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Abstract—

 

Index Terms— field devices: sensor and actuator of the plan. CAN: control area
network. TTP: time-triggered protocol FTT-CAN: Flexible Time-Triggered
Communication on CAN, TCN: Train Communication Network.

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I.    
introduction of fieldbus system

T

Fieldbus systems have been a part of automation
for up to twenty years. In fact, they have made automation what it is today.
The term itself was coined in process automation and refer to the process
field, besides, the origin intention was to replace the star like point
connection between the process control computers and the sensor or actuators
with a single, serial bus system- hence, the terms “FIELDBUS”.

But what is a fieldbus today, and what will it
be in the future? While the reduction of cabling may have been initially ( and
in some applications still is) the most important reason for the use of
fieldbus system go far beyond this.

 

 

II.    
definition of the fieldbus

The augmented term
fieldbus is consisting of two term which are “field” and “Bus”. The meaning of
“field” as defined in industrial world, is a geographical or contextual limited
area. The industrial state that the field is an abstraction of plant levels.

            Meanwhile,
the context “bus” is a well-known word in computer science as a set of common
line that electrically ( or even optically ) connects various unit (circuit) in
order to transfer the data among the circuit. The origin of the fieldbus was to
replace any point to point between the field 

 and their controller ( like PLC & CNC ) by
a digital single link on which all the information is transmitted serially and
multiplexed in time. In the most cases, the fieldbus transfer this information
in small sized packets in serial manner. Choosing the serial transmission has
many merits in comparison with other kids of transmission such as parallel
transmission. For instance, the sequential or serial transmission reduce  the total required number of the connecting
line over greater distance than that of the point to point or parallel
transmission. A set of rules must be defined in order to accomplish data
transfer between the units along the bus. This set of rules is called
Communication Protocol or just the Protocol. This is unlike the case of the
ordinary point-to-point transmission where any two connected entities send and
receive data from each other whenever the data is available. The protocol is
responsible for two important rules on the bus, the mechanism that any unit can
acquire or seize the bus (from the network terminology this means the way of
Medium Access), and the synchronization between those multi-units on the bus.

 The medium access protocol choosing is a
virtual steps in designing the DCCS. This is because of the odd nature of the
brusty traffic of such control system. So the existed LAN protocols such as the
token ring or the CSMA are not appropriate for the control applications. For
the token ring case, the more modes added in circuit, the longer the time each
node to wait till it can transmitted data. Needless to say that the CSMA
protocols, which are contention based protocols, will add randomness to the
overall respond time of the node data. This came from the way that the CSMA
allow only one node to transmit data if and only if no other node is seizing
medium.

The randomness occurs
when a collision happens as the nodes which encountered the collision will have
to wait to for a random time before it start again transmitting. A number of
solution have been proposed to extend the CSMA in order to enhance its
performance. For example, the collision avoiding, collision detection, and
collision resolution are all extensions to the CSMA protocol. None of this
solutions can fit directly for the control network application. In fact,
according to Raji in 1994, stated that, they need some other modifications like
the CAN fieldbus protocol.   

      

 

 

 

III.    
historical aspect

Although the term
” fieldbus” appeared only about 20 years ago, the basic idea of
field-level networks is much older. Still, the roots of modem fieldbus
technology are mixed 4. Both classical electrical engineering and computer
science has contributed their share to the evolution. This early stage is
depicted in Fig. 1. One foundation of automation data transfer has to be seen
in the classic telex networks, and also in standards for data transmission over
telephone lines. Large distances called for serial data transmission, and many
of these comparatively early standards still exist, like V.21 (data
transmission over telephone lines) and X.21 (data transmission over special
data lines). Various protocols have been defined, mostly described in state
machine diagrams and rather simple because of the limited computing power of
the devices available. Talking about serial data communication, we should
notice that the engineers who defined the first protocolsoften had a different
understanding of the expression “serial” and “parallel” than we have today. For
example, the “serial” Interface V.24 transmits the application data serially,
but the control data in a parallel way over separate control lines. In parallel
to this development, hardware engineers defined interfaces for standalone
computer systems: for memories and printers, but also for process control and
instrumentation equipment. The first application was to interconnect
measurement devices, and therefore standards like CAMAC (in nuclear science)
and GPIB (IEEE 488) were developed. To account for the limited data processing
speed and synchronization requirements, these bus systems had parallel data and
control lines. Later, the serial point-to-point connections of computer
peripherals were extended to support longer distances and finally also multi
drop arrangements. The capability of having a bus structure with more than just
two connections together with an increased noise immunity due to differential
signal coding eventually made the RS 485 a cornerstone of fieldbus technology
up to the present Then two big evolutions took place. First of all, computer
systems became more and more common, and soon a need for interconnections
between the computers emerged. Conventional telephone networks were no longer
sufficient to satisfy the requirements. Second, the big communication systems
of the national telephone companies gradually changed from analog to digital
systems. This opened the possibility to transfer large amounts of data from one
point to another. Together with an improved physical layer, the first really
power data transmission protocols were defined, such as X.25 or SS7.

