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, w
Trang 3Digital Interfaces and Bus Systems for Communication
Practical fundamentals
Frank Blasinger
Manfred Schleicher
Trang 4Digital 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 automati-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
Trang 51 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
Trang 62.13 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
Contents
Trang 7JUMO, FAS 603, Edition 07.02
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
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
signals
A measurement, a temperature for example, is converted into a signal corre-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
Trang 81 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
signals
The term “digital” is derived from the word “digit” and comes originally from 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