A review of the architectures of ten types of fieldbuses

After 15 years of debate, the IEC61158 fieldbus international standard for industrial control systems was finally passed in early 2000. The fieldbus dispute gradually subsided, and the IEC/SC65C/WG6 fieldbus standards committee also completed its historical mission. In order to further improve the IEC61158 standard, IEC/SC65C established the MT9 fieldbus revision group to continue this work. The MT9 working group continued to improve and expand on the basis of the original 8 types of fieldbuses, and in August 2001, it formulated the third edition of the fieldbus standard consisting of 10 types of fieldbuses, which are: Type 1 TS61158 fieldbus, Type 2 ControlNet and EtherNet/IP fieldbus, Type 3 Profibus fieldbus, Type 4 P-NET fieldbus, Type 5 FF HSE fieldbus, Type 6 Swift-Net fieldbus, Type 7 WorldFIP fieldbus, Type 8 Interbus fieldbus, Type 9 FFH1 fieldbus and Type 10 Profinet fieldbus. This standard became a formal international standard in April 2003. Due to limited space, this article briefly discusses 10 types of buses and briefly reviews the latest developments of the above buses in the past three years.

1 Type1 TS61158 Fieldbus

The Type 1 fieldbus standard consists of the following parts:

PhL: Superset of IEC61158-2:1993 standard;

A superset of Foundation Fieldbus;

A functional superset of WorldFIP;

DLL：IEC TS61158-3，TS61158-4；

A superset of Foundation Fieldbus;

A functional superset of WorldFIP;

AL：IEC TS61158-5，TS61158-6。

Before 1998, IEC/SC65C only recommended one type of fieldbus, which mainly adopted the basic technology of Foundation Fieldbus and WorldFIP, and formulated the fieldbus standard strictly in accordance with the IEC definition. Due to various reasons, it was not passed after multiple rounds of voting, and could only become the technical report TS61158 according to regulations, and the current Type 1 fieldbus was formed on this basis. The universal fieldbus network structure recommended by the International Electrotechnical Commission is shown in Figure 1. It can be seen from the figure that the fieldbus system can support information processing, monitoring and control systems in various industrial fields, and is used for low-level communication between process control sensors, actuators and local controllers, and can be interconnected with the PLC of factory automation. Here, the H1 fieldbus is mainly used at the field level, with a rate of 31.25Kbps, responsible for the two-wire power supply to the field instrument, and can support the intrinsic safety of bus-powered equipment; the H2 fieldbus is mainly oriented to the process control level, monitoring management level and high-speed factory automation applications, with a rate of 1Mbps, 2.5Mbps and 100Mbps.

Figure 1 General fieldbus network structure

2 Type2 ControlNet and EtherNet/IP fieldbus

The Type 2 fieldbus standard developed by ControlNet International (CI) consists of the following parts:

PhL and DLL: ControlNet;

AL：Control Net和EtherNet/IP。

The architecture of the Type2 fieldbus system is shown in Figure 2. ControlNet uses a new communication mode, the Producer/Consumer mode, which allows all nodes on the network to access the same data from a single data source at the same time. Its main features are enhanced system functionality, improved efficiency and precise synchronization. The media access of the network is controlled by a time-limited access algorithm, that is, the parallel time domain multiple access (CTDMA) method is used to adjust the node's information transmission opportunity within each network refresh interval (NUI).

Figure 2 Type 2 fieldbus system architecture

EtherNet/IP Ethernet Industrial Protocol is an open industrial network that uses an active star topology and can mix 10Mbps and 100Mbps products. The protocol adds the Control and Information Protocol (CIP) on top of TCP/UDP/IP to provide a common application layer. The control part of CIP is used for real-time I/O messages, and its information part is used for message exchange. Both ControlNet and EtherNet/IP use this protocol to communicate, share the same object library, objects and device rules, so that devices from multiple suppliers can be plug-and-play in the entire network mentioned above. The definition of objects is strict, supporting real-time messages, configuration and diagnosis on the same network. In order to improve the real-time performance of industrial Ethernet, ODVA (Open DeviceNet Vendor Association) announced the IEEE1588 "Clock Synchronization for EtherNet/IP Real-Time Control Applications" standard in August 2003.

