other things, are used to link computer networks.

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________________________________________________________________________________________

NETWORKING PROTOCOLS AND STANDARDS ________________________________________________________________________________________

THE OPEN SYSTEMS INTERCONNECTION BASIC REFERENCE MODEL

The Open Systems Interconnection Basic Reference Model, often referred to

as the OSI Reference Model or simply OSI Model, is a layered, abstract

description for communications and computer network protocol design. It was

developed as part of the Open Systems Interconnection initiative (also called

the OSI seven layer model), which was an effort to standardize networking

that was started in 1982 by the International Organization for Standardization

(ISO) and others.

Layer 1, for example, is known as the physical layer and deals with modems

and computer port protocols, etc. Layer 2 deals with Data Links and

‘Ethernet’ protocol falls into this category (though some of its standards apply

to the physical layer). The name comes from the physical concept of the ether.

It defines a number of wiring and signaling standards for the physical layer and

two means of access at the Media Access Control (‘MAP’)/ Data Link layer.

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FIG. 5(a)

FIG. 5(b)

With acknowledgement to Wikipedia

Data Unit

Data

Segments

Packets

Frames

Bits

Host layers

Media layers

Layer

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data link

1. Physical

Function

Network process to application

Data representation and encryption

Interhost communication such as Peer to Peer

End-to-end connections and reliability

Path determination and logical addressing

Physical addressing

Media, signal and binary transmission

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LOCAL AREA NETWORKS (LAN)

A LAN is a series of interconnected computers and associated equipment

linked by line or radio, which form a network usually spanning a relatively

small area, for example, in a building or between a group of buildings. The

network can be increased by interconnecting LANs. A number of LANs

connected in this way is called a Wide-Area Network (WAN).

Wireless LANs use high frequency radio signals or infrared light beams to

communicate between the workstations. Wireless networks can be used

efficiently for allowing laptop computers or remote computers, etc. to connect

to the LAN. Wireless networks may be more convenient in older buildings

where it may be difficult to install cables.

Wireless LANs, standardized by IEEE 802.11, do have some disadvantages.

They are very expensive, provide poor security, and are susceptible to

electrical interference from lights and radios. They are also slower than LANs

using cabling.

ETHERNET

Ethernet has been standardized as IEEE 802.3. It uses a star-topology, twisted

pair wiring form and became the most widespread LAN technology in the

1990s. In recent years, Wi-Fi, the wireless LAN, has been used in addition to

or instead of Ethernet in many installations.

Layer 3 is the Session or Networking layer and contains many standards

dealing with the Internet. Other layers deal with such things as transmission of

data control, file transfer protocols, etc.

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SOME OTHER COMMON NETWORKING STANDARDS AND PROTOCOLS

RS-232 (RETMA) Standard

RS-232 is a layer 1 protocol which is an American standard for serial binary

data interconnection or a serial line interface protocol developed to connect

modems and computer terminals. It is commonly used in computer serial

ports. Another protocol in this layer is that of SDN Integrated Services

Digital Network.

RS is an abbreviation for RETMA Standard. (RETMA stands for Radio

Electronics Television Manufacturers' Association, formed in the US to

standardise the nomenclature of American vacuum tubes in 1953.)

The standard defines:

• electrical signal characteristics

• interface mechanical characteristics, pluggable connectors and pin

identification

• functions of each circuit in the interface connector

• standard subsets of interface circuits for selected telecom applications.

The standard recommended but did not make mandatory the common D-

subminiature 25 pin connector. The standard specifies 20 different signal

connections. Since most devices use only a few signals, smaller connectors

can be used.

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EIA-422 (formerly RS-422)

EIA-422, now TIA-422, has the title TIA-422 Electrical Characteristics of

Balanced Voltage Differential Interface Circuits.

It is an American technical standard which provides for data transmission over

terminated or non-terminated transmission lines.

An advantage offered by this standard includes data rates as high as

10 Mega baud at 12 metres (40 ft). The specification itself does not set an

upper limit on data rate, but rather shows how signal rate degrades with cable

length.

