PROFIBUS is a fieldbus standard used in automation technology, promoted in 1987 by Siemens and fourteen other companies along with five research institutions. PROFIBUS stands for PROcess FIeld BUS. PROFIBUS and PROFINET, used in industrial Ethernet, are two different communication protocols.
2. Origins
The history of PROFIBUS can be traced back to a cooperative project initiated in 1987 in West Germany, involving fourteen companies and five research institutions, aimed at promoting a serial fieldbus that meets the basic requirements of field device interfaces. For this purpose, the participating members agreed to support common technological research related to factory production and process automation.
The earliest proposal in PROFIBUS was PROFIBUS FMS (Fieldbus Message Specification), a complex communication protocol designed for demanding communication tasks, suitable for general communication tasks at the shop floor level. Later, in 1993, a simpler architecture with much higher speeds was proposed: PROFIBUS DP (Decentralized Peripherals). PROFIBUS FMS is used for non-deterministic communication between PROFIBUS masters, while PROFIBUS DP is primarily used for deterministic communication between PROFIBUS masters and their remote slaves, although communication between masters is still allowed.
PROFIBUS can be divided into two types: PROFIBUS DP, which is used by most people, and PROFIBUS PA (Process Automation).
PROFIBUS DP is used in factory automation applications, allowing a central controller to control many sensors and actuators, and it can also utilize standard or optional diagnostic functions to ascertain the status of each module.
PROFIBUS PA is used in process automation systems, monitored by process control systems to control measurement devices. It is an intrinsically safe communication protocol suitable for explosive atmospheres (in the industrial hazardous area classification of Ex-zone 0 and Ex-zone 1). Its physical layer (cable) conforms to IEC 61158-2, allowing power to be supplied to field devices via communication cables, limiting current even in fault conditions to avoid situations that could lead to explosions. Due to network power supply, the number of devices that can be connected to a PROFIBUS PA network is also limited. The communication rate for PROFIBUS PA is 31.25 kbit/s. The communication protocol used for PROFIBUS PA is the same as that for PROFIBUS DP, and with the help of conversion devices, it can connect to PROFIBUS DP networks, using the faster PROFIBUS DP as the backbone to relay signals to the controller. In some applications that require simultaneous handling of automation and process control, both PROFIBUS DP and PROFIBUS PA can be used together.
By the end of 2009, there were already thirty million devices on PROFIBUS networks, of which five million were used in process control.
3. Technology
PROFIBUS Communication Protocol (OSI Model)
Note: *MBP stands for Manchester Bus Powered.
Application Layer
To effectively utilize the functions of PROFIBUS DP, PROFIBUS DP defines several different service levels:
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DP-V0: Cyclic data exchange, diagnostic functions.
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DP-V1: Cyclic and acyclic data exchange, alarm handling.
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DP-V2: Clock-synchronized data mode, data broadcasting between slaves.
Data Link Layer
The Data Link Layer FDL (Fieldbus Data Link Layer) combines communication between general masters and slaves with token-passing messages. In the PROFIBUS network, the controller or control system acts as the master, while sensors and actuators are the slaves.
There are several types of messages sent, identified by their starting character (start delimiter, SD):
Variable Length Data: SD2 = 0x68
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SD2
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LE
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LEr
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SD2
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DA
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SA
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FC
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DSAP
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SSAP
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PDU
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FCS
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ED
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Fixed Length Data: SD3 = 0xA2
Acknowledgement: SC = 0xE5
LE: Length of Protocol Data Unit (PDU) and lengths of all data including DA, SA, FC, DSAP, and SSAP
LEr: Repeated content of LE (due to PROFIBUS‘s Hamming distance HD=4, data outside the FCS check code range needs to be repeated once, hence the repetition of LE content)
DSAP: Destination Service Access Point (SAP)
SSAP: Source Service Access Point
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SAP (Decimal)
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Service
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Default 0
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Cyclic Data Exchange (Write_Read_Data)
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54
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Master to Master Service Point (M-M Communication)
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55
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Change Slave Address (Set_Slave_Add)
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56
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Read Input (Rd_Inp)
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57
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Read Output (Rd_Outp)
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58
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Control Command for DP Slaves (Global_Control)
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59
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Read Configuration Data (Get_Cfg)
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60
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Read Diagnostic Data (Slave_Diagnosis)
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61
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Write Preference Data (Set_Prm)
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62
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Check Configuration Data (Chk_Cfg)
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All service points except SAP55 are essential; if the device does not have non-volatile memory, it may not support changing the slave address with SAP55 service.
