Generally, the industrial communication networks used for control of distributed control of devices and process automation are called the Fieldbus. Of the many Fieldbuses in use today, Profibus, short for Process Field Bus, is a widely used industrial communication protocol that enables reliable and efficient data exchange between devices in automation systems. It was initially developed in the late 1980s/1990s by a consortium of companies in Germany to address the need for a standardized communication solution in the industrial sector. Today, Profibus has become a global standard via IEC 61158 Type 3 that covers the Application Layer service and protocol while IEC 61784 specifies the Communication Profile of Profibus It provides seamless integration of various devices such as sensors, actuators, controllers, and drives. As the first part of a series of articles on Profibus, this will focus on the physical layer while the later ones deal with the higher layer protocols.

Applications of Profibus

Profibus finds applications in various industries, ranging from manufacturing and process automation to building management and transportation systems. Some common applications include:

Factory automation:

Profibus enables seamless integration of various devices in a manufacturing environment, such as sensors, drives, and controllers. It facilitates real-time communication and control, improving productivity and efficiency.

Process control:

Profibus is widely used in the process industry, including sectors like oil and gas, chemicals, and pharmaceuticals. It enables precise monitoring and control of critical parameters, enhancing safety and productivity.

Building automation:

Profibus is employed in building management systems to control and monitor various building services such as HVAC (Heating, Ventilation, and Air Conditioning), lighting, and access control. It allows for efficient energy management and centralized control.

Transportation systems:

Profibus is utilized in transportation systems, including railways and airports, for communication between control systems and devices such as sensors, actuators, and switches. It ensures reliable and timely data exchange for smooth operation and passenger safety.

Profibus Communication Profiles

Profibus is an open field bus standard that uses the common bus topology where all the devices are connected. Though a common name is used, actually Profibus offers different communication protocols/profiles to cater to the specific requirements of different end applications. While there are 3 profiles of Profibus specified - PROFIBUS FMS, PROFIBUS DP and PROFIBUS PA, the last two are in use today. Let us explore all of them briefly.

PROFIBUS FMS

PROFIBUS FMS (Fieldbus Message Specification) is a communication profile primarily designed for complex applications that require a high degree of data exchange. It provides advanced features such as message routing, time synchronization, and event-driven communication. PROFIBUS FMS is commonly used in process automation and manufacturing execution systems (MES). But due to its complexity, a much simpler profile – DP was specified which became the de facto Profibus standard.

PROFIBUS DP

PROFIBUS DP (Decentralized Periphery) is the most used variant of Profibus. It focuses on efficient real-time communication between automation devices and control systems. As the name indicates, network decentralization is allowed where all the devices can be connected to a single bus without need to have a centralized bus master. It employs a master-slave communication mechanism where the master is always in charge of bus control and manages the slave. It is possible to have multiple masters which are managed via a token ring protocol.

PROFIBUS DP is widely adopted in factory automation industries such as automotive, packaging, and food processing. It offers excellent reliability, high-speed data transfer, and easy integration with various devices. It can support up to 32 nodes in a typical linear topology that can be expanded using repeaters.

PROFIBUS PA

PROFIBUS PA (Process Automation) is a communication profile specifically designed for the process industry. It provides a seamless integration of field devices such as sensors, transmitters, and actuators with control systems. PROFIBUS PA operates on a two-wire Manchester-encoded Bus Powered physical layer that is intrinsically sage, allowing for long cable lengths and adoption in hazardous industrial environments. Though the through put is slower compared to DP, it offers advanced features like device parameterization and diagnostics, making it ideal for process control applications with a maximum of 32 nodes.

It is possible to connected both Profibus-DP network and Profibus-PA network together using specialized couplers.

Profibus Mapping to OSI Layer Model

With an introduction to the different Profibus communication profiles, let us see how they are mapped to the standard Open Systems Interconnection (OSI) model. The below picture captures the same.

Profibus Mapping to OSI Layer model

Profibus OSI Mapping

Each layer serves a specific purpose in the data exchange process.

Physical Layer:

This layer defines the electrical and mechanical characteristics of the communication medium used in Profibus networks. It includes specifications for different physical layers such as EIA-485, optical, and MBP (Manchester Coded Bus Powered).

Data Link Layer:

The data link layer handles the reliable transmission of data packets over the physical medium. It is responsible for error detection and correction, as well as flow control.

Application Layer:

The application layer is where the actual application-specific data is exchanged between devices. It defines the protocols and data structures used in the application layer communication. On top of this various Profibus application profiles sit.

None of the intermediate layers like Network, Transport, Session, and Presentation Layers are defined in the specification.

Profibus Physical Layers - EIA-485

EIA-485 is one of the most commonly used physical layers in Profibus networks. It is a differential signaling standard with NRZ (no return to zero) encoding that allows for reliable data transmission over long distances. EIA-485 supports multi-drop configurations, where multiple devices can be connected to a single bus. The communication speed can be from 9.6 to 12,000 Kbit/s over a simple twisted, shielded two wire cable with length ranging from 1200 m to 100 m per segment. It can support up to 32 nodes per segment and a maximum of 126 per network repeaters. This makes it suitable for applications that require communication with multiple devices, such as factory automation systems.

Profibus Physical Layers - Optical

Optical physical layers utilize fiber-optic cables for data transmission in Profibus networks. They offer several advantages over traditional copper-based physical layers, such as immunity to electromagnetic interference and longer communication distances. Data speeds can range from 9.6 to 12,000 Kbit/s over multi-mode and single mode glass/PCF/plastic fibers and coded with NRZ (no return to zero) encoding. The devices in an optical network are typically connected in star and ring topology while line is also possible and the maximum number of nodes in the network going up to 126 nodes. Optical Profibus networks are commonly used in applications where noise and harsh environments are a concern, such as power plants and petrochemical facilities.

Profibus Physical Layers - MBP

MBP, or Manchester Coded Bus Powered, is a physical layer specifically designed for Profibus PA networks. It utilizes a bus-powered approach, where the communication signal is derived from the bus voltage itself. This eliminates the need for external power supplies for field devices, simplifying installation and reducing costs. As the name implies, it uses bit-synchronous, Manchester coding at 31.25 Kbit/s over simple twisted, shielded two wire cables. Like its RS485 counterpart, the count of devices in a segment can go up to 32 nodes with a maximum of 126 nodes per network. Possible bus topologies include line and tree. MBP physical layer is well-suited for process automation applications in hazardous industries where power availability and safety are a concern.

Conclusion

Profibus is a versatile and reliable communication protocol that plays a crucial role in the industrial automation sector. Its variants, such as PROFIBUS FMS, PROFIBUS DP, and PROFIBUS PA, cater to diverse application requirements. Understanding the Profibus protocol, its physical layers, mapping to the OSI layer model, and troubleshooting techniques is essential for successfully implementing and maintaining Profibus networks. In the upcoming articles we will continue covering the higher layers of this popular fieldbus protocol.

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