Education Eng[1]

8/6/2019 Education Eng[1]

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The industrial IT revolution

The new and more effective information flowing in today's corporate

systems can provide many competitive benefits. Shorter delivery times,faster product development, customer focused production and shorter

turnaround times are some of the key benefits of industrial IT

(Information Technology), not to mention rapid access to information

and the potential for accurate process control.

Industry is developing IT tools that require a greater degree of infor-

mation at all stages of the process from purchasing to production to

market. The quality of the information highways is currently one of the

most important prerequisites for improving industry's competitiveness

and efficiency.

The standard of the highway network

New ideas, solutions and systems for the creation of these IT tools are evolving,

but one negative consequence of this dynamism and great diversity is a long-

standing absence of accepted standards, despite several efforts to remedy this.

Every pioneer has created his own standard and the problem of inadequate stan-

dards only becomes evident when computers, machines and other equipment

have to communicate.

This involves standards at multiple levels, not merely cables and connectors, but

such matters as data creation and storage, packaging, addressing and transmis-

sion. As well as the manner in which the medium (e.g. a cable) carries informa-

tion and how it is received, unpacked and read by the recipient.

When all such aspects function, then data communication has been realised,which is the basis of modern IT development.

10

Data communications involvemore than cables and connectors


2/47

11

Industrial data communications

Most of the standardization within data communications has taken place within

office applications in integrated networks for personal computers, mainframecomputers, printers, servers, modems etc.

The same focus has not been given to local data communications within indus-

try. This may be due to the fact that the lack of standardization and diversification

is even greater within industry since computers, lathes, measurement equipment,

scales, robots, transportation systems and different alarm systems must be able to

communicate with each other. Furthermore, greater demands are made on relia-

bility and immunity to interference.

One reason for writing this book is to clarify the concepts used in data com-

munications, to explain how it works and to provide a practical tool for problem-

solving within local industrial data communications. For further information, con-

tact Westermo or attend one of our seminars or courses on data communications.

Data communications more and more importantfor improving productivity

As automation becomes increasingly common, the demands become greater on

effective communication between the units and systems used to control process-

es and those that actually carry out production and measurement. Data commu-

nication is the backbone that assures improved efficiency and competitiveness,

regardless of whether the application is manufacturing, construction, transporta-

tion or the medical services.

Data communicationskeeps the wheels ofindustry in motion.

All products marketed

by Westermo are CE-marked, which means thatthe product complies with ECrequirements in all directivesapplying to the standard.

ContactWestermo for adeclaration of conformity


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The purpose of data communications is to transfer information between

two or more units. As a rule, it is characters (text or numbers) and/or

instructions (commands) which are transmitted, although it can also be

drawings and pictures.

The simplest level of computer language is binary characterswhere

each character is composed of seven to eight 1s or 0s. Most comput-

ers operate at this level.

Data communications is basicallya matter of ones and zeros

The computer processes binary characters, made up of onesand zeros.

Each of the characters is called a bit. By combining several bits, a bina-

ry characterset can be constructed. The most common system, ASCII, contains

128 characters, each of which is made up of 7 bits. Each of these characters (or

bit patterns) is known as a byte. Please note that a kilobyte is made up of 1 024

ASCII characters.

All communication is carried out at this level, internally within the computer as

well as externally with other units. Internal communication within the computer is

simple. However, as soon as the computer has to communicate with external

units, a series of factors must be synchronized and controlled to ensure that the

transmission of data takes place correctly.

See the ASCII table on page 21.

12

How does datacommunications work?

Bits and bytes

byte

bit


4/47

13

One bit at a time, or a whole byte?

There are two ways of transmitting data: by parallel or serial transmission.

Parallel transmission is faster and simpler since the entire character with its

8 bits is transmitted in a single operation using 8 transmission paths, one for each

bit. All communication within the computer itself takes place via parallel paths in

the internal data bus, so that an entire character or several characters can be

simultaneously transmitted.

Parallel transmissionvia a multi-conductor cable (Centronics-type) can only becarried out at short distances for practical and economic reasons. Therefore, the

majority of all external data communications is achieved through serial transmis-

sion, i.e. the bits are sent, one at a time, on a single transmission path.

Serial transmissionplaces higher demands on the receiver and the transmitter

which has to keep track of when a character starts and ends and of the inherent

sequence of the bits. The transmitter and receiver must transmit and receive at

the same rate. This is known as the transmission speed and is expressed in bit/s

(bits per second).

In order to tell the receiver where a character starts and ends, the transmitter

sends out extra bits, a start bitand one or several stop bits.

One character at a time or whole sentences?

There are two methods of serial transmission: asynchronous transmission and

synchronous transmission. In asynchronous transmission, the transmitter transmits

the characters one at a time, with their respective start and stop bits. The receiv-

er knows that each start bit will be followed by a character which has to be inter-

preted. The stop bit completing the message re-sets the receiver. About 90-95%

of serial data transmission is asynchronous.

In synchronous transmissionthe entire message is sent in an even flow. The rate

is maintained by a clock signal on a separate wire or modulated on the data sig-

nal.

The advantage of asynchronous transmission is that it is simple and inexpen-sive. The disadvantage is that it is inefficient in comparison with synchronous

transmission as it contains as much as 2025% of message content comprising

parity bits.

Parallel and serialtransmission

Asynchronous andsynchronous transmission

Parallel transmission

Serial transmission

Start bit Stop bit

In asynchronous transmission,one byte is transmitted ata time. The byte starts with

a start bit and ends witha stop bit.

In synchronous transmission,the whole set of data istransmitted at once, in acontinuous stream.


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TD

RD

SG

TD

RD

SG

TD

RD

SG

237

TD

RD

SG

237

TD

RD

SG

TD

RD

SG

TD

RD

SG

2

37

TD

RD

SG

2

37

Transmitters and receivers

Within the field of data communications, we define hardware as transmitters or

receivers. Two units, e.g. a PC and a robot can both be transmitters and receivers,

although this is seldom possible at the same time.

When communication only takes place in one direction, e.g. a computer which

sends an on/off instruction to a motor, this is called simplex transmission. On

the other hand, if the motor then has to reply that it is functioning and report its

speed, duplex transmission is required.Half-duplextransmission means that the communicating units must take turns

in sending out signals, i.e. communication can take place in both directions but

not simultaneously.

Full-duplextransmission is two-way simultaneous transmission. One example is

a telephone converstation where both parties can speak at the same time.

The right connection

Two terms which recur in data communications are DTE (Data Terminal

Equipment) and DCE (Data Communication Equipment).

Computers and terminals are usually DTEs, modems and communications

hardware are generally DCEs while other equipment such as multiplexers andprinters can be DTEs or DCEs (refer to the relevant equipment manual). DTEs

transmit and receive data on different pins in the connector than DCEs.

Therefore, to avoid common errors when connecting equipment, it is important

to know the definition of the particular item of equipment.

If you connect a DTE with a DCE, the DTE will transmit data on pin 2 while the

DCE will receive data on pin 2 (in spite of the fact that the signal is called TD,

Transmit Data in both cases). If you connect two DCEs, you have to connect pin

2 and pin 3 in order for the transmitter to be connected to the receiver (for more

detailed information turn to page 19).

Transparent communication

When connecting two or more modems together to create a network the

modems do not influence the data. What goes in one end comes out the other

describes why the term transparent communication is used. Transparency also

means that all units will hear all messages.

Master-Slave configuration and addresses

The vast majority of industrial networks are based on a master slave configuration

where one or several masters sequentially send messages to the slaves who in turn

respond. This sequence is call polling. As the system is transparent a prerequisite

for this procedure is that each slave has its own address.

A master sends a message starting with the specific slaves address. The slave

recognizes its address and performs the commands included in the message. In

many protocols an acknowledgment is returned to the master who will proceed

to the next slave.

The format of the address and the message is all part of the protocol used by

the specific control system. The modems are not concerned with this fact as long

as the signals conform to the standard of the communications protocol.

If the slaves are unintelligent (no address) so called addressable modems can

be used.

A message intended for all slaves is called a broadcast message. This can typ-

ically be a message from the master instructing all slaves to perform the same

task. An example would be a number of PLCs controlling sirens. In case of a gen-eral alarm all sirens should sound and this could be achieved by sending a broad-

cast message.

