Foundation Field bus,Profibus

1. Unit-4Basics of Fieldbus and Profibus
Profibus DP (distributed peripheral) allows the use of multiple master devices, in which case
each slave device is assigned to one master.
multiple masters can read inputs from the device but only one master can write outputs to
that device.
Profibus DP is designed for high speed data transfer at the sensor/actuator level (as opposed
to Profibus FMS which tends to focus on the higher automation levels) and is based around
DIN 19 245 parts 1 and 2 since 1993.
It is suitable as a replacement for the costly wiring of 24 V and 4–20 mA measurementIt is suitable as a replacement for the costly wiring of 24 V and 4–20 mA measurement
signals.
The data exchange for Profibus DP is generally cyclic in nature. The central controller, which
acts as the master, reads the input data from the slave and sends the output data back to the
slave.
The bus cycle time ismuch shorter than the program cycle time of the controller (less than
10 mS).

2. ProfiBus FMS (Fieldbus message specification) is a peer-to-peer messaging format, which allows
masters to communicate with one another.
ProfiBus DP, up to 126 nodes are available and all can be masters if desired. FMS messages
consume more overhead than DP messages.
• ‘COMBI mode’ is when FMS and DP are used simultaneously in the same network, and some
devices (such as Synergetic’s DP/FMS masters) support this. This is most commonly used in
situations where a PLC is being used in conjunction with a PC, and the primary master
communicates with the secondary master via FMS. DP messages are sent via the same network
to I/O devices.to I/O devices.
• The ProfiBus PA protocol is the same as the latest ProfiBus DP with V1 diagnostic extensions,
except that voltage and current levels are reduced to meet the requirements of intrinsic safety
(class I, division II) for the process

3. Profibus Protocol stack
All three ProfiBus variations namely FMS, DP and PA use the same data link layer
protocol (layer 2).
The DP and PA versions use the same physical layer (layer 1)
implementation, namely EIA-485, while PA uses a variation thereof (as per IEC 61158-2)
in order to accommodate intrinsic safety requirements

5. Physical layer (layer 1)
The physical layer of the ProfiBus DP standard is based on EIA-485 and has the
following features:
• The network topology is a linear bus, terminated at both ends.
• Stubs are possible.
• The medium is a twisted pair cable, with shielding conditionally omitted depending on
the application. Type A cable is preferred for transmission speeds greater than 500 kbaud.
Type B should only be used for low baud rates and short distances. These are very specific
cable types of which the details are given below.
• The data rate can vary between 9.6 kbps and 12 Mbps, depending on the
cable length. The values are:cable length. The values are:
9.6 kbps 1200 m
19.2 kbps 1200 m
93.75 kbps 1200 m
187.5 kbps 600 m
500 kbps 200 m
1.5 Mbps 200 m
12 Mbps 100 m

6. The second layer of the OSI model implements the functions of medium access control as
well as that of the logical link control i.e. the transmission and reception of the actual
frames. The latter includes the data integrity function i.e. the generation and checking of
checksums.
The medium access control determines when a station may transmit on the bus and
ProfiBus supports two mechanisms, namely, token passing and polling.
Token passing is used for communication between multiple masters on the bus. It
Data link layer (layer 2)
Token passing is used for communication between multiple masters on the bus. It
involves the passing of software tokens between masters, in a sequence of ascending
addresses. Thus, a logical ring is formed (despite the physical topology being a bus). The
polling method (or master–slave method), on the other hand, is used by a master that
currently has the token to communicate with its associated slave devices (passive
stations).
ProfiBus can be setup either as a pure master–master system (token passing), or as a
polling system (master–slave), or as a hybrid system using both techniques.