 

Figure 1

 

IV.    
fieldbus and the network
references model

 Since it
is a network, it is important to know the  relation with the famous OSI7 reference model.
This section, will describe the fieldbus in terms of the layers of the OSI
model.          The definition of the OSI
model from Tanenbaum 96 who said “The OSI model organizes the protocols
used and the services provided by a general communication system in a stack of
layers”. In other words, the OSI is complete layerednetwork model in which
each layer does certain communication service. One can see in Fig. 2-a.  the reference OSI model layers and its
layers.     How it works? From the figure
we can see that if a node wants to send a data packet from the application it
must first call for the sending service of it Is application layer which in
turn will call the sending functions in the next layer, and so on till the data
is sent at the physical medium to the other node. This node will reverse the sequence
till the received data reaches the application layer of its node then to the
application which will used this data.

The list below the seven layers of the OSI model
with their functions: – 1- Application layer is to provide the services that
are required by specific applications. 
2- Presentation layer is responsible for the data interpretation, which
allows for interoperability among different equipment. 3- Session layer is
concerned with any execution of remote actions. 4- Transport layer is responsible
for the end-to-end communication control. 5- Network layer is concerned with
logical addressing process of nodes and routing schemes. 6- Datalink layer is
responsible for the access to the communication medium, and for the logical
transfer of the data. 7- Physical layer is concerned with the way that the
communication is done physically.

  Any
communication system that is based on the OSI seven layers will have both
merits of higher flexibility and compatibility with products from different
vendors. Nevertheless, the same OSI system (due to its complexity) has a
considerable overhead in both, the communications, and the processing.

There exist many protocols and services that are
laid in the 3-layerd hierarchy of the fieldbus network. This at the end will
lead to a great difficulty in evaluating one and unique international fieldbus
standard. In fact there are many different fieldbus protocols in the world.
There are large differences that can be found in the three layers of any
fieldbus protocol and their similar layers in another fieldbus protocol. The
designer of the DCCS communications system has multi-option solutions to
fulfill his system requirements. These requirements are varied from one
situation to another. In most cases the quality of services and the system
throughput in addition to the overall system performance are all a common
requirements any nearly all the DCCS systems. Also a fast response time is
usually required by the real-time computer controlled networks designers. 

 

In addition,  the different types of the fieldbuses that do
exist in the international market and their national origins and standards.

 

Modification to the MAP project was necessary as
the node implementation become more complex in order to support all the
services of the OSI reference model Almeida 99-1. The modification allowed
the short length control data packets, which occurs at high rates, to be
directly transmitted through the application layer to the datalink layer. Which
means that we abbreviated the OSI hierarchy into 3-layer model as can be seen
in Fig.2-b. The resulting fieldbus is referred to as a 3-layered Architecture.
These layers are: the Application layer, the Datalink layer, the Physical
layer.  One may assume that the other
four layers of the OSI model that are not available in the fieldbus hierarchy
have disappeared along with their own functions and services Almeida 99-1.
This is absolutely wrong, as these functions are augmented into the existed
layers. For example, the main function of the presentation layer, which is to
support the interoperability between different equipment, is done now by the
application layer in the fieldbus. What is more, the assembling and
disassembling of data packets which was the function of the transport layer is
done now by the datalink layer in the fieldbus network. If routers to be used
in some fieldbus networks, then the routing service, which was assigned to the
network layer, is done by the application layerin most cases in the fieldbus.

b

a

 

Figure 2: The OSI 7-layers reference model (a),
and the reduced  fieldbus  3-layer structure (b).

