PROFIBUS is an international, open, and device-independent fieldbus standard. It is widely used in manufacturing automation , process industry automation , and automation in other fields such as building automation, transportation, and power. PROFIBUS consists of three compatible parts: PROFIBUS-DP (Decentralized Periphery), PROFIBUS-PA (Process Automation), and PROFIBUS-FMS (Fieldbus Message Specification).
(1) PROFIBUS-DP: A high-speed, low-cost communication method used for communication between device-level control systems and distributed I/O. PROFIBUS-DP can replace 24VDC or 4-20mA signal transmission. The master station and slave station use a polling communication method, and it is mainly used for unit-level and field-level communication in automation systems.
(2) PROFIBUS-PA: Designed specifically for process automation , it allows sensors and actuators to be connected on a single bus and has intrinsic safety specifications. Power and communication data are transmitted in parallel via the bus, and it is mainly used for unit-level and field-level communication in process automation systems.
(3) PROFIBUS-FMS: Used for workshop-level monitoring networks, it is a token-based, real-time multi-master network. It defines the communication model between master stations and is mainly used for process data exchange between the system level and workshop level in automation systems.
PROFIBUS protocol structure
The PROFIBUS protocol structure is based on the ISO 7498 international standard, using the Open System Interconnection (SIO) model as its reference model. The OSI model is the foundation of fieldbus technology. For the underlying networks of industrial control, the amount of control-oriented information per node is relatively small, and the information transmission task is relatively simple, but the requirements for real-time performance and speed are high. Most fieldbus communication models are simplified to varying degrees based on the OSI model.
(1) PROFIBUS-DP: Defines layers 1 and 2 and the user interface. Layers 3 to 7 are not described. The user interface specifies the application functions that users, the system, and different devices can call, and details the device behavior of various PROFIBUS-DP devices.
(2) PROFIBUS-FMS: Defines layers 1, 2, and 7. The application layer includes the Fieldbus Message Specification (FMS) and the Lower Layer Interface (LLI). FMS includes application protocols and provides users with a wide range of powerful communication services. LLI coordinates different communication relationships and provides a device-independent second-layer access interface.
(3) PROFIBUS-PA: PA data transmission uses the extended PROFIBUS-DP protocol. Additionally, PA specifications describe the behavior of field devices. According to the IEC 1158-2 standard, PA's transmission technology ensures intrinsic security and can power field devices via the bus. PA networks can be extended on the DP using connectors.
PROFIBUS basic characteristics
Profibus enables the networking of distributed digital controllers from the field level down to the shop floor. Compared to other fieldbuses , a key advantage of Profibus is its stable international standard EN50170, which has proven its versatility in practical applications. It covers a wide range of applications, including manufacturing, process automation, and digital automation , and can simultaneously implement centralized, distributed, and hybrid control modes. The system consists of a master station and slave stations.
The master station determines the data communication on the bus. When the master station obtains control of the bus (token), it can actively send information without external requests. In the PROFIBUS protocol, the master station is also called the active station.
Slave devices are peripheral devices, typically including input/output devices, valves, actuators, and measurement transmitters. They do not have bus control; they only acknowledge received information or send information to the master station when requested. Slave devices are also called passive stations. Because slave devices require only a small portion of the bus protocol, they are particularly economical to implement.
PROFIBUS communication media access control method
PROFIBUS uses a distributed token access method, a time-triggered network protocol. Tokens are passed between master nodes via a ring mechanism, while master-slave nodes use a round-robin polling method. Once a master node obtains a token, it is allowed to communicate with slave nodes and/or other master nodes for a specified period. The maximum time a token circulates among all master nodes (the set time interval, TTR) is predetermined, determining the specific duration each master node holds its token. Data transmission between master nodes must ensure that each master node has sufficient time to complete the communication task within the predefined time interval, and data exchange between master and slave nodes should be completed as quickly and easily as possible.
To address this, the PROFIBUS Media Access Control (MAC) protocol uses two types of clock timers: one is the token run cycle timer, used to time the actual run cycle (TRR) of the token; the other is the token holding timer, used to time the master node token holding time (TTH). When a token arrives at a master node, the cycle timer of that node begins to count.
When the token arrives at the master node again, the MAC assigns the difference between the periodic timer's TRR value and the set periodic value TTR to the token holder timer, i.e., TTH = TTR - TRR. The token holder controls the transmission of information based on this value.
When the token timer controls the transmission of information, if the token arrives and times out (TTH < 0), the node can only send one high-priority message. If the token arrives on time, the node can continuously send multiple pending high-priority messages until all high-priority messages have been sent or the token holding time has expired. If there is still token holding time after sending all pending high-priority messages, low-priority messages can be sent in the same way. Regardless of whether high-priority or low-priority messages are sent, the node only checks whether the token holding time has expired before sending, rather than checking whether the timeout will occur after sending the message. This detection method means that message transmission inevitably causes token holding timeouts, affecting the implementation of periodic real-time communication.
