Digital Interfaces and Bus Systems for Communication: Practical Fundamentals

153 Pages·2001·4.468 MB·English

by  Schleicher M., Blasinger F.

Digital Interfaces
and Bus Systems
for Communication
Practical fundamentals
Frank Blasinger
Manfred Schleicher
Preface
Digital communication confronts us every day in modern process engineering:
- it is used for configuring and setting the parameters for microprocessor instruments
- serial bus systems, with minimum wiring requirements, are able to acquire a large amount of de-
centralized information and distribute it to the process equipment. Intelligent field and automati-
on devices can communicate directly with one another via a digital bus.
This book is intended as a step-by-step introduction to the subject of digital communications, for
practical engineers and those new to this field. The emphasis is on clarifying generalized topics, as
well as including some JUMO-specific applications.
In this revised edition, the material on bus systems has been extensively updated. The method of
operation of bus systems for which JUMO has field devices available is explained in a practical man-
ner.
Special thanks are due to all our colleagues, who helped us to prepare this book with their coope-
ration and professional input.
Fulda, March 2001
Manfred Schleicher Frank Blasinger
English translation of the 3rd. edition (revised)
M.K. JUCHHEIM GmbH & Co, Fulda
Copying permitted with source reference!
Part No. 00392023
Book No. FAS 603
Printed: 03.01
ISBN 3-935742-03-7
Contents
1 Basic principles of digital interfaces and networks ..................... 3
1.1 Analog/digital signals ......................................................................................... 3
1.2 Data encoding ..................................................................................................... 7
1.3 Types of data transmission .............................................................................. 13
1.3.1 Operating modes of a transmission medium ...................................................... 17
1.3.2 Speed of data transmission ................................................................................ 18
1.4 Media for data transmission ............................................................................ 20
1.4.1 Transmission quality and cable terminating resistance ...................................... 22
1.4.2 Modem ................................................................................................................ 23
1.5 Properties of various interfaces ...................................................................... 25
1.6 Networks and bus operation in automation ................................................... 38
1.6.1 Communication networks and levels .................................................................. 40
1.6.2 Fieldbus topologies ............................................................................................ 44
1.6.3 Centralized and distributed arrangement of automation devices ...................... 48
1.6.4 Access methods ................................................................................................. 49
1.6.5 Bus communication ............................................................................................ 54
1.7 OSI reference model ......................................................................................... 57
1.8 Network management ...................................................................................... 61
1.8.1 Functions of MAC and MAP ............................................................................... 61
1.8.2 The data structure ............................................................................................... 64
1.8.3 Error checking ..................................................................................................... 66
1.8.4 Connection of networks via repeater, bridge, router and gateway ..................... 68
1.9 Operation through application programs ....................................................... 72
1.9.1 Configuration software (setup program) ............................................................. 73
1.9.2 Project design software ...................................................................................... 75
1.9.3 Measurement display and operation using visualization/evaluation software .... 76
2 Important fieldbus systems .......................................................... 81
2.1 HART communication ...................................................................................... 83
2.2 ASI bus ............................................................................................................... 85
2.3 Bitbus ................................................................................................................. 86
2.4 CAN bus ............................................................................................................. 88
2.5 FIP bus ............................................................................................................... 93
2.6 Interbus .............................................................................................................. 94
2.7 LON bus ............................................................................................................. 96
2.8 Modbus .............................................................................................................. 97
2.9 P-Net .................................................................................................................. 98
2.10 PROFIBUS ....................................................................................................... 100
2.11 FOUNDATION fieldbus ................................................................................... 105
2.12 Ethernet ........................................................................................................... 107
Contents
2.13 Summary of the fieldbus systems ................................................................. 111
3 Organization of the data system for JUMO .............................. 113
3.1 The various communications options ........................................................... 114
3.1.1 Physical interfaces ............................................................................................ 114
3.1.2 Transmission protocols and fieldbus systems .................................................. 114
3.2 JUMO instruments with HART ....................................................................... 116
3.3 JUMO instruments with CANopen ................................................................ 118
3.4 JUMO instruments with LON ......................................................................... 120
3.4.1 The JUMO mTRON concept ............................................................................. 120
3.4.2 Network structure ............................................................................................. 122
