What products are involved?
Rheonics SRV viscometer, SRD density-viscosity meter, DVP gas density-viscosity meter and DVM HPHT density-viscosity meter
What is the purpose of this article?
This article describes the fundamentals of PROFIBUS communication protocol.
TABLE OF CONTENTS
- 1. What is PROFIBUS?
- 2. What are the characteristics of PROFIBUS?
- 3. What devices do I need to set up a PROFIBUS network?
- 4. What configuration do I need?
- 5. References
1. What is PROFIBUS?
PROFIBUS (Process Field Bus) is a standardized fieldbus communication protocol widely used in industrial automation and process control. It is based on IEC 61158 and IEC 61784 standards, which define fieldbus communication technologies and their specific protocol implementations, including signal characteristics and medium access requirements. Using serial communication over RS-485 or fiber optic networks, it enables efficient data exchange between industrial devices, including sensors like Rheonics inline viscometers, as well as controllers and actuators.
Although considered a predecessor to PROFINET, PROFIBUS remains widely used due to its reliability, compatibility with legacy systems, and proven performance all over the world.
Figure 1. PROFIBUS. Image Sourced from PROFIBUS & PROFINET International website.
2. What are the characteristics of PROFIBUS?
PROFIBUS has two main variants in use today: PROFIBUS DP and PROFIBUS PA.
2.1 PROFIBUS DP
PROFIBUS DP (Decentralized Peripherals) is designed for high-speed, half-duplex communication between controllers and field devices, making it ideal for factory and industrial automation. It uses an RS-485 physical layer, typically over twisted-pair copper cables.
2.2 PROFIBUS PA
PROFIBUS PA (Process Automation) is designed for hazardous environments, providing intrinsic safety and allowing power and data to be transmitted over the same cable. It uses the MBP (Manchester Bus Powered) physical layer, which supports communication and power delivery on a two-wire system but at lower data speeds compared to PROFIBUS DP.
2.3 Key Features
- Field device integration: Supports a wide range of devices, from sensors to controllers.
- High flexibility: Suits various industries such as manufacturing, process control, and energy.
- Communication speeds: Up to 12 Mbps (PROFIBUS DP).
- Wide address space: Up to 126 devices on a single network.
- Extended cable length: Supports longer wire lengths without the need of repeaters, compared to PROFINET's 100-meter limitation.
- Reduced Signal Loss: Digital signals maintain integrity over long distances, unlike analog signals, which degrade with increasing cable length.
3. What devices do I need to set up a PROFIBUS network?
3.1 PROFIBUS Master
A PROFIBUS Master is a PLC, DCS, or PC-based controller that initiates communication with the “Slaves”. In a master-slave interaction, the master has unidirectional control over all its slave devices and processes. The controller will always be the master, and the IO devices will always be slaves.
In addition to cyclical communication for the fast exchange of input and output data between the master and slaves at regular intervals, need-based data can also be transmitted using PROFIBUS, e.g. device setting data. A master has the initiative, accessing the data of a slave in read or write mode acyclically.
Figure 2. PROFIBUS Master. Image Sourced from PROFIBUS & PROFINET International website.
Figure 2 illustrates a PROFIBUS network with two masters. The PLC/DCS centrally manages regular cyclic communication (green arrows) with slave devices, such as sensors and actuators. The Engineering Station, shown on the right, interacts with the network via acyclic communication (dashed arrows), focusing on tasks like configuration and diagnostics rather than continuous control.
3.2 Slave devices
Sensors, actuators, or other field devices respond to a prompting master with a response message. The master's request message contains commands or queries, while the response message includes process data or feedback from the field device.
In the case of PROFIBUS DP, the protocol supports up to 125 slave devices per master. Each slave is assigned a unique address within the range of 0 to 126, with one address reserved for the master.
In a setup where the Rheonics SME acts as a Modbus TCP server to a Modbus-PROFIBUS gateway, the gateway acts as the Modbus client while serving as the slave to the PROFIBUS master, typically a PLC.
Figure 3. Rheonics SME
3.3 PROFIBUS Cables
Twisted, shielded copper pair RS-485 cables are the standard choice for PROFIBUS DP data transmission in non-intrinsically safe applications. These cables are easily identifiable by their characteristic purple outer sheath.
These cables include a shielding layer, such as a braided or foil shield, to minimize electromagnetic interference (EMI).
While PROFIBUS maintains RS-485 communication standards, connector types can vary. Figure 4 illustrates a PROFIBUS cable using an M12 connector, demonstrating that RS-485 communication is not limited to traditional DB connectors (like DB9).
Figure 4. PROFIBUS Cable. Image Sourced from Siemen’s Website
3.4 PROFIBUS Connectors
The most common connector for PROFIBUS DP is a DB9 serial connector. Typically, since RS-485 transmission is used, all devices are connected in a line structure (or bus). This topology requires proper termination at the beginning and end of each segment to prevent signal reflections, which can cause communication errors or data loss.
Many DB9 connectors have integrated termination resistors, which can be activated using a built-in switch, such as the red switch shown in Figure 5. This design simplifies installation and maintenance by reducing the risk of incorrect termination, ensuring stable and reliable network operation. If a device that is not at the end of the bus has its termination enabled, it introduces an additional resistor into the network, causing an error.
Figure 5. PROFIBUS Connector. Image Sourced from Siemen’s Website
4. What configuration do I need?
4.1. Device Description Files
A GSD file (Generic Station Description) is a standardized text file that describes the communication parameters of a PROFIBUS device. It includes details such as device identification, supported baud rates, operating modes, diagnostic data length, and I/O module configurations. This file is necessary for integrating a PROFIBUS device (like a PROFIBUS Gateway) into a network using tools such as TIA Portal. It is provided by the device manufacturer and can usually be found in the documentation or their website.
4.2. Addressing in PROFIBUS
In a PROFIBUS network, each device, whether it's a master or a slave, must have a unique address within the range of 0 to 126. This ensures that each device is identifiable and avoids communication conflicts. Figure 6 shows a simple PROFIBUS network (purple cable) where an S-300 is a PROFIBUS Master with a unique address of 1.
Figure 6. PROFIBUS Network. One PROFIBUS Master
4.3. How does Rheonics support PROFIBUS integration and development?
The PROFIBUS and PROFINET International (PI) association is one of the largest automation organizations in the world and is responsible for PROFIBUS and PROFINET, two of the leading automation technologies. Its main goals include the continuous development of PROFIBUS and PROFINET technology, know-how transfer, and investment protection.
Rheonics is a member of the PI association as a vendor of hardware, software, and PROFIBUS-compatible systems. You can check our membership on the official PI website at the following link: Members.
Rheonics provides detailed user manuals and technical support for integration. Our inline viscosity and density sensors can be easily integrated into PROFIBUS networks thanks to all the guidelines and steps provided in our Support Portal. In addition, our experienced engineering team can assist with installation and configuration to ensure successful integration into your PROFIBUS network in a timely manner.
5. References
- PI (PROFIBUS & PROFINET International), "PROFIBUS System Description: Technology and Application," Available: PROFIBUS.com. [Accessed: Dec. 2, 2024].