3 Type3 Profibus fieldbus

Type3 fieldbus is supported by the Profibus user organization PNO, and Siemens of Germany is the main supplier of Profibus products. The Profibus series consists of three compatible parts, namely Profibus-DP, Profibus-FMS and Profibus-PA. In order to improve the performance of the Profibus bus, PNO has launched new versions of Profibus-DP-V1 and Profibus-DP-V2 in recent years, and gradually eliminated the Profibus-FMS bus. The extended Profibus-DP fieldbus architecture is shown in Figure 3. Profibus-DP is particularly suitable for high-speed communication between device-level automatic control systems and distributed I/O. Profibus-PA is designed for process automation. It can connect transmitters and actuators to a common bus, and power supply and data communication can be completed using two wires, and intrinsic safety performance can be achieved. Based on this, the extended DP function DPV1 further improves the Profibus-PA function; DPV2 solves major problems such as communication and time synchronization between slave stations.

Figure 3 Type 3 Profibus fieldbus architecture

Profibus-DPV1 mainly adds acyclic services and expands the communication with Class 2 master stations. As we all know, the performance feature of Profibus-DP is to apply data exchange services on the basis of cyclic connection (Mscy-C1) to realize centralized data exchange between a master station and a series of slave stations. Class 1 master station refers to PLC, PC or controller. Class 2 master station refers to operator station and programmer, etc. DPV1 expands the above functions and adds acyclic services on the basis of the existing Mscy-C1 connection. The new services can read and write any data group in the slave station. In the past, Class 2 master stations could only use the connectionless services of DP slave stations, but now they can read and write arrays acyclically through connection-oriented communication, and expand the functions for entering Internet communication.

Profibus-DPV2 can realize cyclic communication, non-cyclic communication and communication between slave stations. Since slave stations can communicate directly, the communication time is shortened by one DP bus cycle and master station cycle, thereby shortening the reaction time by 60% to 90%. At the same time, bus cycle cycles with equal time intervals are established, and the time deviation is less than 1μs, which is suitable for high-precision positioning control and can realize closed-loop control. DPV2 can develop special profiles according to different application needs, such as ProfiDrive for motion control and Profisafe for interlocking protection.

4 Type4 P-NET fieldbus

The P-NET fieldbus was developed by the Danish company Process-Data Sikebory Aps in 1983 and is mainly used in the beer, food, agriculture and animal husbandry industries. It is supported by the P-NET user organization and has more than 5,000 application systems on site.

Figure 4 Type 4 P-NET fieldbus architecture

P-NET fieldbus is a multi-master station and multi-network system. Figure 4 shows the architecture of the P-NET system. The bus adopts a segmented structure. Multiple master stations can be connected to each bus segment. The master stations can be interconnected on the Internet through interfaces. It allows direct addressing in several bus areas without a hierarchical network structure. The bus communication protocol includes 1, 2, 3, 4 and 7 layers, and uses the channel mechanism to define the user layer. The communication adopts a virtual token transmission method. The master station sends a request, and the addressed slave station immediately returns a response within 390μs. Only the data stored in the slave station memory can be accessed. Each station node contains a general single-chip microprocessor, and the matching 2KB EPROM can be used not only for communication, but also for measurement, calibration, conversion and application functions. The P-NET interface chip performs all functions of the data link layer, and the functions of layers 3 and 4 are solved by the software in the host processor. The physical layer of the bus is based on the RS-485 standard, using shielded twisted pair cable, with a transmission distance of 1.2km, and NRZ encoding asynchronous transmission.

5Tpye5FF HSE fieldbus

In 1998, the Fieldbus Foundation (FF) of the United States decided to develop the H2 fieldbus using the High Speed ​​Ethernet (HSE) technology as the backbone network of the communication network above the control level of the fieldbus control system. It is integrated with the H1 fieldbus to form an information integrated open architecture. Figure 5 shows the system structure. The HSE network follows the standard Ethernet specification and appropriately adds some functions according to the needs of process control. However, these added functions can be seamlessly operated within the standard Ethernet structure framework, so the FF HSE bus can use the currently popular commercial (COTS) Ethernet devices. The 100Mbps Ethernet topology uses a switch to form a star connection. This switch has a firewall function to block special types of information from entering and leaving the network. HSE uses standard IEEE802.3 signal transmission, standard Ethernet wiring and communication media. The distance between the device and the switch is 100 meters using twisted pair cables and up to 2 kilometers using optical cables. HSE uses a connection device (LD) to connect the H1 subsystem. LD performs a bridge function, which allows each field device connected to the H1 network to complete point-to-point peer-to-peer communication. HSE supports redundant communication, and any device on the network can be configured redundantly.