14

DTE DCE

DCE DCE

Simplex and duplex

TransparentCommunication

Simplex

Half duplex

Full duplex


6/47

15

Transmission speeds

The optimum transmission speed is not the same as the fastest possible speed

since the risk of transmission errors and interference increases with an increase in

the transmission speed. It is the type of cable and the distance which define the

optimum speed. The aim is always to achieve a highly secure and reliable trans-

mission as well as immunity to interference.

In order to send digital data signals over an ordinary copper wire, the signals

must be transformed. The length of the cable will attenuate and alter the signals.At high speeds, this effect will be critical.

Two terms which are easily confused are those used to describe transmission

speeds: bit/s and baud.

The transmission speed is measured in bit/s(data bits per second). Since

approximately 10 bits are required to transmit one character, it is simple to cal-

culate how many characters are transmitted per second. At a transmission speed

of 9 600 bit/s, about 960 characters per second are transmitted.

In order to transform the digital signal into a signal which can be transmitted

on the network, a modem is used. The modem transforms (modulates) the sig-

nal and the baud rateindicates

how many times per secondthe signal is transformed. Each

transformation is a packet

which is sent along the line to

the receivers modem which

unpacks (demodulates) the information into digital signals

again.

Short-haul modems are transparent and the transmission is not modulated so

that data is received exactly as it was transmitted. The PTT modem can function

as a short-haul modem or with a built-in buffer to hold the bits before they are

sent. For every transmission more than one bit can be sent so the value for trans-

mission speed-bit/s and the transmission times per second-baud differ. If amodem transmits at 2 400 baud and there are four bits in every transmission, the

transmission rate will be 9 600 bits/s.

Modulation

The term, modem, is an acronym of the term modulation, i.e. signal transforma-

tion, and the term demodulation, which is the recreation of the original signal. The

data signals must be transformed and adapted so that they can be transported

over different types of cable. The digital signal levels (1s and 0s) must be trans-

formed into readable changes for the selected cable.

There are three types of modulation:

Frequency modulation, where different frequencies are used to represent a1 and a 0.

Phase modulationwhere the phase of the carrier sine wave is shifted abruptly

to represent the 1s and the 0s. This is the most common method used for PTT

modems which transmit across the telecommunications network.

Amplitude modulationuses the strength of the signal or amplitude peaks to cre-

ate readable 1s and 0s.

Phase/Amplitude modulation is a combination that allows more bits per baud

to be transmitted.

Bit/s and baud

Modulation anddemodulation

PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4

R D TD D CD RTS CTS DTR R D TC

ACCESS


R D TD D CD RTS CTS DTR R D TC

ACCESS

Modulated analogue

electrical signal

Amplitude modulation

Frequency modulation

Phase modulation


7/47

Handshaking is a way for data communication equipment to control the

flow of data between connected equipment. It becomes necessary

when there is a part of the system that is slower than the rest.

There are two common forms of handshaking. Hardware handshak-

ing referred to as (RTS/CTS) which uses separate status lines to control

data flow and software handshaking referred to as (Xon/Xoff) which

uses extra characters in the data flow to achieve the control.

If we have a computer connected to a serial printer and the com-

puter is capable of transmitting its data to the printer faster than the

printer is able to print, it is normal for the printer to have a small buffer

to store this extra data whilst printing, however under certain circum-

stances this buffer will fill up. This is why software or hardware hand-

shaking is needed to tell the computer that it must stop sending data until the

buffer is empty.

Another example is a computer attached to a modem. The data rate between

computer and modem is sometimes much higher than the telephone line can

support so the modem must use handshake signals to tell the computer to slow

down.

Software handshaking.

With software handshaking the printer for example will send a character to be

computer -Xoff when its buffer is full. When the data in the buffer is processed

then the printer will transmit an Xon Character to the computer. The actual char-

acters used have to be defined in some kind of protocol however ASCII 17 (Xon)

and ASCII 19 (Xoff) are commonly used.

16

Handshaking

TD

RD

SG

CTS

RD

TD

SG

DCD

TD

RD

SG

RTS

RD

TD

SG

DTR

Xon/Xoff

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA NN EL 2 P OW ER12-36V DC

1 2 3 4 5 -

MD-42

R + R - T + T -

CHANNEL4

1 2 3 4 5 6 7 81 2 3 4 5 6 7 8

R+ R - T+ T- T+ T- R+ R -

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA NN EL 2 P OW ER12-36V DC

1 2 3 4 5 -

MD-42

R + R - T + T -

CHANNEL4

1 2 3 4 5 6 7 81 2 3 4 5 6 7 8

R+ R - T+ T- T+ T- R+ R -


8/47

Hardware handshaking.

Instead of using extra characters in the data flow, RS-232 provides additional

hardware lines to control communication. The most common lines used areRTS(Request to Send) and CTS (Clear to Send), typically when a computer wish-

es to communicate with a modem.

1. A computer wishes to transmit data so it raises RTS (+3V to +15V).

No data is transmitted

2. The connected modem registers the RTS. When it is ready to receive data

it raises its signal CTS.

3. The computer waits until it sees the CTS line go high and then transmits

its data.

If at any point CTS is dropped by the modem the computer will stop transmitting.

Other hardware lines are sometimes used, for instance serial printers often raise

the DTR line to tell the computer to stop sending data because they have run out

of paper.

Hardware signals are not always used just for handshaking and can be employed

for a number of purposes within data communications. It is also possible that

when connecting two pieces of equipment together a special combination of

crossovers is required to ensure that each piece of equipment sees the right sig-

nals at the right times to ensure reliable data communication.

17

DCE DCE DCE

RTS

1

CT S

DTE DTEDTE

TD

32


9/47

It is not enough to agree on the appearance of the signals and on how

they are to be transformed and transmitted. The next level is

to agree on the appearance of the connectors and the voltage levels for

which they are designed, i.e. the physical and electrical interfaces.

Furthermore, there is a logical interface which defines what a signal

means.

The way in which signals fit together, how the communication is

started, how it is terminated, whose turn it is to send or receive data,

how messages are confirmed etc. are controlled by a protocol. Many

different protocols exist. For example: Profibus, Comli, Modbus.

The physical interfacedefines how units should be connected to each

other and defines the appearance of the connector.

The electrical interface defines the electrical levels and what these mean

(1s or 0s).

The logical interfacedefines how the signal should be interpreted.

RS-232-C/V.24

The most common interface for data communications via the serial port of

the computer is the 9- or 25-pin V.24 standard.

V.24 recommends that the cable should be no longer than 15 metres.

At greater distances, up to several kilometres, short-haul modems are used

to transform the V.24 signal into a signal that is less vulnerable to interference.

V.24 (the ITU-T standard) or RS-232-C(the EIA standard) are two standards

which are similar, in principle, see table. V.24 is the physical standard

while V.28 is the electrical standard. For this reason, the interface is sometimes

described as V.24/V.28.

The interface describes and defines the 25-pin male connector

and the signals and voltages for which they are designed.

Interfaces

18

RS-232/V.24

3 V

-3 V

Startbit

Databits

Paritybit

Stopbit

Not accepted level


10/47

19

Signals in V.24/RS-232-C

Pin V.24 RS-232 Signal Signal name Direct.9/25 Code Code DCE

1 101 AA GND Protective Ground

3 2 103 BA TD Transmitted data I2 3 104 BB RD Received data O7 4 105 CA RTS Request To Send I8 5 106 CB CTS Clear To Send O6 6 107 CC DSR Data Set Ready O5 7 102 AB SG Signal Ground 1 8 109 CF DCD Data Carrier Detector O

9 can be + 12 V 10 can be 12 V 11 126 SCF STF Select Transmit Frequency I12 122 SCB Secondary DCD O13 121 SBA Secondary CTS O14 118 SBA Secondary TD I

15 114 DB TC Transmit Clock O16 119 SBB Secondary RD O17 115 DD RC Receive Clock O18 19 120 SCA Secondary RTS I

4 20 108/2 CD DTR Data Terminal Ready I21 110 CG SQD Signal Quality Detect O

9 22 125 CE RI Ring Indicator O23 111 CH/CI Data Signal Rate Selector O24 113 DA EC External Clock I25 133 RFR Ready For Receiving I

The most common signals used in local communication with modems are print-

ed in bold type. The I/ O direction indicates the direction from the modem (DCE)where I is an input signal and O an output signal.

