Everything you need to know about PROFIBUS
Let’s talk about one of the most popular digital protocols in a wide array of industrial segments. Even though you’ll still find a lot of analogs out there, PROFIBUS has gained ground on it. Its strong user experience, robustness, and advanced technology take most of the credit for that. But its popularity also comes from PROFIBUS & PROFINET International (PI), providing training, certifications, and seminars to advance the protocol’s reach.
Let’s learn more about its history, operation, and pros and cons. Stick with me to discover PROFIBUS!
Late in the 1980s, the German government began working with German companies and institutes to create a proprietary protocol. In 1987, these collaborators developed and tested a new open fieldbus solution. They designed a bit-serial fieldbus to support field devices in their basic needs. Thus began the PROFIBUS protocol.
In 1989, they completed the project and founded the PROFIBUS Nutzerorganization e.V. (PNO). This group started with only 10 members. In 1995, they expanded to PROFIBUS International and now have more than 1400 members.
According to PI, today more than 50 million field devices use this protocol, making it a global leader in industrial communications.
PROFIBUS uses modular design and different transmission technologies, making it flexible to set up and use. The system’s building blocks can cover all types of industries and applications.
Decentralized peripherals (DP) make up the core of the system’s communication. All PROFIBUS applications use DP to communicate between centralized and decentralized devices.
Most often you’ll find RS485, a popular copper cable, used to transmit data. However, the protocol offers other possibilities. An application in a non-explosive area can use RS485, but an explosive area should use RS485-IS (intrinsically safe).
You can also find MBP (Manchester coded Bus Powered) and its intrinsically safe version, MBP-IS. You’ll use one of these two when you need power and communication on the same cable. And let’s not forget optical and wireless communication.
So you can use PROFIBUS in many applications, but each requires different devices. The application profile gives you the right protocol, always keeping DP as the common communication for all of them.
The OSI layer model
PROFIBUS began based on the Open Systems Interconnection (OSI) layer model. The OSI model standardizes communication using seven layers, as you see here.
PROFIBUS only uses the first, second, and seventh layers. Here, the first layer (transmission technology) defines the physical transmission – RS485, MBP, wireless, and so on.
The second layer defines the bus access model and its data security. For PROFIBUS, we have the master-slave method using the token principle. And the seventh layer has the interface connecting the communication and application. DP lives here.
Profibus DP (Decentralized Peripherals)
PROFIBUS DP works with cyclical and acyclical communication using particular rules for each.
As I mentioned above, PROFIBUS uses the master-slave method. The master (controller, PC, etc.) uses the cyclical mode to exchange data (output, input, etc.) with slaves (field devices, drives, etc.). The master also needs to exchange setting data with the slaves. For that, it uses the acyclical mode.
You can have class 1 and class 2 masters, with authorization passing from the active master to the second master using the token-passing principle.
- PROFIBUS DP class 1 master: This master exchanges cyclical data with the slaves to communicate.
- PROFIBUS DP class 2 master: This master can access and set up slaves without connecting permanently to the bus. It uses acyclical communication.
- PROFIBUS slaves: These devices connect to the network passively, sending data only in response to commands from a master. Slaves range from transmitters to valves and much more.
Additional information about PROFIBUS DP:
- Electrically balanced network
- Up to 126 nodes
- 9.6 Kbp/s to 12 Mbp/s
- Cable, wireless, or optical fiber
- Profisafe profile
- Segments up to 32 nodes
- Gateway repeaters and terminators
- Network length from 100 to 1000 meters
- Linear topology
PROFIBUS PA (Process Automation)
PROFIBUS PA focuses on implementing devices in process automation. It resembles DP in areas such as cable length, intrinsic operation, and more. However, device specs here use function blocks, where we find classifications such as input, output, and internal parameters.
It has a simpler setup than a standard analog system, with transparent access to device data using description files.
Additional information about PROFIBUS PA:
- IEC 61158-2 standard
- Support for explosive applications
- Signal and power in the same cable
- Up to 126 nodes
- GSD configuration file
- 3.25 Kbp/s
- Segments up to 32 nodes
- Couples or links, junction boxes, and terminators
- Maximum length is 1900 meters with spurs up to 120 meters
- Different topologies
PROFIdrive shows up in factory automation, defining access methods, device behavior, and data formats. It communicates on simple frequency converters to servo controllers.
This one, as you can probably guess, assists in safety applications. You can use this protocol in applications up to SIL 3.
Couplers and links
Couplers send physical bus characteristics between DP and PA networks. Furthermore, they do so transparently, bringing the networks together rather than directing the action.
You can use couplers in explosive and non-explosive places, which we in the industry call ex and non-ex. Keep in mind, though, that in ex, you can only have so many devices in each segment. You don’t want too many players in a volatile environment, right?
Links act as slaves for DP and masters for PA. They also support larger networks than couplers. Each link has a physical address and can connect with up to five couplers.
The general station description (GSD) file contains all the data you need for your PROFIBUS or PROFINET field device. If you want to integrate the device into your control system, then you must have the GSD file.
You can imagine the GSD file as a form, filled in with all the data on the device and the type of information it provides. It also has an editor with a syntax and semantics check to set up the file according to the GSD specifications.
Terminators on the network
Every PROFIBUS network runs at high frequencies on RS485. Each segment of this copper cable needs a terminating resistor at both ends to absorb signal reflections that corrupt messages and trip the network.
A terminator basically consists of three resistor circuits that match the impedance of the PROFIBUS cables. You can use passive or active terminators, but active terminators will make your network more robust.
Difference between PROFIBUS and PROFINET
Basically, PROFIBUS is the traditional digital protocol based on serial communication. PROFINET is a new protocol based on the industrial Ethernet. PROFINET supports a faster communication with more bandwidth. That means you can send larger messages than on PROFIBUS.
This spreadsheet from the PROFIBUS Association has a comparison between these protocols. It offers a good overview of the differences between PROFIBUS and PROFINET:PROFINET has the advantage of wireless communication as well. In fact, as a standard Ethernet protocol, so you can use WiFi and Bluetooth too if you want. To learn more about PROFINET, check out our article here.