Figure 5 System structure

Layers 1 to 4 of FF HSE are defined by existing Ethernet, TCP/IP and IEEE standards. HSE and H1 use the same user layer. The Fieldbus Information Specification (FMS) defines the service interface in H1, and the Field Device Access Agent (FDA) provides an interface for HSE. The user layer specifies function modules, device descriptions (DD), function files (CF) and system management (SM). FF specifies 21 function modules for basic and advanced process control. FF also specifies new flexible function modules (FFBs) for complex batch processing and mixed control applications. FFBs support data acquisition monitoring, subsystem interfaces, event sequences, multi-channel data acquisition, and network connectors for PLC and other protocol communications.

6 Type6 SwiftNet fieldbus

Type 6 SwiftNet fieldbus was developed by the US SHIP STAR Association and supported by Boeing. It is mainly used in aviation and aerospace. This bus is a simple structure with high real-time performance. The protocol only includes the physical layer and data link layer. SwiftNet fieldbus adopts a layered bus topology. Its communication architecture is shown in Figure 6.

Figure 6 Type 6 SwiftNet fieldbus architecture

The physical layer transmission rate is 5Mbps, at which time 105 different messages are transmitted per second. The bus uses TDMA (Slotted time division multiple access) slot time slice multiple access method to provide dedicated high-speed, low-jitter synchronous channels and channels specified on request. Dedicated channels are suitable for the distribution or exchange of automatic status data; channels specified on request are suitable for non-scheduled messages. The TDMA method divides the time on the bus into equal length intervals, called slots. Only when a station on the bus is assigned to a slot can it * and send. Each station can also coordinate bus access, data transmission and reception according to the proportion occupied.

The nodes (slaves) of the SwiftNet bus are connected to intelligent sensors through I/O channels. Due to the high requirements for sensors and actuators in the aerospace field, the bus began to use network sensors that comply with the IEEE1451.1 and IEEE1451.2 standards. The IEEE1451.2 "Intelligent Converter Interface for Sensors and Actuators" standard is an open standard that defines a standard interface between the control network and the sensor, making the network sensor a digital sensor at the field level of the fieldbus system, and allowing users to choose intelligent sensors produced by different manufacturers according to their needs, realizing true plug-and-play.

7 Type7 WorldFIP fieldbus

The WorldFIP Association, founded in 1987, developed and vigorously promoted the Type 7 WorldFIP fieldbus. The WorldFIP protocol is the third part of the EN50170 European standard, and the physical layer adopts the IEC61158.2 standard. Its products occupy 60% of the market in France and about 25% of the market in Europe. It is widely used in power generation and transmission and distribution, processing automation, railway transportation, subway and process automation.

The WorldFIP fieldbus architecture is shown in Figure 7. The system is divided into three levels, namely process level, control level and monitoring level. It can meet various needs of users and is suitable for various types of application structures, centralized, decentralized and master/slave types. A single WorldFIP bus can meet the needs of process control, factory manufacturing and various drive systems. In order to meet the requirements of low cost, a low-cost Device WorldFIP (DWF) bus was developed. It is a device-level network that can adapt well to various harsh environments in industrial sites and has intrinsically safe explosion-proof performance. It can realize communication between multiple master stations and slave master stations.

Figure 7 Type7 WroldFIP fieldbus architecture

The WorldFIP protocol consists of a physical layer, a data link layer, and an application layer. The data link layer uses a bus arbitration method. At any given moment, only one station can perform the bus arbitration function. It uses an ID-DAT frame to broadcast an identifier on the bus. The link layer connected to all stations on the bus records the ID-DAT frame at the same time. Only one station is identified as the producer of the identifier, and the other stations are only users. The application layer provides MPS and SubMMS services. MPS is a factory cycle/non-cycle service, and SubMMS is a subset of industrial messages.

8 Type8 Interbus fieldbus

Type 8 Interbus fieldbus is developed by Phoenix Contact of Germany and supported by Interbus Club. Interbus has more than 1,000 bus equipment manufacturers in the world, providing more than 2,500 products. So far, Interbus has more than 500,000 application systems widely used in the global automation field.

Interbus fieldbus is an open serial bus that can form various topological forms and allows 16 levels of nested connection. The bus can connect up to 512 field devices, with a maximum distance of 400 meters between devices and a maximum distance of 12.8 kilometers without a repeater network. Interbus includes remote bus and local bus. The remote bus is used to transmit data over long distances and uses RS-485 transmission. The network itself is not powered and has a communication rate of 500Kbps and 2Mbps. Interbus has its own unique loop structure, which uses standard cables to transmit data and power at the same time. The loop can connect analog, digital devices and even complex sensors/actuators, and also allows direct access to intelligent terminal instruments.