The TD (Transmit Data) signal is an outlet in a DTE and an inlet in a DCE.

1

2345678910111213

141516171819202122232425

12

3

4

5

67

8

9

DTE to DTE or DCE to DCE DTE to DCE

9 WayD-sub9 WayD-sub

327865149

25 WayD-sub

25 WayD-sub

123456782022

25 WayD-sub

25 WayD-sub

123456782022


327865149

327865149

123456782022

123456782022

327865149


327865149

25 WayD-sub

25 WayD-sub

123456782022

25 WayD-sub

25 WayD-sub

123456782022


327865149

Cable configuration

The picture below shows how the pin configuration for 9- and 25-pole

connectors should be made for all combinations of DTEs and DCEs.

Male D-subs


11/47

Explanation of the most important signals

Protective Ground Connector no. 1 is reserved for protective ground

between the devices.

Signal Ground Signal ground is a signal reference and must always be

connected to connector 7 (25-pin)/connector 5 (9-pin)

in V.24.

Transmitted Data This signal transmits data from a DTE to a DCE.

Received Data This signal is the data that a modem or a DCE

transmits to a DTE.

Request to Send This signal is a request to send data from a DTE.

The device waits for the CTS answer signal.

Clear to Send The answer signal which tells the DTE that it is ready to

transmit data.

Data Set Ready The signal from a DCE which indicates that the device isswitched on, connected and ready.

Data Terminal Ready The same as DSR, although from a DTE.

Data Carrier Detect The output signal from a DCE which indicates that there

is a carrier between the devices and that the connection

is ready for communication.

External Clock This signal is used in synchronous transmission when

it is necessary to clock data. The signal is the input in

the DCE.

Transmit Clock Transmits the DCE clock in synchronous systems.

Receive Clock Clock received in the DTE for decoding data.

Ring Indicator Output signal from a modem indicating that it has

received a ring signal.

20

GND

SG

TD

RD

RTS

CTS

DSR

DTR

DCD

EC

TC

RC

RI


12/47

21

ASCII

BINARY b6

b5

b4

b3 b2 b1 b0 HEX 0 1 2 3 4 5 6 7

0

0

0

0

0

1

0

1

0

0

1

1

1

0

0

1

0

1

1

1

0

1

1

1

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

NUL DLE SP 0 @ P p

SOH

`

DC1 ! 1 A Q a q

STX DC2 " 2 B R b r

ETX DC3

4 D T d t

ENQ NAK % 5 E U e u

ACK SYN & 6 F V f v

BEL ETB ' 7 G W g w

BS CAN ( 8 H X h x

HT EM ) 9 I Y i y

LF SUB * : J Z j z

VT ESC + ; K k

FF FS , < L l

CR GS - = M m

SO RS . > N n

SI US / ? O _ o DEL

$

# 3 C S c s

EOT DC4

[

{

\

|

]

^

}

~

ASCII is the acronym used for American Standard Code for Information Interchange.

Different varieties of the ASCII code exist for different languages as well as an Extended

ASCII where the 8th data bit is used.


13/47

TD

RD

RTS

DTR

GND

TD

RD

RTS

DTR

GND

RS-232DEVICE

A

B

RS-485DEVICE

RS-485DEVICE

RS-485DEVICE

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA NN E L 2 P OW ER12-36V DC

1 2 3 4 5 - +

MD-42

R+ R-T+ T-

CHANNEL4

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R- T + T - T + T- R + R -

TD

RD

RTS

DTR

SG

TD

RD

RTS

DTR

SG

TX A

TX B

RX A

RX B

RX A

RX B

TX A

TX B

RS-232DEVICE

RS-422DEVICE


R D T D D C D R TS C T S D T R R D T C

ACCESS MA-42

TD

RD

RTS

DTR

SG

TD

RD

RTS

DTR

SG

RS-232DEVICE

TX A

TX B

RX A

RX B

RS-422DEVICE

RS-422DEVICE

RS-422DEVICE

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA N NE L 2 P OW ER12-36V DC

1 2 3 4 5 - +

MD-42

R+ R-T+ T-

CHANNEL4

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9R + R- T + T - T + T- R + R-

V.11/RS-422

V.11/RS-422 is a standard interface which is suitable for industrial appli-

cations. It was created for the construction of multidrop data buses,between a main computer and a number of terminals. The interface is

balanced, uses a four-wire format and is relatively immune to interfer-

ence. The interface changes the polarity of the wire pair depending on

whether a 1 or a 0 is to be transmitted.

RS-422 was originally designed to handle 10 units and can now be

set up with up to 32 units connected. The maximum recommended

distance is 1,200 m (100 kbit/s) or 50 m (10 Mbit/s). RS-422 can be

integrated with RS-485, RS-423-A and RS-449 via converters.

RS-422 on four-wire

In the RS-422 four-wire system the master transmiter can always be active/

energized irrespective of the state of the slaves. The standard allows simultaneous

two-way communications.

22

Interfaces for industry

RS-485

RS-422

Termination


14/47

Meter

10 kb/s

10 000

1 000

100

10

1 200

100 kb/s 1 mb/s 10 mb/s

+5 V 0 V

R+

R-

23

RS-485 communicationdistance

Failsafe

RS-485

RS-485 is an updated version of RS-422 and is used more and more often nowa-

days as the standard interface for different units. It is designed for data buses with

up to 32 units and is suitable for multidrop networks with master/slaves. It is rec-

ommended for distances of up to 1 200 m.

The great advantage of using this interface is that it can reverse the direction

of communication which allows for half-duplex transmission on two-wire lines.

RS-485 is the transmission method underlying many popular fieldbus standardsfor instance, Profibus, Bitbus and Interbus-S.

RS-485 on 2-wire

RS-485can be used with two-wire in various master/slave systems, where every

slave is addressable. In a two-wire solution the data direction must be controlled.

This can be achieved with a handshaking signal (RTS/DTR) or by means of the

data flow. Connected units must be capable of what is termed tri-state, i.e. a lis-

tener mode, in which the inactive transmitter enters a high-impedance state thus

not loading the line.

Termination and fail-safeIt is recommended to terminate the line with a circuit of equal impedance to thecharacteristic impedance of the line. For RS-422 and RS-485 a 120 resistance isrecommended. The termination should be placed as shown on page 22.

The purpose of the termination is to prevent the reflection of data at the endsof the cable. If there is no active driver on the network the line can be forced intoa known idle state with a fail-safe circuit. Without this circuit it is likely that the linewill pick up noise and falsely trigger the receivers leading to problematic commu-nications.

RS-232 to RS-422/485 converters The RTS question

When using an RS-232 to RS-422/485 converter it is important to remember that

an RS-485 driver sometimes has to enter tristate or become a receiver. Normally

the RTS signal from the RS-232 circuit is used to control the state of the convert-

er. To work correctly the RTS signal from the RS-232 device must go high for the

duration that data is being transmitted from it and go low to allow the converter

to receive any message back. If this signal is not available then it is necessary to

use converters that can control the data direction from received data alone.

Installation

If a twisted wire pair is used, it should be terminated with a 120-ohm resistor.

The RS-232 cable should not be longer than 15 metres and keep the RS-422/

485 stubs as short as possible.

RS-422/485 guarantees transmission distances up to 1 200 m for data rates up

to 100 kbit/s. Longer distances can be allowed at lower speeds.

RS-485

+5 V

0 V

B

A

Tristate

Startbit

Databits

Paritybit

Stopbit


15/47

Distance and short-haul modems

We mentioned earlier that V.24 does not recommend the use of cables longer

than 15 metres. Consequently, short-haul modems are used for longer connec-

tions. Short-haul modems are sometimes called line-drivers or base-band

modems. These modems transform the V.24 interface into defined signals which

are transmitted over twisted pair or fibre-optic cable for distances up to several

kilometres. The receiving short-haul modem transforms the signals into V.24

again. The modems must be of the same standard and must share a commoninterface in order to communicate via the cable.

Current loop (TTY)

One problem associated with using ordinary copper wire for long-distance com-

munication is that transmission is relatively unreliable and the risk of interference

is high. The transmission speed often has to be reduced to maintain reliability. A

tried and tested method of improving reliability which has been in use for a long

time is to transmit an electrical current over the network.

