Field Bus Networking

Flexible and Secure Field Bus Networks

PROFIBUS - BITBUS - DIN-MESSBUS - SINEC L2 - MODBUS - ...

Index for this page:
Introduction  
In process automation and industrial communication, field busses build the basis on the lowest layers of automation technology. Todays technology cannot be thought without them anymore and they will gain even more significance with the increasing decentralization in automation. On of the most widespread interface norms is the EIA-Standard 1 RS-485, which describes the electrical properties of the field bus. Importance of RS-485
It offers good EMV properties (noise immunity, noise radiation) and cost effective implementation possibilities and is not expected to be replaced by other systems in the forseeable future. Its protocol independence makes a broad variety of applications possible. EMC Properties
Tody several different field bus systems have been implemented and established upon the RS-485 interface norm, e.g. PROFIBUS, SINEC L2, BITBUS, DIN-MESSBUS, FIP, etc. RS-485 Field Busses
Limitations of Pure RS-485 Systems  
An important part of an RS-485 interface are the driver or coupling components. They define the major part of the physical properties und limit the capabilities of an RS-485 field bus system. Driver Component
  • max. line lenght 300 to 1200 meters (depends on transfer rate und bus norm)
  • max. 32 (128) 2 participants
  • only line topology network3
  
The figure shows the simple network topology resulting from the physical limitations of the RS-485 norm.  
 
Problems in Network Management  
This strict network topology cannot always be kept for modern field bus systems. Network management is made difficult with pure RS-485 systems.
Because ... 		 
  • building subnets is not possible
  • no tree or ring structures possible
  • the whole network is vulnerable to noise
  • maintenance is difficult
  
Single segments of the network cannot be switched off for maintenance purposes, making service and extension of existing networks a particularly cost intensive task.  

DIA-LOG RS-485 Repeater

Applications for Repeaters  
In order to raise the number of possible participants as well as the maximum line length, repeaters are used in RS-485 field bus systems. They are basically integrated into the network as a passive participant, enabling 31 further participants to be connected. To achieve this, they build a new bus segment which again can have the maximum line length. That way a limited tree topology is possible. Bus Segments

Tree Topology

Properties of the DIA-LOG Repeater  
On top of the basic properties of a general repeater, the DIA-LOG RS-485 star topology repeater offers further advantages:

It is fully cascadable.

An almost arbitrary number of DIA-LOG repeaters kann be connected in order to build new bus segments. This means practically 		 
  • an unlimited number of participants
  • unlimited line lengths
  
Suppression of Line Noise  
Using integrated active and passive components, line noise occuring even in smaller networks are suppressed.
  • Suppression of transfer disturbances
    through galvanic separation of the bus segments.
  • Surge protection
    of single bus segments preventing overvoltages
    from spreading and distroying participating devices.
  • active noise supression
    - through exact quarz-time signal detection
    - through reshaping the use-signal in terms of amplitude and time
 
Integration into Exitsting Networks  
The DIA-LOG RS-485 repeater can be seamlessly integrated into existing networks. No restructuring of the network is needed whatsoever.
  • automatic direction switching
    no need to adjust the transfer protocol
    or use control lines
  • no delay of the use-signal
    through internal run times below the baudrate
  • absolute code transparency
    the use-signal is not modified by the repeater
 

DIA-LOG RS-485 Star Repeater

Applications of Star Repeaters  
Star topology repeaters (couplers) are used to establish tree or star shaped networks respectively. Opposed to traditional repeaters they provide six rather than only two equal RS-485 interface ports. Star Topology
Properties of the DIA-LOG Star Repeater  
  • six equal RS-485 interfaces
    That way five new galvanically isolated line segments are created.
  • an optional RS-232 interface
    together with five RS-485 interfaces
  • an optional line echo
    some RS-232 protocols require an echo on the line
    (e.g. collision detection)
  • full cascadability
  • suppression of line noise
  • integration into existing networks
 6xRS-485

