PROFIBUS is an open, vendor-independent protocol that became part of the international standard organisation model OSI. PROFIBUS links controllers or control systems to several decentralized field devices (sensors and actuators) via a single cable. PROFIBUS is the fieldbus-based automation standard.
For any communication protocol explanation we have to consider OSI (Open System Interconnection) model of ISO, this gives us a general architecture of network specification,
OSI model has 7 layers
1) PHYSICAL LAYER- SAYS ABOUT ELECTRICAL CONNECTIONS
2) DATA LAYER SAYS ABOUT DATA TRANSFER FOR ADJACENT NETWORK
3) NETWORK LAYER SAYS ABOUT ENCAPSULATION OF DATA INTO PACKETS FOR TRANSMISSIONS AND RECEPTION
4) TRANSPORT LAYER SAYS ABOUT DATA HANDLING OVER ENTIRE NETWORK
5) SESSION LAYER SAYS DATA ORGANIZING, START AND END OF SPECIFIC TRANSMISSION
6) PRESENTATION LAYER DEFINES CHARACTER SETS AND CONTROL AND GRAPHIC CONTROLS.
7) APPLICATION/SOFTWARE LAYER SAYS ABOUT ACTUAL SOFTWARE PROGRAMS.
Pd nt spa
The profibus uses three layers namely Layer 1, 2 and 7 as in foundation fieldbus.
Profibus can use foundation fieldbus, RS 485 or fibre optic or AS-i RF/IF radio frequency communication, Ethernet or TCP/IP for communication depending on the complexity of network. For simple case profibus can be set up using AS-i protocol. For lesser number of masters profibus use RS 485 protocol and for multiple master communications, uses Ethernet or TCP/IP. In that case profibus will be replaced by a higher level of communication technique known as Profinet.
Communication methods are Master slave, multi master slave or Publisher subscriber method, FF uses Manchester encoding techniques.
In other words in physical layer it uses RS 485 or fibre optic or AS-i RF/IF radio frequency communication.
Let me explain layer by layer
Layer 1 tell us about how data is “transmitted” by slaves or corresponding transmission technology
Layer 2 tells us how “communication” is taking place between masters or corresponding communication technology
Layer 7 tells us what the applications of profibus are like software
First thing to remember is pro“fibus” is a communication technique similar to “fieldbus”.
Whereas its profi “net” is a more enhanced version of profibus and works as a sophisticated communication technique similar to “ethernet” which can incorporate all communication technique as shown below.
There are different profibus versions depending on applications layer 7 like:
Profibus PA (Process Automation) for process automation
Profibus DP (Decentralized periphery as in DCS) for factory automation
Profidrive (like VMS vibration monitoring system) for motion and motor-pump- machinery related systems
Profisafe for safety and ESD (Emergency Shutdown system) related systems etc
Layer 1 tells us about how data is “transmitted” by slaves or corresponding transmission technology. Profibus is almost compatible with any communication protocol like RS 485, Optic fibre, MBP (Manchester coded bus powered as in FF), wireless communication, HART, AS-i model etc, but most commonly used technology is MBP, RS 485 and optic fibre.
RS 485 transmission is the transmission technology most frequently used by PROFIBUS. The application area includes all areas in which high transmission speed and simple, inexpensive installation is required. Twisted pair shielded copper cable with one conductor pair is used. The RS 485 transmission technology is easy to handle. All devices are connected in a bus structure (i.e. line). Up to 32 stations (master or slaves) can be connected in one segment. The bus is terminated by an active bus terminator at the beginning and end of each segment as shown below.
To ensure error-free operation, both bus terminators must always be powered. The bus terminator can usually be switched in the devices or in the bus terminator connectors.
In the case of more than 32 users, or to enlarge the area of the network, repeaters (line amplifiers) must be used to link up the individual bus segments.
Transmission as per MBP and MBP-IS (Manchester coded bus powered- Intrinsically Safe) is used in FF
The transmission medium is a shielded, twisted pair cable. The signal is transmitted Manchester-coded at 31.25 kbps. In general, the data line is also used to supply power to the field devices. The main advantage of this technology is that it can be used in hazardous area and; bus, star and tree topology can be used for transmission.