 

 

 

V.    
FUTURE FIELDBUS SYSTEM

Like
all other technological products fieldbus is up to continuing process of
updating. The further developments are going on in the fieldbus technology
especially in the vehicle system. A new era has been born which some
specialists called the “X-by-wire”. This technology tends to replace all the
mechanical linkage that are found in the vehicle with digital links and wire
all these link into one network protocol that entirely run the vehicle. Rising
now in the horizon of this era are two protocols. The first is called “FlexRay”
and secondly is called “TTP” . This two are based on the older kin. The
controlled area network (CAN). Among the anticipated benefits :better fuel
economy, better vehicle performance in adverse conditions, and advances in
safety features such as collision warning and even automatic collision
avoidance systems. Figure 3 shows one of the configuration topologies of the
FlexRay protocol which called the active star.

                          The reason of the new protocol
based on the older CAN because these protocol try to resolve problem s
encounter when designing the new X-by-wire automobiles using the old CAN
protocol. These problem arise from the arbitration method that is used to
divide the bus between the competing messages. The CAN used a priority scheme
to assign the highest priority to the oldest que message regardless of the
transmitter making the attempt. FlexRay is a hybrid protocol that allocate
portions of network time to both a time triggered protocol and to prioritized
message access. While the CAN prioritization scheme is based on dominants and
recessive bit values, FlexRay uses timing offset values proportional to priority.
As its name suggest, FlexRay adds flexibility permitting coexistence of both
prioritized and time- triggered messages on the same network. There are other
proposed protocols such as TTP ( time-triggered protocol), FTT-CAN ( Flexible
Time-Triggered Communication on CAN) and TCN which is Train Communication
Network.

 

 

Figure 3: FlexRay Active Star Topology

 

 

 

VI.    
Fieldbus cabling

Various types of cable are useable for fieldbus.
Table 1 contains the types of cable indentified by the IEC/ISA Physical Layer
Standard.

Table 1:
Fieldbus Cable Types & Maximum Lengths

 

The most preferred
fieldbus cable that being specified in the IEC/ISA Physical Layer Standard was
a Type A fieldbus cable. (This cable will probably be used in new
installations). Other types of cable can be used for fieldbus wiring. The
alternate preferred fieldbus cable is a Type B cable which is multiple, twisted
pair cable with an overall shield. (This cable will probably be used in both
new and retrofit installations where multiple fieldbuses are run in the same
area of the user’s plant).

 

A less preferred
fieldbus cable is a single or multiple, twisted pair cable without any shield
referred to a Type C cable. The least preferred cable is Type D cable because
it is a multiple conductor cable without twisted pairs but with overall shield.
Types C and D cables will mainly be used in retrofit applications. They will
have some limitations in fieldbus distance as compared to Type A and B. This
may preclude the use of Type C and D cable in certain applications.

 

The wiring rules of
foundation fieldbus:

 

1)    
Building the Network

Two wires usually carry a signal voltage or current to or from the
field area. In fieldbus, the wire pair is called a network. This definition of
a network is purposely narrow and includes only 31.25 kbit/s devices and
signaling.  Notice that neither wire is grounded because this is one of the
absolute rules of fieldbus.

 

 

 

 

 

 

 

 

 

Figure 1: Simple Fieldbus Network

 

2)    
Adding to the Network

We can add to the network by tapping into the trunk at any point or
by extending it. We can have a total of 32 devices on each segment of a network
with some restrictions. One restriction is the total wire pair length in a
given segment.

 

3)    
Spurs

Shorter spurs is better. The total spur length is limited according to the number of spurs
and number of devices per spur. A spur can be up to 120m in length if there are
few of them.

 

4)    
Repeaters

If you need a lot more than 1900m of cable, it can be done by using
repeater. The repeater takes the place of one of the field devices. To
increasing the length of network, repeaters can be used to increase the number
of devices in a network beyond the limit of 32 on one segment.

 

5)    
Mixing Cables

Occasionally you might need to mix cable types. For communications
purposes, the combined lengths of cable should work OK but in fieldbus, it may
not be possible to supply devices at the opposite end of the bus from the power
supply with the operating voltage current they require (due to effects of Ohms
Law and cable resistance).

 

6)    
Shielding

For best performance, fiedlbus cables should be shielded. Common
multi-conductor (mutli-core) “instrument” cable can be used. When used shielded
cable, connect each spur’s shield to the trunk shield and connect the overall
shield to ground at one point.

 

7)    
Polarity

The Manchester signal used by fieldbus is an alternating voltage
that changes polarity once or twice per bit. The fieldbus receive circuits look
at only the alternating voltage. Field devices must be connected to see the
signal in correct polarity.