Current loopis the oldest known method. V.24 signals are represeted as a pulse

of electrical current or no electrical current in the wire pair. Current loop is some-

times refered to as TTY. To supply each wire pair with current, the transmitter iseither connected as active and the receiver as passive, or vice versa. Current Loop

results in more reliable communication but is relatively vulnerable to interference

since the current loop is not balanced, and there is no accepted standard for

Current Loop.

A more advanced method is the balanced 10 mA current loop described

below.

10 mA balanced current loop (W1)

Westermo has developed its own improved interface for its short-haul modems. The

aim is to improve the reliability and performance of the modems and reduce their

vulnerability to interference.Westermos modems transform the signal to be transmitted into a balanced 10

mA current loop. The connected units are always electrically isolated from each

other by means of galvanic isolation. The method involves changing the direction

of the current in the wire pair depending on whether a high or a low signal is to

be transmitted from the V.24 interface. On the transmitter side, there are two

amplifiers which drive 10 mA into the line and on the receiver side there are two

optocouplers which read this signal.

Current always flows in the loop, even when no data is being transmitted, with

the exception of when you chose to use a hardware handshake line to activate the

transmitter and hence provide a way of sending hardware handshake information

across a link.This method is very reliable, there is very little risk of interference and data can

be transmitted at distances of up to 18 km.

24

Current Loop

T+

T -

T+

T -

R+

R -

R+

R -

R+

R -

R+

R -

T+

T -

T+

T -

20 mA

20 mA


16/47

25

Why the 10 mA balanced current loopis less sensitive to interference

A balanced current loop is less sensitive to interference than a non balanced sys-

tem because when noise is applied to the line both wires are effected the same

way and hence the differential between the two wires that encodes the data is

maintained. See drawing.

1. Data in to the transmitter.

2. Voltages on lines A and B are inverted depending on data hence driving

current either one way or the other through the circuit.

3. Common mode interference on the line.

4. Noise superimposes onto data stream.

5. Data is received and decoded unchanged from when it was transmitted (1)

+

-

+

-

A

B

2

1

3

4

5

TD

RD

Wire A

Wire B


17/47

26

Unfortunately, all of our problems are not solved once we have identi-

fied the right method of transmission and the right interface to use. The

biggest problem of data communications still remains to be tackled

external interference. Interference can cause data losses, transmission

errors and, worst of all, it can damage equipment. Advances in com-

puter technology have led to the use of smaller circuits and units which

are operated at lower voltages. This is very favourable in terms of ener-

gy conservation and limiting the heat that is given off by the equipment

but it also means that the equipment is more sensitive and vulnerable

to power surges. Surveys have shown that up to 70% of all interference

during data transmission is due to defective installation or interference

from sources near to the equipment, such as neighbouring machinery

or cables. Only 20% is due to defects in the hardware or software. Therefore, most

of the sources of interference are located in the same room as, or near to the

equipment. The other sources of interference are external often originating from

nature itself in the form of lightning.

The most significant sort of interference is a type known as transients. These are

short but high pulses of electrical current in the network. The service life of com-

puter equipment which is exposed to transients, from 1 000 V up to 10 kV, for a

period of a few milliseconds, is bound to be short.

Lightning, machinery and fluorescent lights

We know that very high levels of current are discharged when objects are direct-

ly hit by lightning. The discharged electricity can propagate and damage power

and PTT lines and worst of all, cause fires. Even if the line is not directly hit by

lightning, electrical equipment can still be affected because the current propagates

great distances along the power lines. Thats why a bulb flickers even when the

lightning is a long way off.

However, external transients are not only caused by lightning. Bulbs can also

flicker when an industrial plant in the neighbourhood starts up or shuts down its

machinery. This can also cause transients, or power surges, in the electricity net-

work.

As a rule, most transients originate right on your own premises. Machinery, equip-

ment and fluorescent lighting can cause power surges.

Switching off a fluorescent light can, for example, result in a power surge, due to

stored charge, of up to 3 000 V. Lightning striking an object near to a power line

could result in a surge of up to 610 kV.

The problemof interference

Transients


18/47

27

A normal communications circuit board in a computer is designed for 12 V.

Therefore, transients are often the reason why computer equipment breaks down

without apparent reason or why there is temporary interference in transmission.

Transients are the most common causes of disturbances. Only in 10% of the

cases is the interference due to failures, i.e. long-term undervoltage or power

surges and power cuts.

Interference in the network

One cause of interference during data transmission, which is almost as common,

is problems with earth currents. This is a particular problem if the network consists

of units which are electrically connected to different fuse panels. The return cur-

rent can take different routes, either the intended route, via earth to the fuse panel

that the unit is connected to, or it can travel via the serial ports signal ground to

another fuse panel.

Earth currents which move through the network can cause interference as well

as cause damage to the circuits driving the line.

A communications network consists of physical lines which are many metres

long. These lines are often laid together with other power and PTT lines. An elec-

tromagnetic field is created around each cable conduct-

ing current and this effects adjacent or crossing cables.

Together, these lines create large aerialswhich can pick

up different types of disturbance. Recommendations exist

for the installation of different types of cables in order to

minimize electromagnetic interference.

The simplest way of tackling both transients and the

problem with earth currents is to use modems with gal-

vanic isolationwhich electrically isolate the lines and units

without preventing the transmission of signals. Galvanic

isolationprevents transients, lightning and earth currentsfrom reaching the equipment.

In the above example, theearth currents can take thewrong route, via the com-puter networks signalground to a fuse panel, andthereby causing interference.

Earth currents

Zero

Protective ground

Zero

Protective ground


19/47

28

In any system, electronic signals are always prone to inter-

ference. Analogue signals tend to be more prone due to

the fact that all points on the signal carry information- i.e.

amplitude and frequency. Small disturbance to the signal

will cause the receiving system to interpret the signal dif-

ferently to that of the original transmitted signal and give

an incorrect output.

Digital signals are less prone to interference as there

are only two basic states; high or low. However due to

the interaction of the capacitance, resistance and induc-

tance of the cables used to carry the digital signals and

the effects of external noise, the information contained in the signals can be dis-

torted until the signal is unrecognisable.

Industrial signalsMany techniques have been developed to overcome the problems associated

with transmitting data over long distance. For example RS-422 and RS-485 are

designed to operate up to 1 200 m.

Even with busses specially designed to be used in long distance data commu-

nications applications, electronic interference will always be a problem and in

extreme cases the expected transmission distance will not be achieved.

There are many practical steps that can be taken to improve the susceptibility

of the signals to interference.

Isolation

In all data communications it is essential to galvanically isolate equipment and net-works from each other to prevent the propagation of transients and other forms

of interference that can cause transmission errors or damage equipment.

There are several methods ensuring isolation for example relays, transformers,

isolation amplifiers and optocouplers. Incoming transients can also be removed

using protective components such as varistors, capacitors, RC filters and zener

diodes.

Westermo use optocouplers for isolation in their receivers. Optocouplers pro-

vide better performance than for example differential amplifiers. Transformers pro-

vide isolation on the power source and varistors and zener diodes are used to sup-

press transients.

Interference suppression

+

+

multidrop

Fast balancedcommunication


20/47

29

Shielding

Shielded or double shielded cables can be used to increase the resistance to exter-

nal interference. Under normal circumstances the cable shield should only be

connected to ground at one end.

In some extreme circumstance where high frequency noise is a problem, the

cable can be connected to ground at both ends. However this method introduces

a potentially larger problem if there is a potential difference between the points.

If this is the case current will start to flow through the shield of the cable and carry

with it any noise on the ground plain.

As an alternative it is sometimes possible to connect one end of the shield to

ground and the other to ground via a small value, high voltage capacitor

Telephone modems and interference

For very long distance data communication PTT modems are used. These devices

rely on analogue signals to carry the digital information over the PTT network . Asstated earlier the analogue signals are prone to the effects of noise. To counter

these effects error checking algorithms and filtering are used.

Optical fibre cables

An increasingly common means of reducing the effects of noise in an industrial

environment is to use fibre optic cables.

Data transmission via fibre-optic cable tends to be insensitive to electrical inter-

ference and provide total isolation between systems, so making it an excellent

media for industrial data communications.

