The PROFIBUS DP Protocol (Process Field Bus) forms the link between the automation system, peripheral modules, and field devices. This link corresponds to a logical assignment between communication partners in order to carry out a communication.
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Introduction to PROFIBUS DP Protocol
PROFIBUS allows the entire periphery to be connected to a central controller. The main advantages are savings in cabling and flexibility in the architectures, not to mention exploiting the intelligence embedded in the field equipment.
Thanks to its different profiles, PROFIBUS can respond effectively to all the automated installation needs. This explains the presence of PROFIBUS in the entire industry: from the automotive sector to cement factories, including the food industry and water treatment.
- PROFIBUS DP meets general automation needs
- PROFISafe integrates security aspects
- PROFIDrive takes into account the unique requirements of Motion Control
- PROFIBUS PA is intended for connecting sensors & actuators in continuous or batch processes.
All these profiles operate according to the same protocol. Indeed, they do not constitute a communication solution but complete the standard to facilitate and guarantee the interoperability of stations dedicated to specific applications, regardless of the manufacturer.
PROFIBUS is an open communication system accepting devices from various manufacturers. It is the ideal cell as well as field scale network. It, more than that, enables the rapid transmission of medium amounts of data (approximately 200 bytes).
Origin of PROFIBUS DP Protocol
The PROFIBUS network was developed from 1987 to 1990 by a working group of 14 companies and four institutes. The first products appeared on the market in 1990. Further, the first installations fitted with PROFIBUS couplers were installed in 1991. Therefore, it is not surprising that PROFIBUS is a field added to being a cell network widely distributed worldwide, which is currently standardized in the IEC 61158 and EN 50 170.
To date, in the industry, there are basically two variants:
- Profibus-DP (Decentralized Peripherals) connects active equipment and detectors to a central PLC controller in industrial production applications. The bus offers several diagnostic means as standard.
- Profibus-PA (Process Automation) is used for measuring and monitoring equipment. That is to say; it is particularly designed for risk areas (particularly explosion). The physical layer complies with IEC 61158-2, making it possible to supply power to the instruments connected via bus topology while limiting the currents that would allow the explosion risk, even in a fault event. However, this functionality limits the number of devices that can be connected to the same PA bus segment, and the speed is limited to 31.25 kbit/s.
Main technical characteristics
- The standard communication medium is RS485.
- For applications in hazardous areas, we also have an intrinsically safe version, RS485-IS.
- The optical fiber is commonly used to increase distance or overcome CEM problems.
- For the process, the PROFIBUS PA profile defines an MBP transmission of a Manchester Bus Powered type.
- Baud rate: 9600 baud – 12 Mbaud.
- Number of stations / segment: 32.
PROFIBUS Communication protocols
Profibus DP and its DP-V0 to DP-V2 protocols offer a range of options in order to optimize inter-application communications.
PROFIBUS-FMS (FMS = Fieldbus Message Specification)
This variant is the universal solution for communications tasks carried out at the cell and field level in the industrial communication hierarchy. The powerful FMS services offer a wide range of applications and great flexibility, allowing for extensive communication tasks with cyclic or acyclic data traffic at moderate speed.
PROFIBUS-DP (DP = Decentralized Peripherals)
Optimized in terms of speed, this variant is specially adapted for communication between automation systems and decentralized devices. Furthermore, it also offers “Plug and Play” connectivity for field devices.
PROFIBUS-FMS and DP use the same transmission technology and the same bus access protocol. These two variants can, therefore, simultaneously operate side by side on a single cable.
PROFIBUS-PA (PA = Process Automation)
The PROFIBUS-PA variant is intended for applications related to the automation of processes. PROFIBUS-PA uses intrinsically safe transmission technology specified in IEC 1158-2. In addition, it allows remote powering from stations by bus.
The functions of the PROFIBUS-FMS, DP, and PA protocols are supplemented by profiles describing the PROFIBUS use in specific application areas such as process automation, building management systems, or manufacturing automation.
The equipment profiles define the specific functions of the equipment implemented artwork. In addition, the DDL language (Device Description Language) and the functions ensure full interoperability of equipment.
PROFIBUS Physical Layer
The PROFIBUS protocol architecture is based on the OSI reference model (Open System Interconnection) / ISO corresponding to the international standard ISO 7498.
In this model, each layer performs specific tasks. Layer 1, known as the physical layer, defines the techniques used to ensure the transfer of data on the physical medium. Layer 2 (Data Link Layer) describes the mechanism of access to the bus. Layer 7 (Application Layer) defines the layer closest to the user.
The field of application of a field bus depends mainly on the choice of the transmission medium and the physical interface of the bus. In addition to the requirements for transmission security, the cost of acquiring and installing the bus cable plays a determining role. Taking this into account, the PROFIBUS standard offers several transmission techniques while ensuring a uniform bus protocol.
Cable transmission: This variant was chosen as the basic version for production, building management, and driving technology applications by the American standard EIA RS-485. It uses a twisted pair cable copper shielded.
Optical fiber: Based on optical fiber, this transmission technique is perfectly suitable for highly parasitized environments and extension broadband reach.
Intrinsically safe transmission: PROFIBUS-PA guarantees intrinsic safety while allowing the stations to be powered by the bus. The transmission technique is described in the international standard IEC 1158-2.
Access method of Profibus DP Protocol
Data access is done according to the master/slave principle, and several masters can share the network. Also, a token (Token) is used to regulate the traffic between the different participants. Actually, there are two categories of masters: Master class 1 and Master class 2.
- Class 1 master: this is the process controller, usually an automate or SNCC, but it can also be a PC. It constantly exchanges input and output data with the slaves according to a predetermined
- Cycle Class 2 master: This PC-based tool allows configuration and maintenance operations for PROFIBUS slaves. Data exchanges do not occur regularly, but according to needs, it is an acyclic communication that does not disturb the cyclic exchanges of the class 1 master.
PROFIBUS masters
- A class 1 master is mandatory in a PROFIBUS architecture
- A class 2 master is not essential. In fact, in most installations, it is non-existent. It is only relevant if the slaves support acyclic exchanges, according to DP-V1. Currently, class 2 masters are mainly used in process systems with PROFIBUS PA.
Conclusion
I hope we have answered your question, “What is a Profibus DP Protocol?”.
Even though newer technologies, such as Profinet, are readily available, the Profibus DP protocol is still relatively cost-effective and, therefore, still in use today.
FAQ
Profibus DP vs. Profinet: What are the differences?
Profibus DP is an old computer network that was developed in 1984. Since then, newer technologies have been developed that offer better functionality at a lower cost. Some of these newer technologies include Profinet (which also supports Ethernet) and EtherNet/IP (which offers very similar functionalities to Profinet at a lower cost).