3.4.3 Hardware architecture of a LON device ........................................................... 123
3.4.4 Communication procedure ............................................................................... 124
3.5 JUMO instruments with Modbus/Jbus ......................................................... 126
3.5.1 Physical interface and data flow ....................................................................... 126
3.5.2 Master/slave principle ....................................................................................... 127
3.5.3 Transmission mode ........................................................................................... 128
3.5.4 Format of the data blocks ................................................................................. 129
3.5.5 Connection via an interface converter .............................................................. 130
3.6 JUMO instruments with PROFIBUS .............................................................. 132
3.7 Checklist for fault-finding in serial interfaces .............................................. 134
4 Outlook ......................................................................................... 137
4.1 Standards and technologies in automation engineering ............................ 137
4.1.1 NOAH (Network Oriented Application Harmonization) ..................................... 137
4.1.2 OPC (OLE for Process Control) for communication ......................................... 138
4.1.3 Ethernet fieldbus equals system bus ................................................................ 141
4.2 Long-distance data transmission ................................................................. 143
4.3 Distributed systems ........................................................................................ 145
Index ............................................................................................. 147
1 Basic principles of digital interfaces and networks
This chapter deals first of all with some basic principles. The aim here is to
achieve this without over-complex theoretical or mathematical treatment.
Amongst other things, the basic facts about data encoding, types of data
transmission, properties of different interfaces, construction of networks etc.
are explained for practical engineers, who are increasingly faced with the sub-
jects of digital communication and bus systems in modern automation engi-
neering.
1.1 Analog/digital signals
In today’s automation engineering, more and more devices operate digitaIly.
This is in contrast with the more familiar analog measurement technology and
data transmission. This means that digital process instruments are increasing-
ly replacing analog type instruments in modern process control, part because
of the technological advances and the advantages offered. Nowadays, digital
transmission is even superseding the use of familiar standard signals such
as 4 — 20mA, 0 — 10V, etc. for the transfer of analog measurements. The
main features of different data transmission technologies are explained in
more detail below.
Analog A measurement, a temperature for example, is converted into a signal corre-
signals sponding to this temperature by a measuring device. The signal could be, for
instance, a 4 — 20mA current. Every temperature value corresponds clearly to
a value of electrical current. If the temperature changes continuously, the ana-
log signal also changes continuously. In other words, a characteristic feature
of analog transmission is that the amplitude of the selected signal varies con-
tinuously over time (see Fig. 1).
Fig. 1: Analog signal with continuously changing amplitude
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1 Basic principles of digital interfaces and networks
In automation engineering, such standard signals (4 — 20mA) are transmitted
in pure analog form as a normalized current signal. A temperature value is
measured by a Pt100 resistance thermometer, then converted into a current
proportional to the measurement by a transmitter, and subsequently transmit-
ted to a controller, indicator and recorder (see Fig. 2). By means of the current,
every change in measurement value is immediately recorded by each instru-
ment connected in the circuit.
Fig. 2: Analog signal transmission
In measurement engineering, the information content of an analog signal is
very limited in comparison with acoustic (sound) or optical (light) data trans-
mission. Apart from the advantages of an unambiguous, continuously repro-
duced measurement, with simultaneous supply of power to the measurement
recorder (e.g. two-wire transmitter), the information content of the analog sig-
nal consists only of the magnitude of the measurement, and whether or not
the signal is available at the connected device.
Digital The term “digital” is derived from the word “digit” and comes originally from
signals the Latin “digitus = finger”. Digital means sudden or step changes, i.e. a digital
signal does not vary continuously.
In the example of temperature measurement, this means that the analog mea-
surement is divided into specific value bands, within which no intermediate
values are possible. The values are read at fixed time intervals, the sampling
time. The task of conversion is carried out by an analog to digital converter (or
ADC). Here, the accuracy or resolution of the signal depends on the number of
value bands and the sampling frequency.
In the example shown in Fig. 3, samples are taken every 20msec, with a sub-
division into 10 value bands.
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Fig. 3: Digitized measurement signal
The digitized quantity has only the two values “high = 1” and “low = 0” and
must now be transmitted as a data packet by a microprocessor (µP)-transmit-
ter with an interface (see Fig. 4). The measurement is transmitted encoded as
a packet, and has to be decoded by the receiver (see also data encoding,
Chapter 1.2). The transmission mode can vary: by different voltage levels, light
pulses or a sequence of notes.
Fig. 4: Digital signal transmission
Digital data transmission has a number of advantages compared with conven-
tional analog technology. As well as the actual measured value, the field de-
vice, with its integral microprocessor, can also transmit additional information
(designation, dimensions, limit values, service interval etc.) to the automation
system. Furthermore, data can be transmitted to the field device. The fact that
several devices can communicate with the automation system over one cable
results in a reduction in materials and less expenditure on installation, hence
reducing the overall costs (see Fig. 5).
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Fig. 5: Analog and digital cabling
One disadvantage of the conventional technology using 4 — 20mA signals
with µP-devices is the unnecessary D/A conversion. A digitized value available
in the microprocessor must be converted to an analog current signal and then
digitized once again in the automation device for further processing.
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