There are limitations as to the maximum distance that can be achieved. This is

due to the amount of available power at the transmitter, the loses within the cable,the type of fibre and the sensitivity of the receiver.

Data communications toRS-422 for 10 Mbit.

Data communications toRS-232/V.24

CMW=0

Improved transmission with amodem and twin-screenedcable, with each screengrounded at one end.


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30

Designing networks

A local network for data communications is usually called a LAN or

Local Area Network. Whether the network is within one building or con-

nects several buildings, it is considered to be local because it is owned

and operated by the user. The local network can, in turn, be connected

through a leased line or by calling PTT circuits, with a public network,

such as a regional, national and global network, which is sometimes

called a WAN, Wide Area Network or a MAN, Metropolitan Area

Network.

A local network can consist of data communications for office appli-

cations or for industry, hospitals, mining operations or traffic control.

The particular network design which is selected, also known as the

topology, is important since it is a long-term infrastructure which must

handle and transport important data without problems. It must also be possible to

modify or expand the network when necessary.

Serial point-to-point

Point to point data communications, i.e. between two communicating units on

a line, is the most common application. This is the case in simple applications, such

as computer-printer, as well as in more complex applications, where each user

communicates via his own line for security reasons. The common RS-232 stan-

dard interface is not recommended for distances exceeding 15 metres. Therefore,

modems are used as repeaters and as protection against transients for communi-

cation at distances of up to 18 kilometres.

Star network

A network with many point-to-point connections is called a star network. Each unitcommunicates, over its own line, with the central processing unit at the hub. The

advantage of a star network is that it is highly reliable. If one line is down, the other

lines are not affected. The disadvantage is that since more cable has to be used,

this kind of network is more costly. Furthermore, all communication must

take place via the central processing unit.

Point to point

Star network


R D T D D C D R T S C TS D T R R D

ACCESSMX-14

C H A N N E L 3

PWR

RD

TD

D CD 2

D CD 3

D CD 4

C H AN N E L 2 P O W ER

LD-01 DC

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R - T + T - T + T - R + R -


R D T D D C D R T S C T S D T R R D

ACCESSMX-14


R D T D D C D R T S C T S D T R R D

ACCESSMX-14


R D T D D C D R T S C TS D T R R D

ACCESSMX-14

C H A N N E L 3

PWR

RD

TD

D CD 2

D CD 3

D CD 4


LD-01 DC

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R - T + T - T + T - R + R -

C H A N N E L 3

PWR

RD

TD

D CD 2

D CD 3

D CD 4

C H A NN E L 2 P O W ER

LD-01 DC

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R - T + T - T + T - R + R -

C H A N N E L 3

PWR

RD

TD

D CD 2

D CD 3

D CD 4

C H A NN E L 2 P O W ER

LD-01 DC

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R - T + T - T + T - R + R -

C H A N N E L 3

PWR

RD

TD

D CD 2

D CD 3

D CD 4


LD-01 DC

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R - T + T - T + T - R + R -


22/47


23/47

32

Communications layers

Besides transmitting data (characters, numbers, commands) data communications

equipment must also handle a large quantity of peripheral data which is neces-

sary for communication to take place. For example, such information includes data

on the transmitter and the receiver (addressing), on what is to be transmitted, on

how it is to be transmitted and on the form into which it is to be transformed and

sent.

For this information to be processed correctly, independently of the type ofcommunication device used and manufacturer, a reference model, known as the

OSI (Open Systems Interconnection) model, exists which defines seven different

layers of data communications.

1. The physical layer, which defines the electrical and mechanical interface.

2. The data link layer, for control and monitoring of the data traffic.

3. The network layer, for handling addressing, paths, performance etc.

4. The transport layer, which handles point-to-point communication, and

also checks that it is free from errors.

5. The session layer, which controls the data flow and buffering.

6. The presentation layer, which is responsible for code transformation, for-

matting, conversion and encryption.7. The application layer, which handles information for the application,

secrecy and identification etc.

The OSI model is not a standard. Instead it is a reference model for the develop-

ment of different standards.

Industrial fieldbus systems

Ethernet-type buses are most commonly used for office communications and

computer-to-computer communications. This standard is suitable for the type

of transmission which takes place between several users.

Industrial applications have different requirements. Industrial communications

requirements are often less complex while the needs for reliability and perform-ance are higher. At the same time, communication must be carried out in a harsh-

er environment where there is a high chance of interference. Furthermore, the

communication distances are long and many different interfaces are involved.

The specification determines which network design is selected and which com-

munications protocols are used. The specification also determines which fieldbus

system or systems are most suitable. Fieldbus systems, such as the simple ASIand

Cansystems, handle simple communications with simple I/O devices. The more

complex Interbus-Sand Profibussystems handle communications between one or

several control systems and between computers and remote-controlled modules.

Furthermore, more or less standardized fieldbus systems exist as well as a num-

ber of control system buses which are unique to each supplier.Good industrial data communications systems combine different fieldbus stan-

dards (see examples in Applications).

An industrial system which is commonly used is the multidrop networkwhere

a main computer communicates with a large number of terminals, units, transmit-

ters or measurement systems.

OSI Open SystemsInterconnection

Field Bus Systems

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

7

6

5

4

3

2

1

Communication takes placevia the different OSI layers.

67490 89

ASI

CAN

Profibus

Interbus S

Ethernet


24/47

33

Communications productsfor industrial networks

The building blocks of a communications network consist of physical cables, the

computer hardware carrying out the communication, computer software as well

as a number of communications products which enables the data to be transmit-

ted reliable.

Transforms and packs digital data into signals which are defined

for the media which is to transmit the data (4-wire, fibre optic

cables etc).

Amplifies and restores signals for long-distance transmission.

RS-422 and RS-485 allow connections to be made at a maxi-

mum of 1 200 m with a maximum of 32 loads. By installing a

repeater, you can add a further 1 200 m and 31 load segments

to the network.

Used to save wiring. For example, instead of installing 16 con-

nections with modems and cables, the same function can beobtained using two multiplexers and one line.

The multiplexer, recreates the 16 channels

and each channel can communicate as if it

was an independent permanent connection

with full-duplex transmission and an optional

transmission speed.

A unit which provides galvanic isolation to isolate connected

devices from each other, often via optical transmission. An iso-

lator does not function as a modem. (With a few exeptions all

of Westermos products are equipped with galvanic isolation).

Used to enable devices with different interfaces to communicate

with each other, e.g. RS-422/485 to RS-232/V.24 or from fibre

optic cables to RS-422/485 and RS-232/V.24.

A modem with three or four channels, where each channel has

a separate modem function. Used to create multidrop networks.

A router is used to separate different segments in a network to

improve performance and reliability.

An intelligent connection between two local networks with the

same standard but with different types of cables.

An intelligent connection between a local network and external

networks with completely different structures.

Multiplexer

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA N NE L 2 P OW ER12-36VDC

1 2 3 4 5 - +R+ R-T+ T-

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R- T + T - T + T- R + R-

CHANNEL3

PWR

RD

TD

DCD2

DCD3

DCD4

C HA N NE L 2 P OW ER12-36VDC

1 2 3 4 5 - +R+ R-T+ T-

1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 9

R + R- T + T - T + T- R +

R-

Modem

Repeater

Multiplexer

Isolator

Interface

converter

Line-sharing

device

Router

Bridge

Gateway


25/47

34

Data communications overthe telecommunications network

An important complement to local data communications is external

communications. This is for example the possibility of connecting up to

external databases in order to search for information on markets, to find

out stock-exchange rates or to access public registers etc.

The number of databases which can be accessed has increased enor-

mously and these databases are linked through global networks. For

example, you might connect up to a national database and end up in

an international financial database in New York.

External data communications can be justified for many reasons. For

example, the telecommunications network is one way of gaining access

to your office and your computer when you are engaged in field work.

Dial-up connections

External communication via the telecommunications network means that you call

the receivers modem which answers. Both modems then set up a carrier over thePSTN line. The carrier is a signal which the modems listen for. When the two

modems connect, it means that they can hear each others carriers and lock into

or synchronise on the signals.