EIA-422 only specifies the electrical signalling characteristics of a single,

balanced, signal. Protocols and pin assignments are defined in other

specifications. The maximum cable length is 1200 m. Maximum data rates

are 10 Mbit/s at 1.2 m or 100 kbit/s at 1200 m.

A common use of EIA-422 is for RS-232 extenders often found in video

editing studios. EIA-422 uses a nominal 0 to 5 V signal.

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IEEE-488

IEEE-488 is a short-range, digital communications bus specification.

IEEE-488 is also commonly known as HP-IB (Hewlett-Packard Instrument

Bus). This came about in the late 1960s, when Hewlett-Packard, a

manufacturer of test and measurement instruments, such as digital multi-

meters, oscilloscopes, etc. developed an Interface Bus (HP-IB) to enable easier

interconnection between instruments and controllers such as computers. The

bus is also known as the GPIB (General Purpose Interface Bus). As other

manufacturers conformed to the HP system, the US Institute of Electrical and

Electronics Engineers used it as the IEEE Standard Digital Interface for

Programmable Instrumentation, IEEE-488-1975 (now 488.1). The protocol

consists of several parts. IEEE-488.1 formalized the mechanical, electrical,

and basic hardware protocol parameters of GPIB.

In addition to the IEEE, several other standards committees have adopted HP-

IB. The American National Standards Institute’s corresponding standard is

known as ANSI Standard MC 1.1, and the International Electrotechnical

Commission has its IEC Publication 625-1.

The IEEE-488 bus employs 16 signal lines — eight bi-directional used for data

transfer, three for handshake* [ not ready for data (NRFD), Data Valid (DAV)

and Not data accepted (NDAC)], five for bus management, plus eight ground

return lines.

________________________________________________________________________________________

*In data communications, a sequence of events governed by hardware or software, requiring mutual agreement

of the state of the operational modes prior to information exchange.

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IEEE-488 therefore uses 24-pin Amphenol-designed micro ribbon connectors

(often incorrectly termed Centronics-type) as shown in FIGURE 6. They are

most commonly manufactured in a stackable male/female combination that

allows for easy daisy-chaining by stacking cables. Mechanical considerations

limit the number of stacked connectors to four or less.

FIG. 6

With acknowledgement to Wikipedia

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Manufacturers Automation Protocol (MAP)

MAP developed jointly by the Electronic Industries Association (EIA) and the

International Standards Organisation (ISO) is based on the seven layer

International Standards Organization's (ISO) Open Systems Interconnect (OSI)

network model. It was designed to standardise the connections of the many

different types of manufacturing equipment.

MAP 3 defines two specifications for the application layer of the OSI; File

Transfer Access Management (FTAM) and Manufacturing Message

Specification (MMS). It is the latter that manufacturers of PLCs and

associated equipment are expected to adopt and it has the backing of the

Department of Trade and Industry in the UK. MAP implementation, however,

has proved to be not without difficulty with many delays, in part due to the

manufacturers ‘learning curve’ delay which accompanies new technological

innovation.

One of the disadvantages of MAP products is that they have to be flexible

enough to carry different types of information, yet fast enough to keep pace

with modern factory production.

Because MAP networks carry information from various intelligent devices,

such as computers and programmable logic controllers, the user group chose a

broadband, token-bus topology as the basis for its network. Broadband

networks are capable of simultaneously supporting many applications. By

comparison, a token-ring network and an Ethernet network support only one

application at a time.

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PROFIBUS

A popular European standard for industrial process control networking is

PROFIBUS (Process Field Bus). This is a popular type of field bus (an

industrial network system for real-time distributed control) which was

developed in 1989 as the result of a German research project involving 15

firms and research institutions.

PROFIBUS was defined in 1991/1993 as DIN 19245, changed in 1996 to EN

50170 and is, since 1999, included in the IEC 61158/IEC 61784 Standards.

The PROFIBUS standard is maintained, updated and marketed by PROFIBUS

International, a non-profit organisation in Germany.

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________________________________________________________________________________________

SCADA NETWORKS ________________________________________________________________________________________

SCADA is the acronym for Supervisory Control and Data Acquisition.