FCS: Frame Check Sequence, obtained by directly summing the bytes in the message, including PDU, DA, SA, FC, DSAP, and SSAP, without considering overflow.
ED: End Character, which is 0x16
When sending bytes, asynchronous sending mode is used, with even parity bits, and one start bit and one stop bit. When sending data, there should be no pause between the stop bit of one byte and the start bit of the next byte. When the master detects that there has been no data on the network for at least 33 bits (logical state 1), a new message will be submitted.
Bit Transmission Layer
The Bit Transmission Layer has three different transmission methods:
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If the electrical transmission method complies with the EIA-485 standard (formerly known as “RS-485”), a twisted pair with a 150-ohm impedance is used, with a bit rate range from 9.6 kbit/s to 12 Mbit/s. The length of the network line between two repeaters is also limited, varying from 100 meters to 1200 meters depending on the bit rate. This transmission method is mainly used with PROFIBUS DP.
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If using optical fiber as the transmission medium, star, bus, or ring network topologies can be used, with the length of the network line between two repeaters reaching up to 15 kilometers, allowing for redundant use of ring network topologies, which can still operate normally even if there is damage to the network.
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If using the “Manchester Bus Powered” (MBP) transmission method, the network not only carries signals but also provides power to devices. This method reduces power consumption by devices, making it suitable for explosive environments. The maximum length of this bus topology can reach 1900 meters, allowing for 60 meters of network branches connected to devices, with a fixed bit rate of 31.25 kbit/s, specifically designed for PROFIBUS PA used in process control.
Many manufacturers’ products can use PROFIBUS in mobile devices or wireless data transmission scenarios, although these products do not yet have corresponding technical standards.
Application profiles are predefined configurations by PROFIBUS for specific applications or devices, including special functions and features. Application profiles are established by working groups of the PROFIBUS International organization (PI) and published by the PROFIBUS International organization. Application profiles allow users to ensure that similar devices provided by different manufacturers have consistent functionalities, which is crucial for product openness, interoperability, and interchangeability. This expands users’ choices and drives manufacturers to enhance product performance and reduce costs.
There are many existing application profiles for PROFIBUS, such as those for encoders, measurement devices, intelligent pumps, robots, and CNC machine tools. There are also application profiles for special applications, such as those for HART, wireless PROFIBUS, and using PROFIBUS PA in process automation devices. Other application profiles include motion control’s PROFIdrive and functional safety’s PROFIsafe.
PROFIBUS became the German industrial standard DIN 19245 in 1991/1993, the European standard EN 50170 V.2 in 1996, and a part of the international fieldbus standard IEC 61158/IEC 61784 (TYPE 3) in 1996. In 2006, PROFIBUS also became the mechanical industry standard GB/T20540-2006 in the People’s Republic of China.
6. Organizations
The PROFIBUS User Organization (PROFIBUS Nutzerorganisation e.V., PNO) was established in 1989. This organization is mainly composed of PROFIBUS manufacturers and users in Europe. In 1992, the first regional user organization was established (PROFIBUS Schweiz in Switzerland), followed by other regional user organizations (RPAs).
In 1995, all regional user organizations were integrated into an international umbrella organization, the PROFIBUS & PROFINET International Association (PI). As of 2010, PROFIBUS had 25 regional user organizations worldwide (including PNO) with over 1400 members, including many key automation equipment service manufacturers and numerous end customers.