Transmission speeds over the telecommunications network have increased, and

nowadays, 2 40056 000 bit/s are common. It is not only the modem itself which

determines the speed, but also the PSTN line. The quality of the line is largely

affected by the distance, the number of stations and relays. Most high-speed

modems can automatically reduce their speed in order to maintain a high trans-

mission quality.

Within telephone communications, it is very important to comply with the

accepted standards, since the transmitter and receiver have no way of knowing

what devices are being used on either side. The transmission speeds which are

used in certain standards are shown in the table on the left.

Telephone modem

Standards and speeds

V.21 300 bit/s V.22 1 200 bit/s V.22 bis 2 400 bit/s V.32 9 600 bit/s V.32 bis 14 400 bit/s

V.34 28 800 bit/s V.34 bis 33 600 bit/s V.90 56 000 bit/s


26/47

35

The language of the telephone modem

In order to communicate via the telecommunications network, in addition to a

standardized modem, you need a terminal or a computer with communications

software installed which uses the serial port of the computer.

Instructions are required to control the telephone modem. Hayes Micro-com-

puter Products has developed such a language of instruction which has become

the standard and which is known as the Hayes commands. These are a set of

instructions to the PSTN modem which can either be manually sent from the com-puter, via the keyboard, or which are automatically sent from the communications

program when different tasks are carried out.

Error correction and data compression

Most telephone modems transmit data synchronously between the modems,

even if the communication between the computer and the serial port is asyn-

chronous. In order to monitor the transmission, the data can be divided into

blocks and each block can be assigned a check sum. If there is interference, the

check sum will be incorrect and the receiver will request re-transmission, also

known as an ARQ (Automatic Repeat reQuest). The most common method of

error correction using the ARQ approach is in accordance with the ITU-T V.42error correction standard which is supported by both MNP (Microcom

Networking Protocol) and LAPM (Link Access Procedure for Modems).

On-line services

The telephone modem can be used to connect to other computers, directly

or indirectly via a network. The internet has rapidly expanded into the largest

world-wide network with millions of users. The TCP/IP protocol used on the

Internet allows electronic mail, discussion groups, world wide web (databases,

information and marketing), file transmission and retrieval, telephony, video con-

ferences, chat etc. However, there are also other networks and services available

via a modem such as MEMO, Lotus Notes, CompuServe, etc. The PSTN modem

can be used for distance working by connection to a company's computer.

Superhighways

Intensive work is being conducted on creating international standards and on con-

structing "Superhighways" for data communications. Fast digital high-speed net-

works, such as broadband ISDN, can rapidly convey large quantities of informa-

tion containing data, sound and graphics, across the continents. The huge capac-

ity of the cable TV networks can also offer a new resource for faster data traffic.

However, it is important to remember that the

foundations of such efficient highways must first

be laid locally, through efficient local data com-

munications. With such a vital infrastructure in

place, it will then be possible to open up and

access national and global networks.

The fastest communicationsroute is always in what isknown as direct mode.

Every stage of compression,error correction and buffer-ing causes a time delay.

ARQ and MN PMNP Level 1:asynchronous protocol,half duplex

MNP Level 2:asynchronous protocol, fullduplex. Data divided intoblocks. Actual data transmis-

sion speed somewhat lowerthan normal.

MNP Level 3:synchronous protocol, fullduplex. Data in blocks. 10%higher speed with error-freetransmissions.

MNP Level 4:data in blocks, block sizeaccording to line quality.Smaller blocks than Level 3

which results in a 20% fastertransmission rate, when freefrom interference.

MNP Level 5:as in Level 4, but with datacompression which results inup to double the speed.

M NP Level 10:a further development ofMNP 5 which monitors theline dynamically and guar-antees error-free transmis-

sion.

COMP ERRORCORR

ERRORCORR

COMP

BUFFER

BUFFER

DSP

DSP


27/47

36

Analogue Leased Lines

A leased line is provided by a telecom company and provides you with

direct connection between either two sites or multiple sites. Unlike a

dial- up line the leased line is available at all times, but still can go

through exchanges.

There are many configurations of leased lines available. Typically

these will be 2-wire or 4-wire circuits.

On 2- and 4-wire leased lines, pairs of modems are used to provide

point to point full duplex communications. These modems will typical-

ly use the V.22bis, V.32bis or V.34bis modulation standards to provide

connections between 2 400 bit/s and 33 600 bit/s. One modem will be

set to originate and one modem to answer. Ones a connection is estab-

lished it will remain in place until power is removed or the line is bro-

ken.

Leased Line V.23V.23 is an old standard that uses FSK modulation to provide communication cir-

cuits up to 1 200 baud on 2 or 4 wire circuits.

One advantage is that V.23 can be used in multidrop configurations, either on

dedicated wires or on specially provided multipoint circuits. V.23 modems are nor-

mally terminated with a 600 Ohm impedance. This restricts the number of mul-

tidropped units on a dedicated circuit to about 6 unless these terminators are

removed or line equalisers are employed.

On V.23 multipoint systems only one modem can have an active carrier at one

time so communications normally have to be controlled by an external control sig-

nal such as RTS.

The Westermo solution for V.23

Westermos V.23 modem allows speeds up to 1 200 baud. Both the 600

impedance and complex line impedances are supported by this modem.

The carrier, transmit, and input levels are all adjustable. To avoid problems with

the line being locked by a faulty unit the modem automatically disconnects from

the line when there has been no activity for a period of time.

A built-in switchable termination makes it possible to connect many more

modems on to one line than the V.23 standard describes.


28/47

37

Westermos products providefor secure transmission and

protection against interference

Balanced transmissionfor secure data communications.

Balanced transmission results in error-free transmission over long

distances. Both the Westermo 10 mA balanced current loop and

RS-422/485 use this technique.

Reliability that shows

Surveys show that 70% of the interference during data transmission is

due to internal interference from installations near to the equipment(transients, earth currents, electromagnetic fields etc.). 10% is due to

lengthy interference in the electricity distribution network (especially in

rural areas) and only 20% is due to defective software or hardware.

Therefore, a network with Westermos modems which are equipped

with galvanic isolation provides protection against the 70% of the inter-

ference which is due to transients, earth currents and electromagnetic

fields. Furthermore, our modems protect your equipment from light-

ning and external transients. Under are the three symbols which are

found on modems equipped with:

Galvanic isolationwhich through optocouplers electrically isolates the net-

work from your computer equipment. The optocouplers have an isolation

voltage of up to 4 000 V and the optical signal transmission takes placevia LEDs to transistors which detect the signals. Consequently, the optical

isolation of the modems prevents interference and earth currents from

propagating or damaging the equipment.

Transient protectionTransient protection in the network consists of varis-

tors and on the line, fast two-way Zener-diodes which effectively divert

the transients to protective ground, thereby protecting your equipment

against power surges.


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38

ISDN, Integrated ServicesDigital Network.

What is ISDN?

ISDN (Integrated Services Digital Network) is the all-digital equivalent

of the conventional telephone network PSTN (Public SwitchedTelephone Network), or POTS (Plain Old Telephone System).

ISDN technology is standardized according to recommendations of

the International Telecommunications Union (ITU),

Signalling

Instead of the phone company sending a ring voltage signal to ring the

bell in your phone (In-Band signal), it sends a digital packet on a sep-

arate channel (Out-of-Band signal). The Out-of-Band signal does not

disturb established connections, and call setup time is very fast. The sig-

nalling also indicates who is calling, what type of call it is (data/voice),

and what number was dialed. Available ISDN equipment is then capable of mak-

ing intelligent decisions on how to direct the call.

Services

Logically, ISDN consists of two types of communications channels: bearer service B-

channels, which carry data and services at 64 kbit/s; and a single D-channel, which

usually carries signalling and administrative information which is used to setup and

tear down calls. The transmission speed of the D-channel depends on the type of

ISDN service subscribed to. ISDN services available today can be divided into two

categories: Basic Rate Interface (BRI) service, which gives the subscriber access to two

B-channels and a 16 kbit/s D-channel; and Primary Rate Interface (PRI) service, which

provides a 64 kbit/s D-channel and 30 B-channels in Europe and most of Asia, in

North America and Japan the PRI service gives 23 B-channels.

When more than one device is connected through a single ISDN BRI connec-

tion, individual devices are distinguished from one another through the use of

multiple subscriber numbers, (MSN) whereby a different ISDN number is

assigned to each device served by the ISDN subscription.