optional RS-232 converter

optional Echo

cascadable

Why Use DIA-LOG Repeaters and
Star Repeaters 		 
Economic Efficiency
DIA-LOG repeaters and DIA-LOG star topology repeaters can of course be uses together in the same network. Through the extended possbiilites for structuring the network and the integrated mechanisms for noise suppression the economic efficiency and flexibility of the network is significantly increased. 		Economic Efficiency
Transfer Security
The use of DIA-LOG repeaters does not only pay off when exceeding the maximum allowed number of participants (32) or line length (300-1200 m) for a field bus network. Through the built-in noise suppression mechanisms, the signal transfer quality can be improved or even only made possible for smaller or less branched networks. This is especially befenicial in critical sections (strong disturbances, networks close to the theoretical limits, potential differences between the participants). The integrated surge protection offers additional safety even under exceptional circumstances. 		Transfer
Security
Network Security
In a pure line topology, any bus failures (e.g. mechanical bus line damage) have an immediate effect on the entire network. In tree or star topologies, bus failures are limited to the affected network segment only. 		Network Security
Ease of Service
Through the separation into single network segments, service and maintenance, e.g. at finding an error, extending the network or doing test runs, are made a lot easier. 		Ease of
Service
Wide Area Network
Due to the full cascadability, the DIA-LOG repeater permits the creation of heavily branched networks with subnets of varying topology. Even distant buildings become interconnectable. In combination with an external lightning protection, aerial lines are supported. 		Wide Area
Networks
 

Advantages of DIA-LOG Repeaters and Star Repeaters

Problem Resolved by use of DIA-LOG repeater through...
compensation currents due to different earth potentials of the bus participants galvanic isolation of the bus segments
signal deterioration through external influences signal reshaping through the repeater with respect to signal duration
signal attenuation through damping signal reshaping through the repeater with respect to signal amplitude
unclean use-signal from participants signal enhancement by repeater
propagation of signal disturbances separation of the bus into segments
limited cascadability of traditional repeaters unlimited cascadability through protocol independence
 
Line Parameters, Line Length and Transfer Rate, Bus Termination  
a) Line Parameters
The bus line for the RS-485 data transmission is specified in EN50170 and has the following characteristics: 		Line Parameters
  Line Type A
(according to EN50170) 		Line Type B
(lesser wire) 		 
wave resistance (in Ohm) 135...165 100...130  
specific capacity (pF/m) < 30 < 60  
total line resistance (Ohm/km) 110 ---  
wire diameter (mm) 0,64 0,53  
wire cross-section (mm2) > 0,34 > 0,22  
 
   
b) line length, baud rate
the specific line parameters yield the following length extension of a bus segment (according to Profibus): 		Line Length

Baud Rate

Baudrate 9600 Baud 19,2 kBaud 93,75 kBaud 187,5 kBaud 500 kBaud 1,5 MBaud 12 MBaud
line length
type A 		1200 m 1200 m 1200 m 1000 m 400 m 200 m 100 m
line length
type B 		1200 m 1200 m 1200 m 600 m 200 m - -
 
   
c)Termination Resistor
In order to avoid reflexions, the line must be terminated with a resistor at both ends of the field bus. The transmission line can transmit the energy correctly only when a correct resistor is applied. The resistance of the terminator at every end of a network segment must match the wave resistance of the line, which is independant of the line length. If a type A according to EN50170-Profibus data transmission wire is used, the following combinations of resistances apply: 		Termination Resistor

Wave Resistance

RS-485 Busabschluss  
   
CAUTION!
This termination resistor ist not suited for e.g. line type B. For different line types, the termination resistor must always be adjusted.
Anmerkungen:
1. EIA = Electronic Industries Association
2. With newer driver components
3. Strictly speaking, the term 'Network' is not applicable to a pure bus system but is nevertheless used generally.