 

8)    
DC Power for Two-Wire Field Devices

We can’t use just any off-the-shelf power supply, because it would
short-circuit the fieldbus signals.  If
you have 2-wire field devices in your network, you have to make sure they have
enough voltage to operate. Each device should have at least 9 volts.

 

9)    
Intrinsic Safety

The number of field devices may be limited due to power limitations.
A special fieldbus barrier and special terminators may be required. The amount
of cable may be limited due its capacitance or inductance per length.

 

10)    
Live Wire

Fieldbus devices are designed for connection to a live network. This
is done so that the network needs not to be shut down to service a device.

11)    
What Not to Connect

You can’t connect non-fieldbus device such as light bulbs, analog
4-20 mA field devices to the network.

 

12)    
Connecting to Higher Speed
Fieldbus Networks

These networks may be part a larger network operating at speeds of
1.0 or 2.5 Mbit/s. the 31.25 kbit/s network must never be connected directly to
a higher speed network. A special device called a bridge must be placed between
them.

 

13)    
What If It Is Not Wired
Correctly?

The nature of digital communication systems (including fieldbus) is
that they slow down if you don’t have things quite right. If a master device or
bus analyzer tells you that there are numerous retries, this is a clue that
something’s wrong.

 

 

 

VII.    
Fieldbus Power Supply

Power is one of the
areas in which Foundation Fieldbus is quite similar to conventional analog
networks.

Foundation fieldbus
power is:

·        
Shared across many devices and
segments

·        
Quite iften redundant

·        
Multiplexed either through a
separate power multiplexer or through multiplexers integrated with the
supplies.

·        
24 volts is the most common
supply.

Most existing analog
bulk power supplies will work with FOUNDATION fieldbus and are good sources for
FOUNDATION fieldbus bulk power.

Power Source
Configuration For Multiple Segments.

 

Practical Pointer

Make sure the voltage at
the farthest point of the segments powered is at least 9 vdc when the batteries
are at their lowest expected operating voltage. To ensure this, higher voltage
is required at the power supply. Some plants have backup batteries that float
on a 24 vdc bus. These batteries take over if the AC/DC power supplies are lost.
A margin of several volts is recommended.

 

Power Conditioners

It’s critical that
communications on a segment not cross to other segments through the power
supplies. Power conditioners prevent this “cross-talk” between
multiple segments using the same power supply. The power conditioner limits
maximum segment power. A typical value is 400 mA.. If a typical device draws
15-20 mA, a power conditioner could supply about 20 devices and still have some
reserve capacity. Power conditioners also current-limit the segment, so that
grounding of one segment won’t affect other segments attached to the same bulk
power source.

Practical Pointer

Cable shields can be
grounded to the power conditioner. But make sure the shield doesn’t touch a
field device housing; letting it do so can create a second grounding point and
thus cause a ground loop. Also, FOUNDATION fieldbus signaling uses a balanced
line to provide more robust communications than a signal and ground. Balanced
lines require that the individual signal wires NOT be grounded.

 

Terminators

Terminators are simple
resistor-capacitor circuits used to prevent problems like signal reflection
from the end of the wires. They’re installed in pairs, with one terminator as
close as practical to each end of a fieldbus segment. Fieldbus segments have
been known to work with terminators incorrectly installed or missing, but this
situation dramatically increases the chances of segment problems. Power
conditioners frequently include a terminator, eliminating the need for a separate
external terminator on that end of the segment. However, terminators are
usually NOT built into or installed in field devices. That’s because the
segment could be left without proper termination if the device that contains
the terminator is removed from service.

 

Practical Pointer

Put a terminator in the
junction box that’s closest to the far end of a segment. Even if there are
individual devices farther out, the junction box is usually close enough to the
end of the segment for the terminator to function properly. Because terminators
are very simple circuits, it’s tempting to make your own. But homemade units
frequently fail in installation, checkout, or service. Whatever you saved by
making the terminators will be spent many times over fixing or retrofitting
them.

 

Repeater

Repeaters are optional
components used either to extend the length of a fieldbus segment or to
increase the number of devices on a segment. They provide power and a clean
communication signal for the extended part of the segment. A segment can have
as many as four repeaters dividing the segment into five pieces. Electrically,
each piece acts as a separate segment — but devices can communicate with each
other as though they were on the same segment, even if there are up to two repeaters
between the devices. Although a fieldbus segment can have up to 32 devices
without repeaters, H1 segments typically don’t have more than 12-16 devices
even if repeaters are used.

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