The term refers to large-scale, distributed measurement and control systems

commonly known as a Distributed Control Systems or DCS.

DCS was a development from Direct Digital Control where a central computer

monitored and controlled an industrial process.

The use of large computers in Direct Digital Control and in earlier systems

presented two major problems to the designer and user. These problems were:

• additional computers were required in order to ensure continuous

production on failure of one computer; this added significantly to system

costs

• the extended data communication paths, for signal transmission to and

from the computer and their respective end elements, resulted in data

corruption and loss of system integrity.

Distributed Control Systems are used to monitor and/or control entire

industrial processes from Chemical, Services to Transport, etc. Such systems

are manufactured by large process control companies such as Honeywell,

Rosemount, Taylor, Siemens, etc. They consist essentially of distributed

controllers or microprocessors situated throughout an industrial plant

connected to a common network (a LAN, for example, with Honeywell

system.) The controllers receive input signals from process sensors and

deliver output signals to the process actuators. The LAN networked

controllers are observed and accessed for control and maintenance, etc. by a

central control operator interface console. Many sensors are ‘smart sensors’,

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by which is meant they contain a microcontroller. Such sensors can be set,

programmed and maintained by connecting a laptop computer to them. Many

d.p. cells, for example, are ‘smart sensors’.

A SCADA system is essentially a DCS that uses PLCs as the preferred method

of distributed control. The PLCs are cheaper to purchase than the DCS system

components but lack their versitility.

SCADA systems include input/output signal hardware, controllers,

communication network, database and software. Like a DCS the SCADA

monitors and controls a complete site or a system spread out over a long

distance (kilometres/miles). Process control is often performed automatically

by Programmable Logic Controllers (PLCs) distributed around the site. A

PLC may control the flow of cooling water through part of an industrial

process, but the SCADA system may allow an operator to change the control

set point for the flow, and will allow any alarm conditions such as loss of flow

or high temperature to be recorded and displayed. The feedback control loop

is closed through the RTU (Remote Terminal Unit) or a PLC allowing the

SCADA system to monitor the overall performance of that loop.

A typical SCADA process control system is shown in FIGURE 7.

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FIG. 7

With acknowledgements to Wikipedia

An RTU, or Remote Terminal Unit, is a microprocessor-controlled device

that interfaces objects in the physical world to a distributed control system or

SCADA system by transmitting telemetry data to the system and/or altering

the state of connected equipment based on control messages received from the

system.

RTUs, PLCs and DCS are increasingly beginning to overlap in responsibilities,

and many vendors sell RTUs with PLC-like features and vice versa. The

industry has standardized on the IEC 61131-3 functional block language for

creating programs to run on RTUs and PLCs.

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Data acquisition begins at the PLC level and includes meter readings and

equipment statuses that are communicated to SCADA as required. Data is

then compiled and formatted in such a way that a control room operator can

make appropriate supervisory decisions that may be required to adjust or over-

ride normal PLC controls. Data may also be collected in to a Historian

(essentially a continuously updated database), to allow trending and other

analytical work.

COMMUNICATION INFRASTRUCTURE AND METHODS

SCADA protocols are designed to be very compact and many are designed to

send information to the master station only when the master station polls the

RTU. Typical SCADA communication protocols, of which there are several,

are all vendor specific, developed historically to lock customers into the

vendor customer base.

However, standard protocols are recognized by all major SCADA vendors.

Although SCADA can be connected to the internet it is considered inadvisable

for security reasons to do so as SCADA based systems are increasingly seen as

extremely vulnerable to cyberwarfare/cyberterrorism attacks.

Whether such concerns will cause a move away from the use of SCADA

systems for mission critical applications, towards more secure architectures

and configurations remains to be seen, however, the benefits and lower initial

costs of SCADA based systems still outweigh potential costs and risks.

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A LAN ARRANGEMENT IN A DCS SYSTEM

In order to get an idea of a networked system of microprocessor control and of

the layout of a basic controller and interfaces used in a distributed control

system, a typical block diagram of the Honeywell DCS data highway and

interface system is shown in FIGURES 8(a) and 8(b) (DEP is the ‘data entry

panel’).