Up to eight ISDN devices can be connected on single bus, as signals on the D-

channel automatically take care of contention issues, and route calls and services

to the appropriate ISDN device. Alternatively, a separate sub-address (SUB) value

can be used to differentiate between devices.


30/47

39

Speed

The modem was a big breakthrough in computer

communications. It allowed computers to commu-

nicate by converting their digital information into an

analogue signal to travel through the public phone

network. There is an upper limit to the amount of

information that an analogue telephone line can

hold. Currently, it is about 56 kbit/s. Commonly

available modems have a maximum speed of 56

kbit/s., but are limited by the quality of the analogue connection and practically

4550 kbit/s is reached.

The high throughput offered by ISDN 2 x 64 kbit/s, rapid call setup, less than 2 s

and the high level of accuracy inherent to digital transmission, are the main attractions

to ISDN technology.The two channels can be bundled to give a virtual 128 kbit channel or used as

two separate channels enabling simultaneous data and voice calls.

ISDN Components/Interfaces

ISDN components include terminals, Terminal Adapters (TA), Network-

Termination devices (NT), line-termination equipment (LT), and exchange-termi-

nation equipment (ET). ISDN defines terminals of two types. Specialized ISDN ter-

minals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals,

such as DTE are referred to as terminal equipment type 2 (TE2). TE1s connect to

the ISDN network through a 4-wire, twisted-pair digital link. TE2s connect to the

ISDN network through a TA. The TE2 connects to the TA via a standard physical-

layer interface such as RS-232/V.24 or RS485/V11.

Beyond the TE1 and TE2 devices, the next connection point in the ISDN net-

work is the network termination type 1 (NT1) or network termination type 2(NT2)

device. These are network-termination devices that connect the 4-wire subscriber

wiring to the conventional 2-wire local loop. In North America, the NT1 is a cus-

tomer premises equipment(CPE) device. In most other parts of the world, the NT1

is part of the network provided by the carrier. The NT2 is a more complicated

device that typically is found in digital private branch exchanges(PBXs) and that

performs Layer 2 and 3 protocol functions and concentration services. An NT1/2

device also exists as a single device that combines the functions of a NT1 and a

NT2.


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Layer 1 Physical layer

The signalling between the telecom switch and the user is according to the U-inter-

face and the signalling in the user building is normally according to the S-interface.

The U-interface uses frames of 240 bit length. At a rate of 160 kbit/s, each frame is

therefore 1.5 ms long. Each frame consists of:

Frame structureU-Frame when 2B1Q coding

ISDN specifies a number of reference points that define logical interfaces between

functional groupings, such as TAs and NT1s. ISDN reference points include the

following:

R---The reference point between non-ISDN equipment and a TA.

S---The reference point between user terminals and the NT2.

T---The reference point between NT1 and NT2 devices.

U---The reference point between NT1 devices and line-termination equipment

in the carrier network. The U reference point is relevant only in North America,

where the NT1 function is not provided by the carrier network.

40

TE1

TA

S/TUVNT-1

TE1

TE2R

Network Termination.Used to convert U to S/T interface

Supplied in Europe by TelcoISDN equipment

that can connect directlyto ISDN line

S/T interfaceTermination point in Europe

ISDN equipment thatcan connect NOTdirectly to ISDN lineEquipment at phone company switch

Used to connect TE2devices to ISDN line

Standard PSTN equipmenthas an R interface

Switch

O/M W12 W11 W1W2S S

DB1B2

8 bits 8 bits 2 bits

240 bits, 1.5 ms

12 words, 216 bits

S = Synchronitation pattern 18 bitsO/M = Operation and Maintance 6 bits


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41

Frame Format S interface

ISDN physical-layer (Layer 1) S frame formats differ depending on whether the

frame is outbound (from terminal to network) or inbound (from network to ter-

minal). Both physical-layer interfaces are shown below.

The frames are 48 bits long, of which 36 bits represent data. The bits of an

ISDN physical-layer frame are used as follows:

Layer 2Data Link Layer

The ISDN Data Link Layer is specified by the ITU Q.920 through Q.923. All of

the signalling on the D channel is defined in the Q.921 spec.

Link Access Protocol D channel (LAP-D) is the Layer 2 protocol used. This is

almost identical to the X.25 LAP-B protocol.

Here is the structure of a LAP-D frame:

Flag (1 octet)

This is always 7E16 (0111 11102)

Address (2 octets)

SAPI (Service access point identifier), 6-bits (see next side)

C/R (Command/Response) bit indicates if the frame is a command or a

response

EA0 (Address Extension) bit indicates whether this is the final octet of the

address or notTEI (Terminal Endpoint Identifier) 7-bit device identifier (see next side)

EA1 (Address Extension) bit, same as EA0

A = Activation bitB1 = B1 channel

(2 x 8 bits / frame)B2 = B2 channel

(2 x 8 bits / frame)D = D channel (4 x 1 bit / frame)

E = Echo of previous D bitF = Framing bitL = DC balancingS = S-channelN = Inverted F from NT to TEM = Multiframing bit

B1 B2 B1 B2L E

1 1 8 1 1 1 1 1 8 1 1 1 1 1 1 1 8 1 1 1

D A D M D S DE E E L

F

B1 B2 B1 B2D L F L L D L LF L D L L D L L D L

F N

48 bits 250s

NT to TE

TE to NT

Flag Address Control Information CRC Flag

8 7 6 5 4 3 2 1

SAPI (6 bits) C/R EA0

TEI (7 bits) EA1


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The figure above gives aview of usage of the SAPIfield,where SAPI = 0 isused for switch control andSAPI = 16 is used for pack-

age routing when X.31,X.25 over D-channel is used

Control (2 octets)

The frame level control field indicates the frame type (Information, Supervisory, or

Unnumbered) and sequence numbers (N[r] and N[s]) as required.

Information

Layer 3 protocol information and User data

CRC (2 octets)

Cyclic Redundancy Check is a low-level test for bit errors on the user data.

Flag (1 octet)

Closing flag, always 7E16 (0111 11102)

SAPI

The Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point

where Layer 2 provides a service to Layer 3.

TEIs

Terminal Endpoint Identifiers (TEIs) are unique IDs given to each device (TE) on

an ISDN S/T bus. This identifier can be dynamic; the value may be assigned stat-

ically when the TE is installed, or dynamically when activated.

42

SAPI value Related layer 3 or management entity

0 Call control procedures

111 Reserved for future standardization

12 Teleaction communication


16 Packet communication conforming to X.25 level 3 procedures


63 Layer 2 management procedures

All others Not available for Q.921 procedures

TEI Value User Type

063 Non-automatic TEI assignment user equipment

64126 Automatic TEI assignment user equipment127 Broadcast to all devices

Package data viaD-channel SAPI-16

Package data viaB-channel

TE ET PH TE

Switch controlSAPI-0


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43

These are the fields in a Q.931 header:

Protocol Discriminator (1 octet)

Identifies the Layer 3 protocol. For a Q.931 header, this value is 0816.Length (1 octet)

Indicates the length of the next field, the CRV.

Call Reference Value (CRV) (1 or 2 octets)

Used to uniquely identify each call on the user-network interface. This value is

assigned at the beginning of a call, and this value becomes available for another

call when the call is cleared.

Message Type (1 octet)

Identifies the message type (i.e., SETUP, CONNECT, etc.). This determines what

additional information is required and allowed.

Mandatory and Optional Information Elements (variable length)

Are options that are set depending on the Message Type.

CAPI

COMMON-ISDN-API (CAPI) is an application programming interface standard

used to access ISDN equipment connected to basic rate interfaces (BRI) and pri-

mary rate interfaces (PRI). By adhering to the standard, applications can make use

of well defined mechanism for communications over ISDN lines, without being

forced to adjust to the hardware vendor implementations.

To reflect on the actual situation it can be stated that the international protocol

specification is finished and almost every telecommunication provider offers BRI /PRI with protocols based on Q.931 / ETSI 300 102. CAPI Version 2.0 was devel-

oped to support all Q.931 based protocols.

CAPI is designed to be the base of a whole range of new protocol-stacks for net-

working, telephony and file-transfer and is embodied in European standard ETS

300 838 Integrated Service Digital Network (ISDN); Harmonized Programmable

Communication Interface (HPCI) for ISDN.