Fiber optic conductors may be used in PROFIBUS for applications in environments with very high electromagnetic interference, for electrical isolation or to increase the maximum network distance for high transmission speeds. PROFIBUS segments using fiber-optic technology are designed using either a star or a ring structure.
Different topologies used in profibus are like linear, star, ring, tree, etc depending on the transmission techniques used. For example if RS 485 is used then line or linear topology is used. But here we use profibus DP,
If MBP (FF) is used we can use line or tree topology may be used and if optic fibre transmission technique is used then
line, star, ring and tree topology may be used. Apart from that Profibus PA can be configured as trunk, star, trunk and spur, ring topologies as shown below
Profibus DP allows master slave data exchange also. Thus it is evident that profibus support a versatile topology to be applied. Furthermore we can notice that profibus utilizes foundation fieldbus trunk-spur technology communication technique.
The most commonly used network components of profibus are similar to that of FF namely:
Master control device
It is the controller which controls the data between input and output devices and also processes the data for various functions.
Power supply offers power (with redundancy) and communication on the same twisted pair cable. Higher voltage allows longer cable and higher current allows more devices.
PROFIBUS PA segments are attached to the PROFIBUS DP backbone through some sort of coupler or link. A number of companies supply such kind of equipment with different technical features and designations:
“PROFIBUS DP/PA Segment coupler”
“PROFIBUS DP/PA Link”
“PROFIBUS DP/PA linking device”
Repeaters are devices that repeat an electrical signal thereby returning it to its full strength but introducing a delay in the signal. It acts like an amplifier for long transmission of signals. Repeaters extend the total length of a network and the number of devices on the network. Repeaters are mainly used in DP-networks with their daisy chain topology to allow more devices connected to the network. In PA-networks, a Coupler with a new PA-segment can be added in case of an overloaded segment and to add more devices
Junction boxes (also fieldbus coupler, field barrier, multi barrier) are used to connect spur lines to the trunk and offer numerous special features that vary between models and manufacturers. Junction boxes and field barriers are used in networks to connect trunks (main line) to spurs.
Installed on an easy accessible location of the plant,
mounted in a cabinet to protect against humidity and dust,
and are coupled to the trunk which either terminates or continues to the next junction box. Spurs to the field devices are applied in the box. Electronics provides functional protection (e.g. short-cut at the spur) and explosion protection (e.g. intrinsically safe)
Trunk is the main cable which runs along the entire segment. It is similar to a trunk of a tree which supports all branches. This reduces the cable laying cost as only one main trunk cable is used for communication as in FF.
A spur is a wire connecting field devices to the main trunk line. All spur lines come from the junction box.
The field device the end measuring instrument used in the process. The process variable collected by the field device are transmitted on the bus for use in the controlling the overall process.
Every segment requires two terminators to operate properly. The terminators are equivalent to 1micro F and 100 ohm resistance in series. The terminators serve as shunt for field bus current and protect against reflections or noise signals. A typical profibus model is as shown below.
Profibus device communicate using the profibus DP communication protocol and is the same for all the application and allows
Acyclical communication and
Functional Block Diagram communication.
A brief explanation is as below:
The core of the communication is master-slave method where a master (PLC, PC or control system) cyclically asks the connected slaves (field device, I/O, drives) to exchange data. The respective slave answers the master with a response message. A bus cycle comes to an end once all the slaves have polled in order.
Also there can be more than one Master in a profibus system, in that case the access authorization passes from one Master to another using token passing principle.
The functions of profibus DP communication protocol are distributed over 3 performance levels
This version includes cyclic data exchange between PLC and slave device
In addition to this acyclic communication can also take place.
This version supplements the V0 version with additional acyclic communication such as parameterization, operation, monitoring and alarm handling.
This has additional features as an extension to V1 like data exchange broadcast as in Publisher /Subscriber broadcast as in foundation fieldbus communication, uploading downloading, redundancy, cycle synchronizing and time stamping.
Functional block diagram communication
There is also a method apart from cyclic and acyclic communication method and is known as Standardized Functional Block communication.