8. The token is passed from master station to master station in ascending
order.
• When a master station receives the token from a previous station, it may
then transfer messages to slave devices as well as to other masters.
• If the token transmitter does not recognize any bus activity within the slot time, it repeats
the token and waits for another slot time. It retires if it recognizes bus activity. If there is no
bus activity, it will repeat the token frame for the last time. If there is still no activity, it will try
to pass the token to the next but one master station. It continues repeating the
procedure until it identifies a station that is alive.
Each master station is responsible for the addition or removal of stations in
the address range from its own station address to the next station.
Whenever a station receives the token, it examines one address in the address range
between itself and its current successor. It does this maintenance whenever
its currently queued message cycles have been completed.
Whenever a station replies saying that it is ready to enter the token ring it is then passed
the token. The current token holder also updates its new successor.

9. •After a power up and after a master station has waited a predefined period, it claims the
token if it does not see any bus activity.
•The master station with the lowest station address commences initialization. It transmits
two token frames addressed to itself. This then informs the other master stations
that it is now the only station on the logical token ring.
• It then transmits a ‘request field data link status’ to each station in an increasing address
order. The first master station that responds is then passed the token. The slave
stations and ‘master not ready’ stations are recorded in an address list called
the GAP list.the GAP list.

10. When the token is lost, it is not necessary to re-initialize the system. The lowest address
master station creates a new token after its token timer has timed out. It then proceeds with
its own messages and then passes the token onto its successor.
• The real token rotation time is calculated by each master station on each cycle of the token.
The system reaction time is the maximum time interval between two consecutive high
priority message cycles of a master station at maximum bus load. From this, a target token
rotation time is defined. The real token rotation time must be less than the target token
rotation time for low priority messages to be sent out.
•There are two priorities that can be selected by the application layer, namely ‘low’ and
‘high’. The high priority messages are always dispatched first.‘high’. The high priority messages are always dispatched first.
Independent of the token rotation time, a master station can always transmit one high
priority message. The system’s target token rotation time depends on the number of stations,
the number of high priority messages and the duration of each of these messages. Hence it is
important only to set very important and critical messages to high priority. The predefined
target token rotation time should contain sufficient time for low priority message cycles
with some safety margin built in for retries and loss of messages.

11. Layer 2 provides data transmission services to layer 7. These services are as
defined in
DIN 19241-2, IEC 955, ISO 8802-2 and ISO/IEC JTC 1/SC 6N 4960 (LLC Type 1 and
LLC Type 3) and comprise three acyclic data services as well as one cyclic data
service.
The following data transmission services are defined:
• Send-data-with-acknowledge (SDA) – acyclic.
• Send-data-with-no-acknowledge (SDN) – acyclic.
• Send-and-request-data-with-reply (SRD) – acyclic.
• Cyclic-send-and-request-data-with-reply (CSRD) – cyclic

12. 32 Stations are allowed without repeaters, but with repeaters this number
may be increased to 127.
• The maximum bus length is 1200 meters. This may be increased to 4800 m
with repeaters.
• Transmission is half-duplex, using NRZ (non-return to zero) coding.
• The data rate can vary between 9.6 kbps and 12 Mbps, with values of 9.6,
19.2, 93.75, 187.5, 500, 1500 kbps or 12 Mbps.
• The frame formats are according to IEC 870-5-1, and are constructed with a
Hamming distance of 4. This means that despite up to 4 consecutive faulty
bits in a frame (and despite a correct checksum), a corrupted message will
still be detected.
• There are two levels of message priority.

13. Application layer
Layer 7 of the OSI model provides the application services to the user. These services
make an efficient and open (as well as vendor independent) data transfer possible
between the application programs and layer 2. The ProfiBus application layer is specified
in DIN 19 245 part 2 and consists of:
• The Fieldbus message specification (FMS)
• The lower layer interface (LLI)
• The FieldBus management services – layer 7 (FMA 7)
Fieldbus message specification (FMS)
From the viewpoint of an application process (at layer 8), the communication system is a
service provider offering communication services, known as the FMS services. These areservice provider offering communication services, known as the FMS services. These are
basically classified as either confirmed or unconfirmed services.
Confirmed services are only permitted on connection-oriented communication
relationships while unconfirmed services may also be used on connectionless
relationships. Unconfirmed services may be transferred with either a high or a low
priority.