Layer 3Network Layer

The ISDN Network Layer is specified by the ITU Q.930 through Q.939. Layer 3

is used for the establishment, maintenance, and termination of logical network

connections between two devices.

Information Field Structure

The Information Field is a variable length field that contains the Q.931 protocol

data.

Information Field

8 7 6 5 4 3 2 1

Protocol Discriminator

0 0 0 0 Length of CRV

Call Reference Value (1 or 2 octets)

0 Message Type

Mandatory & Optional Information Elements (variable)


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44

Radio communications

Wireless data communications via a radio modem provide a means of

maintaining communications with

remote units measuring stations

external buildings and unmanned installations

temporary or mobile sites

The purpose may be that of gathering test readings, controlling or reg-

ulating equipment or recording various kinds of alarms.

Radio communications technology and how to plan, dimension and

cope with noise and interference, differ greatly from local communica-

tions in a data network.

How it worksCommunication equipment is provided using a radio modem that converts the

data signal into radio waves for a specific channel with a specific bandwidth. The

data signal may require some form of signal processing or filtering before it can

be transmitted by the radio channel. In addition, the signal is modulated (by a

modem) to a correct carrier frequency and can be transmitted via a radio link to

the receiver. Irrespective of whether the source is analogue or digital, the trans-

mission is nearly always analogue. The receiver equipment decodes and recon-

structs the original signal.

The available frequency range for radio communications is limited and regu-

lated by an international agreement (ITU).

Radio waves are propagated in the atmosphere in the layer between the iono-

sphere and the surface of the earth. Communication conditions can vary greatly,

depending on the frequency band, ranging from the longest wavelengths of up

to 1 000 metres in the ELF band to shortest ones of l0 mm in the EHF band.

Radio modems operate in the UHF band at around 440 mhz. The UHF band

between 300 and 3 000 mHz also contains radar, radio, TV, NMT mobile teleph-

ony, mobile radio, satellite communications, amateur radio and both GSM and

wireless telephones.

Frequency band

ELF 3003000 Hz

VLF 330 kHz

LF 30300 kHz

MF 3003000 kHz

HF 330 MHz

VHF 30300 MHz

UHF 3003000 MHzSHF 330 GHz

EHF 30300 GHz


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45

Attenuation and noise

A propagated radio wave is affected by both the ground and the air layers

through which it passes. In the frequency bands in which radio modems operate,

with wavelengths of around 1 metre, there are many objects such as hills and

buildings that can cause a radio shadow (cf. Mobile telephony). This is in addition

to intermittent interference from other equipment. Such interference caused by

objects is termed shadow or interference fading, and causes signal attenuation or

distortion.

The signal reaching the receiver is often very weak compared with the trans-

mitted signal but this in itself does not imply any quality deterioration of commu-

nication. What may cause problems is interference outside our control, noise that

is added to the signal. This not only occurs in the receiving equipment but also

exists in the form of thermal noise (thermal motion of particles), atmospheric noise

(electrical phenomena such as lightning), cosmic noise (incipient radio-frequencyradiation from the sun or other so-called galactic noise) and locally generated

noise (electrical equipment in the receivers surroundings).


37/47

Terminology

When discussing radio communications and antenna it is vital to under-

stand a few basic terms and expressions. The first basic formula to

remember relates frequency (f) to wavelength (l) by the equation: l [m] =

300 / f [MHz].

The Radiation pattern is the three dimensional radiation characteristics

of an antenna in 2 planes, the Electric field (E) and magnetic field (H).

The gain of the antenna is its capability to force radiation in a specific

direction in space at the expense of other directions. Gain is expressed in

dB compared to some reference: for example dBi refers to gain com-

pared to an isotropic antenna and dBd to a dipole antenna.

The polarization is defined as the plane of antennas electric field E and

can be vertical, horizontal, slanted or circular. Typically the antennas physical orien-

tation equals the antennas polarization. Orthogonal polarizations have a crosspolarization loss of 21 dB. In practice all the antennas in one system should use the

same polarization.

The Impedance of an antenna is its AC-resistance and reactance within the oper-

ating band. Nominal impedance of 50 ohms is a standard. The bandwidth is the fre-

quency range where the antennas characteristics like impedance, gain and radiation

pattern remain within the specifications. The commonly used term attenuation is

mainly related to feeders and radio propagation and is also expressed in dB.

Antenna circuit components

An antenna is an electromechanical device whose purpose is to radiate as effec-

tively as possible the power from the feeder in a specific manner.

A power splitter matches and combines multiple loads or sources and equally

splits the power between them without disturbing the characteristic impedance of

the system. Splitters are used in antenna arrays to combine multiple antennas or

in RF distribution harnesses. A feed-line is an interconnecting cable between radio

equipment and antenna. Feeders tend to be lossy components so the type has to

be carefully selected depending on the required length and operating frequency.

Lightning protectors can be inserted between the radio equipment and feeder to

help protect the radio against a lightning strike. Typically a lightning protector is a

DC short-circuited quarter wave stub. When interconnecting antenna circuit com-

ponents, impedance match has to be maintained in order to provide ideal flow of

power without additional losses due to reflections. Impedance match is common-ly measured as VSWR (Voltage Standing Wave Ratio) where a VSWR of 1:1 is

ideal and 1:1.5 is more realistic in practice.

46

Antennas and propagationin radio communications


38/47


39/47

48

The Echelon Corporation through its LonWorks technology has provid-

ed a platform for developing open control systems offering distributed,

network intelligence.

A LonWorks network is usually a peer-to-peer network where each

device controls its own actions and shares information with its neigh-

bours as needed to control the entire system.

Normally the nodes will exchange datarather than commands. In this

approach, application data items such as temperatures, pressures, states,

and other data items can be sent to multiple devices, each of which may

have a different application for using that data item. These data items

may be considered as global data variables on the network and are in

LonWorks technology referred to as Network variables. If a device

updates the value of a network variable this new value will automatically be prop-

agated on the network so other devices may be aware of the new value.

Interoperability is the keyword in the LonWorks technology. It is a condition that

ensures that multiple devices (from different manufactures) talk the same language

and understand each other on the network.

To achieve interoperability it is not enough to just be on the same network, have

the same type of transceiver or be able to send and receive network variables. The

nodes must also be able to understand the content of the network variable. For

instance the nodes need to know if the temperature is in degrees, Fahrenheit or

Celsius or if a flow value is in litre/sec or ml/sec. Hence, a condition for interoperabil-

ity is that the data items are represented in a standardized way. This has been accom-

plished by the LonMark Association, an independent organization of LonWorks

developers, system-integrators, and end-users. Members in the LonMark Associationwork together to define the standards, ensuring interoperability between LonWorks

devices from multiple manufacturers.

In LonWorks technology, interoperability is promoted by the use of Standard

Network Variable Types (SNVT). For a SNVT type network variable, the unit, reso-

lution and range are defined. For example, if a SNVT_speed network variable is

used every LonWorks interoperable node knows that the unit is m/s, the resolution

is 0.1 m/s and the range is 0 to 6553.5 m/s. Today there are more than 140 dif-

ferent SNVT:s.

LonWorkstechnology


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49

1 2 3

4 5 6

7 8 9

0

Open

1020

30

4050

5

4

3

2

1

BV

1 4 8 5 4 6 0

SNVT_stateSNVT_switch

SNVT_temp

SNVT_lux

SNVT_time_stamp

SNVT_alarm

The most prevalent transceiver is the FTT-10A free topology transceiver. It com-

municates at a data rate of 78 kbit/s over a twisted pair cable in any topology

including star, bus ring or combinations. The convenience of the free topology

makes it desirable for the interconnection of sensors and controllers in todays

control networks. The added benefit is a non-polarized interface eliminates one ofthe biggest problems in installation today reversing the communications wires.

Offering the same flexibility in topology, the LPT-10 link power transceiver can

be powered from the same pair as it communicates on.

There is also a 1 250 kbit/s twisted pair bus transceiver as well as the PLT-22

power line transceiver which has advanced signal processing, error correction and

an unique dual carrier frequency feature making it possible to communicate very

effectively in the presence of electric noise, appliance motor noise, dimmers, PCs

and televisions.

LonWorks a

Documents

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