Functional block performs an important role when using cross manufacture application profiles. This contains complex functions of field devices in encapsulated form, thus functioning as representative of corresponding field devices placed in control program. This uses simpler programming languages like Ladder diagram, Functional block diagram, Instruction list etc.
The Profibus devices are classified into 3 types based on their functions:
Profibus DP Master (Class 1)
This is a master which uses cyclic communication to exchange process data with associated slaves. This type of device is integrated in a memory programmable controller or an automation station.
Profibus DP Master (Class 2)
This initially was used as Profibus system commissioning; gradually this is used for setting device parameter via acyclical communication
This is a passive communication node which reacts to master by sending a response message.
Layer 7 Application layer
This is the layer is called a software layer or application layer and details about the blocks being used as software for integration of different parts or blocks.
To ensure good interaction between the bus nodes of an automation solution, the basic functions and services of the nodes must match. This means that they have to speak the “same language” and use the same concepts and data formats. This is achieved by the use of same profiles related to device families and specific industry sector.
The different application layer used in conjunction with Profibus is
HART on profibus:
This describes the integration of HART devices in profibus systems. The “Profibus profile HART” offers an open solution for this. It defines the use of communication mechanism without changes to protocol and services of Profibus. Profile of profibus is implemented in the Master and Slave which enable the mapping of the client server model of HART on Profibus. The HART client application is integrated in a profibus Master and the HART Master in a Profibus slave. Thus the latter serves as a multiplexer and takes over the communication with HART devices.
This application defines the properties of process automation devices on Profibus. This is the basis for using the Profibus in process automation. This application is also characterized by frequently intrinsically safe operation and device power supply via the bus cable. This software defines the functions and parameters for process control devices such as transmitters, actuators, valves and analysers. These functions and parameters are used to adapt the devices to the respective application and process conditions. These are based on functional blocks and the parameters are classified as input, output and internal parameters. This application also provides a status providing information about the quality of value and possible limit violations.
This application describes the device behaviour and access behaviour to data for variable speed electric drives on Profibus. This is used in production automation. It defines the automation behaviour, the access method and the data formats for the drive data of the electrical drives on Profibus, from simple frequency converters to highly dynamic servo controllers.
This application describes the safe communication of safety related device with safety controllers via Profibus. Safety related automation technology which reduces the risk of human injury, damage to production systems and environment harm is used in every industrial processes. This demand is also satisfied by the fieldbus technology and the Profisafe communication profile serves this purpose for Profibus.
Apart from this, there are various other application software for various purposes like Dosing, encoder, Fluid Power, Identsytems, LabDevices, LiquidPumps, LowVoltage Switchgears, Remote I/O, SEMI, Identification&Maintenace, iParserver, Redundancy, Timestamp etc.
The advantage of PROFIBUS is that, it allows incorporating field devices and controlling system from different manufactures in one plant which causes different types of user interfaces. Such device integration is performed by mapping the device functionality to operating software together with consistent data retention and identical data structures for all devices. Various device integration technologies have been developed and are used on the market like:
1.General Station Description (GSD)
GSD is a mandatory textual description of any PROFIBUS field device. It is used for field device integration into the master. It is also used for cyclic data exchange of data. The defined file format permits the configuration system to simply read in the GSD files of any PROFIBUS device and automatically use this information when configuring the bus system.
2.Electronic Device Description (EDD)
It is used in addition to a GSD to textually describe application related functionalities and parameter of complex field device. It is also used to allow exchange of additional information with the master for e.g. diagnosis or asset management.
3.Device Type Manager (DTM) and Field Device Tool (FDT) interface.
It is a software-based method of device integration. A DTM is a field device related software component. A DTM communicates with the engineering system in a “Frame application” via the FDT-interface.
FDI is a new field device integration technology which combines best elements of both EDD and FDT/DTM.FDI has been developed by FDI Corporation LLC (FDT Group, Fieldcomm Group, Profibus & Profinet International, and OPC Foundation).
A detailed explanation of the above is beyond the scope of this article.Wait for my next article on profinet, a high level communication, which incorporates profibus and other communication protocols.