15. Context management services allow establishment and release of logical
connections, as well as the rejection of inadmissible services
• Variable access services permit access (read and write) to simple variables,
records, arrays and variable lists
• The domain management services enable the transmission (upload or
download) of contiguous memory blocks. The application process splits the
data into smaller segments (fragments) for transmission purposes
• The program invocation services allow the control (start, stop etc) of
program execution
The event management services are unconfirmed services, which make the
transmission of alarm messages possible. They may be used with high or
low priority, and messages may be transmitted on broadcast or multicastlow priority, and messages may be transmitted on broadcast or multicast
communication relationships
• The VFD support messages permit device identification and status reports.
These reports may be initiated at the discretion of individual devices, and
transmitted on broadcast or multicast communication relationships
• The OD management services permit object dictionaries to be read and
written. Process objects must be listed as communication objects in an
object dictionary (OD). The application process on the device must make its
objects visible and available before these can be addressed and processed by
the communication services

16. Fieldbus management layer (FMA 7)
This describes object and management services. The objects are manipulated locally or
remotely using management services. There are three groups here:
• Context management
This provides a service for opening and closing a management
connection
• Configuration management
This provides services for the identification of communication components
of a station, for loading and reading the communication relationship list
(CRL) and for accessing variables, counters and the parameters of the lower
layerslayers
• Fault Management
This provides services for recognizing and eliminating errors

17. Handheld testing device
These are similar to the ones available for DeviceNet, and can be used to check the
copper infrastructure before connecting any devices to the cable. A typical example is the
unit made by Synergetic. They can indicate:
• A switch (i.e. reversal) of the A and B lines
• Wire breaks in the A and B lines as well as in the shield
• Short circuits between the A and B lines and the shield
• Incorrect or missing terminations
The error is indicated via text shown in the display of the device.
These devices can also be used to check the EIA-485 interfaces of ProfiBus devices,
after they have been connected to the network. Typical functions include:after they have been connected to the network. Typical functions include:
• Creating a list with the addresses of all stations connected to the bus
(useful for identifying missing devices)
• Testing individual stations (e.g. identifying duplicate addresses)
• Measuring distance (checking whether the installed segment lengths
comply with the Profibus requirements)
• Measuring reflections (e.g. locating an interruption of the bus line)

18. D-type connectors with built-in terminators
For further location of cable break errors reported by a handheld tester, 9-pin D
connectors with integrated terminations are very helpful. When the termination is
switched to ‘on’ at the connector, the cable leading out of the connector is disconnected.
This feature can be used to identify the location of the error, as follows:
If, for example, the handheld is connected at the beginning of the network and a wire
break of the A line is reported, plug the D connector somewhere in the middle of the
network and switch the termination to ‘on’. If the problem is still reported by the tester, it
means that the introduced termination is still not ‘seen’ by the tester and thus the cable
break must be between the beginning of the network and the D connector.
Configuration utilitiesConfiguration utilities
Each ProfiBus network must be configured and various products are commercially
available to perform this task. Examples include the ProfiBus DP configuration tool by
SST, the Allen Bradley Plug & Play software and the Siemens COM package. In many
cases, the decision on the tool to be used for configuration is made automatically by
choosing the controlling device for the bus. The choice of configuration tool should not
be treated lightly because the easier the tool is to use, the less likely a configuration error
will be made.

20. Reference:
Book:Practical industrial data Networks(Designing ,Installation,Trouble shooting)
Authors:Steve Mackay CPEng, BSc(ElecEng), BSc(Hons), MBA
Edwin Wright MIPENZ, BSc(Hons), BSc(Elec Eng)
DeonReynders Pr.Eng, BSc(ElecEng)(Hons), MBA
John Park ASD