FIG. 8(a)

Operator station

Interface 'A' 'B'Operator

station

Hiway traffic

director

Data hiway

To remote devices

master/slave communications

Interface 'A' 'B'

Interface Interface

Process interface unit

Controller

Data entry panel

Analog displays

Controller terminations

'A' 'B'

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FIG. 8(b)

With acknowledgement to Honeywell Control Systems Ltd

The fact that there are many such connection and data transmission protocols

highlights the gross incompatibility between different manufacturer ’s

equipment that existed prior to such protocols being formulated. There is still

some way and time to go before a universal protocol replaces the several

diverse protocols that now exist.

CPU micro proc.

ROM

RAM

RAM back up

DEP I/F

Hiway I/F

MUX and

ADC and

filters

Output station

Field inputs 4 - 20mA DC

Span zero

⎛ ⎝

Field outputs 4 - 20mA DC

Mode and

manual control

1 x

3 x

4 x

5 x

6 x

7 x

8 x

1 y

2 y

3 y

4 y

5 y

6 y

7 y

8 y

PV RV

Selection and data signals

Data hiway A

Data hiway B

Data hiway A

Data hiway B

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. State the difference between Master/Slave networking and Peer to Peer.

2. State:

(a) two types of twisted pair cable

(b) the purpose of the shield in a coaxial cable

3. State the seven layers of the OSI Protocol model and into which layer

would fall:

(a) Peer to Peer protocol

(b) Ethernet protocol.

4. State:

(a) the meaning of RS in the RS 232 standard

(b) the OSI layer into which RS 232 standard falls.

5. (i) What is ‘Profibus’?

(ii) What does the acronym ‘SCADA’ stand for?

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Master/slave networking involves a communication protocol in which one

device or process, known as the master, controls one or more other

devices or processes known as slaves. A peer-to-peer architecture is a

type of network that involves a protocol in which each workstation has

equivalent capabilities and responsibilities.

2. (a) Unshielded TP and Shielded TP Cable.

(b) The shield minimises electrical and radio frequency interference and

forms a barrier to external electric and magnetic fields which would

otherwise interfere with the signal being carried by the coaxial cable.

3. See FIGURE 5(b) on page 12.

(a) Layer 5 ‘Session’ deals with the Peer to Peer protocol.

(b) Layer 2 deals with Data Links and ‘Ethernet’ protocol falls into

this category.

4. (a) RS is an abbreviation for RETMA Standard. (RETMA stands for

Radio Electronics Television Manufacturers' Association of the US.)

(b) RS-232 is a OSI model layer 1(physical)

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5. (i) Profibus stands for Process Field Bus and is a popular type of field

bus or industrial network system in Europe for distributed process

control.

(ii) SCADA is the acronym for ‘Supervisory Control and Data

Acquisition’.

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

In this lesson we have examined different types of networking of PLCs and

microprocessors, that is, connecting several together so that they can

communicate with each other and with other useful pieces of external

hardware. This enables PLC control of a process to be controlled and

monitored in a more effective manner.

Interconnecting PLCs and microprocessors can increase the computing power

and process control ability of a system such that major parts of industrial

processes can be individually controlled by a PLC or PLCs, yet form part of a

complex total plant control system.

The actual networks connecting the PLCs are sometimes known as data

highways or peer ways and usually consist of either UTP cable, coaxial cables

or optic fibres, which form a Local Area Network (LAN) or Ethernet, etc.

Which connection is used will depend on such considerations as cost,

bandwidth and data speeds required, though often a protocol will determine

what connection is or is not permissible.

We have also seen the attempts to overcome the widespread disparity and

incompatibility between manufacturers of PLCs and related equipment. As a

consequence, several protocols or standards have been instituted both in the

USA and Europe in an attempt to r ationalise and impr ove pr oduc t

compatibility, which ultimately will benefit companies and industries.

Teesside University Open Learning (Engineering)

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setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice