1. Introduction to Fieldbus
Lecture Number 7
Introduction to Fieldbus

2. In this session…
In this session, you will learn about fieldbus systems and how they are applied to PLC systems.

3. Learning outcomes At the end of this session you should be able to:
Discuss the technicalities of a fieldbus system
Describe the function of a fieldbus system
Understand basic fieldbus systems such as ASi and Profibus.

4. Introduction To Fieldbus
By C Mayhew

5. Communications Concepts and Terms
By C Mayhew

6. Communications Concepts
Encoding Methods: Parallel Communication
Cross Talk 1 1

7. Communications Concepts
Encoding Methods: Serial Asynchronous Communication
Stop 1 1
Start
Separately Synchronised Clocks

8. Encoding Methods: Serial Synchronous Communication
Communications Concepts
Encoding Methods: Serial Synchronous Communication 1 1
Separate Clock Channel

9. Communications Concepts
Encoding Methods: Manchester BiphaseL Encoding

10. Communications Concepts
Encoding Methods: Baseband 1 1

11. Communications Concepts
Encoding Methods: Carrierband 1 1
MODEM
MODEM

12. Communications Concepts
Signaling Methods: Differential 1 1
Noise
Data High
Data Low
Subtraction of Differential Signal at the Receiver Negates Noise

13. Communications Concepts
Connectivity: Point to Point Simplex
Data Between Two Nodes in One Direction Only.

14. Communications Concepts
Connectivity: Point to Point Duplex
Data Between Two Nodes in Two Directions

15. Communications Concepts
Connectivity: Multi Drop
Multiple Access networks require some form of access method.
Node 1
Node 2
Line
Termination
Media
Node 3
Node 4
Node 5

16. Communications Concepts
Access Methods: CSMA/CD
If Node 1 wishes to send data to Node 3
It first ‘listens’ to the communication line to detect any data transmission.
(Carrier Sense)
All machines are capable
of this. (Multiple Access)
If two Nodes attempt to transmit simultaneously, the collision is detected and the two transmitting machines wait a random time until they try again.(Collision Detection)
Node 3
Node 1
Node 2
Data can eventually be sent.
This method is not deterministic.

17. Communications Concepts
Access Methods: Token Passing
Token Passing is a Deterministic access method.
A digital signature called a Token is passed from one communicating
device to the next in sequence.
Only the device with the Token can send data.
Node 3
Node 1
Node 2
Token

18. Communications Concepts
Control: Master Slave
Master Polls Slaves cyclically.
Slave 1
Slave 2
Slave 2
Master

19. Communications Concepts
Control: Peer to Peer
All Nodes Have Equal Priority. Some Sort of Access Method is Required.
Node 1
Node 2
Line
Termination
Media
Node 3
Node 4
Node 5

20. Communications Concepts
Some Terms
Bit Rate, Baud Rate - Relates to the speed of the data
RS A simple Full Duplex, Single Ended, Point to Point protocol.
Old but still used on devices in Industry
RS A Differential Point to Point protocol. Used in Industry due to its noise immunity.
RS A Differential Multi Drop protocol. Profibus and many other systems are based on this protocol.

21. Fieldbus Systems
By C Mayhew

22. Fieldbus Ethernet, Profibus, ASi and Interbus
The use of Fieldbus technologies greatly reduces the amount and cost of wiring individual devices.
It enables I/O to be distributed around a machine or site.
Connecting controlling devices facilitates information sharing between the devices and enables resources to be shared.
Collection of production data is also more easily facilitated from interconnected devices.
Many systems are promoted as Open Systems (OSI).
This means that they are not manufacturer dependent.
One of the main standards for Fieldbus Systems is IEC1158

23. Fieldbus Hierarchy
There are various levels where Fieldbus can be employed:
Process/Sensor Level Sensing and Measuring Devices
Field Level PLC’s Robots, Drives
Cell Level Workcell Areas
There are also many Fieldbus and networking protocols, these include:
Ethernet
Profibus
ASi
Modbus
Interbus
Futurebus
Foundation Fieldbus

24. Sensor/Actuator Level
Fieldbus Hierarchy
Higher Systems
Internet
Management Level
Cell Level
Field Level
Sensor/Actuator Level

25. OSI Model
The OSI (Open Systems Interconnection) Model is used to implement the system interconnection process.
The interconnection problem is so complex it is broken down into seven areas or Layers.
For example, the lower layer defines the requirements of the physical aspects of connection such as plugs, wires, signalling types etc.

26. Fieldbus Profibus, ASi and Interbus Profibus
Profibus is a Fieldbus standard that has three protocols:
Profibus PA (Process Automation) Sensor Level
Profibus DP (Distributed Periphery) Field/Cell Level
Profibus FMS (Fieldbus Message Specification) Cell Level Forerunner of DP
The S7 314C-2 DP CPU has a Profibus DP interface that can be used to connect other CPU’s, Distributed I/O and any other equipment with a Profibus interface, such as Robot and drive Systems.
The Profibus system can interconnect with other bus systems via Gateways. For example, the Siemens Link20E Module will Gateway from Profibus to ASi and also act as an ASi Master. Profibus is RS485 based and can currently run at speeds of up to 12 MBaud. Wireless (infrared) and Fibre Optic is also available.
The bus currently supports over 100 Nodes and can be configure as a Master/Slave, Peer to Peer or Multi Master bus.
Distance is limited to 2km (copper media).
Access to the bus is via a deterministic Token Passing Sequence.
Devices are connected via 9 pin D Type Fieldbus plugs with switchable terminating resistors.
Nodes are configured via hardware, software or programmer depending on the type of node.
The system is promoted as an Open System and uses layers 1,2 and 7 of the OSI model.

27. Fieldbus Profibus, ASi and Interbus Profibus Network Example
Node 3
PLC
Node 11
Node 20
Node 12
Inverter
Node 25
Robot
Node 10
HMI
Node 5
Bus Gateway
Other Network
Profibus DP Media
Node 8 PC
Profibus PA Media
DP/PA
Coupler
Node 30
Pressure Cell
Node 35
Node 40

28. Fieldbus Profibus, ASi and Interbus ASi
ASi (Actuator Sensor Interface) is a bus system that works at the sensor level.
It is a Master/Slave MBP (Manchester Bus Powered) System, i.e. it has power and data on the same bus.
The supply voltage is
The current system can support up to 62 nodes and has analogue capability.
The system employs a polling technique with 4 bit data frames and can access all 62 nodes in 10ms (31 nodes in 5ms).
Nodes are generally 4 bit I/O but nodes with more I/O are available.
The system is promoted as an Open System.
Safety devices such as Safety Relays and Emergency Stop buttons can be used with the ASi Safe nodes.
The Nodes connect to the unshielded, non terminated ASi cable via IDC connectors. The cable is self healing to IP67 when nodes are removed.
The system will automatically number nodes when a new slave is added and will automatically give a replaced node the address of the old node when a slave is replaced after failure.
Devices can be replaced while the system is energised (hot swapping).
Distance is limited to 100m per segment. With repeaters 300m.

29. Fieldbus Profibus, ASi and Interbus ASi Network Example
ASi Master
Polls Slaves
ASi PSU
Slave 5
8 I/O Module
Slave 8
Slave 2
4 Input Module
Slave 1
Slave 6
8 I/O Module
Asi Media
Slave 7
Pneumatic Valve

30. Fieldbus Profibus, ASi and Interbus Interbus
Interbus is RS422 based (differential signalling) and supports up to 512 devices.
Topology is Ring.
The Ring is required for the bus access method which relies on a deterministic Serial Ring Shift Register type action.
Each device in the ring amplifies the signal and sends it on.
Unlike many other bus system, the physical position of the node in the ring determines the device address. This is because of the Shift Register principle.
The maximum distance is limited to 13km with no more than 400m between bus slaves.
Transmission speed is currently 2MBaud.
Because data frames do not need addresses, there is more room for data which makes the transmission system more efficient.
Transmit and receive data can be sent simultaneously (Full Duplex).

31. Fieldbus Profibus, ASi and Interbus Interbus
Slave 1
1010
Slave 2
1011
Slave 3
0011
Slave 4
1111
Slave 5
1110
Slave 6
0001
Slave 7
1001
Slaves Data
1001
0001
1110
1111
0011
1011
1010
Overhead
Data Clocked Around
Slave 1
Slave 2
Slave 3
Slave 4
Slave 5
Slave 6
Slave 7
10101

32. Ethernet
Ethernet is a fast bus system compared to most other bus systems.
Speeds of 10MBit, 100Mbit and 1GBit are common.
Its addressing system is more complicated than node addresses on many other bus systems, addresses being arrange in Octets.
The communication protocol is TCP/IP (Transport Communication Protocol/Internet Protocol). The addressing format is commonly called IP addressing.
The IP Address has two parts:
The IP Address
The Net Mask
The IP Address is assigned to a device. The Net Mask defines which part of the IP address is for the node and which part is for the network.
In order to understand this we have to remember that many networks can be interconnected and each network must have an address, as well as the individual node on the network.
DHCP (Dynamic Host Configuration Protocol) or equivalent protocols can be used to assign IP addresses. This means that addresses may be assigned automatically as nodes are attached. Alternatively addresses may be fixed.
Access protocol is by non deterministic CSMA/CD (Carrier Sense Multiple Access/Collision Detection).

33. Ethernet Net Mask Example
Machine IP Address
Gives theoretical network addresses with 256 node addresses on each network.
IP =
IP =
Ethernet Media
Net Mask =
Node 82 on network
Ethernet Media
IP =
Net Mask =
IP =
Node 82 on network
Router

34. Mapping Fieldbus I/O Must be ‘Mapped’ to PLC I/O
ASi
Master
Slave 10
4 Input Module
Slave 20
8 I/O Module
Slave 15
Fieldbus Devices
Fieldbus Master has a list of all active nodes and maps them to PLC I/O.
Profibus Node 3
PLC
Control Devices
PLC communication setup has Node 4 (Gateway) as a connected device.
GSD (General Station Description) files are used to set up device parameters in Connection Manager.
Profibus Node 4
Gateway
I:7.0, I7.1, I7.2, I7.3
Q7.0, Q7.1, Q7.2, Q7.3
Node 15
I:10.4, I10.5, I10.6, I10.7
Q10.4, Q10.5, Q10.6, Q10.7
Node 20
I5.4, I5.5, I5.6, I5.7
Node 10
PLC I/O Address
Node Address
Node Mapping
The mapping may be pre defined or by software.

35. Mapping Example: ASi

36. What PLC I/O address would be used for Slave 12?
Mapping Example: ASi
What PLC I/O address would be used for Slave 12?
I/Q 6.4, 6.5, 6.6, 6.7

37. Full System Example ASi Media Profibus Media Ethernet Media Internet
Profibus Node 5
Bus Gateway
ASi Media
ASi Master
Slave 1
Slave 2
Slave 3
Slave 4
Profibus Media
Profibus Node 10
HMI
Profibus Node 12
Inverter
Profibus Node 3
PLC
Profibus Node 11
Profibus Node 2 PC
PC Station running
SCADA software
IP =
Net Mask = PC
PLC
Ethernet
IP =
Ethernet Media
IP =
Internet
Internet Gateway

38. Test Your Knowledge
Proximity sensors are at which level in the Fieldbus Hierarchy?:
Sensor/Actuator level
The ISO model has seven levels. To what does level 1 refer to?:
Physical connections such as plugs and wiring.
The Profibus DP protocol supports devices such as?:
PLC, PC, Robots
The ASi protocol supports devices such as?:
I/O blocks, Pneumatic valves
The Ethernet protocol is a fast bus with speeds up to?:
1GBit
A Gateway is a device that can interconnect?:
Two totally different communication systems

39. Summary
You should now have a basic understanding of fieldbus systems and how they apply to PLC systems.