Figure 8 Type8 Interbus fieldbus architecture

The protocol includes physical layer, data link layer and application layer. The data link layer adopts the process data-oriented transmission method and the lumped frame protocol, which can transmit cyclic process data and non-cyclic data. The frame information includes a start signal, loopback information, and data safety/end information. The lumped frame has a very high transmission efficiency, which is as high as 52%. Although the INTERBUS communication rate is 500Kbps and 2Mbps, the communication speed and efficiency are very high. The application layer service is used to realize real-time data exchange, VFD support, variable access, program call and 12 related services.

The INTERBUS bus has a strong monitoring and diagnostic function. The bus monitoring function monitors the operating status of the entire network system. It also provides a variable structure function for the bus network, which can shut down and connect a sub-bus segment of the bus in time according to design requirements. The monitoring function is a powerful tool for on-site installation, commissioning, diagnosis and maintenance. The specific function is to identify and determine installation errors and component errors. The fieldbus module has an input/output status display, and the output status can be set during commissioning, and the parameters of certain intelligent devices can be saved.

In order to improve the security of industrial networks and meet the requirements of fault-safe communication in manufacturing and process industry automation, the function of safety bus is given priority, especially in the automotive industry. The INTERBUS Club began to study the INTERBUS safety bus in 1999, and obtained the license certificate of the German BIA organization EN954-1 KAT.4 standard in 2001, and complies with the IEC61508 safety communication standard. The INTERBUS safety bus consists of a Safe Control control module and a field distributed intelligent module. The only function of the control module is to control safety data. This solution is superior to other solutions. It completely separates the control system from the safety bus function. The control system structure is clear and simple, does not affect future expansion and modification, and the cost is significantly reduced. This is a development direction of the field bus.

9 Type9 FF H1 fieldbus

H1 fieldbus is developed by FF Fieldbus Foundation. FF Foundation members are composed of world-renowned instrument manufacturers and users. The transmitters, DCS, actuators, and flow meters produced by its members account for 90% of the world market. They have a thorough understanding of the functional requirements of process control field industrial networks and have accumulated rich experience in process control. The proposed fieldbus network architecture is relatively comprehensive, and its communication architecture is shown in Figure 9. Type 9 fieldbus is a subset of Type 1 fieldbus.

Figure 9 Communication architecture

The FF H1 fieldbus protocol consists of a physical layer, a data link layer, an application layer, and a user layer that is added to take into account the control functions and specific applications of field devices. The H1 bus supports a variety of transmission media: twisted pair, cable, optical cable, and wireless media. The transmission rate is 31.25Kbps, and the maximum communication distance is 1,900 meters. The bus supports bus power supply and intrinsic safety.

The data link layer is responsible for implementing link activity scheduling, data reception and transmission, activity status response, link time synchronization between devices on the bus, etc. Here, the bus access control adopts the Link Active Scheduler (LAS) method. LAS has a list of all devices on the bus and is responsible for the operation of the bus by each device on the bus segment.

The fieldbus application layer consists of the fieldbus access (FAS) sublayer and the fieldbus message specification (FMS) sublayer. The FAS sublayer provides message services in three modes: publisher/subscriber, client/server, and report distribution. The FMS sublayer provides object dictionary (OD) services, variable access services, and event services. The fieldbus user layer has standard function blocks (FBs) and device description functions. The standard specifies 32 types of function blocks, which are used by field devices to complete control strategies. Since the device description function includes all the information required to describe device communication and is independent of the master station, it enables field devices to achieve true interoperability.

10 Type10 PROFI net fieldbus

The PNO organization published the Profinet specification in August 2001. Profinet organically integrates factory automation and enterprise information management IT technology, while fully retaining the existing openness of Profibus.

The Profinet fieldbus architecture is shown in Figure 10. As can be seen from the figure, the solution supports open, object-oriented communication, which is based on the commonly used Ethernet TCP/IP. The optimized communication mechanism can also meet the requirements of real-time communication. The DCOM communication protocol based on object application is established through this protocol standard. Profinet components represented in the form of objects exchange their automation data according to the object protocol. Automation objects, namely COM objects, appear on the communication bus as PDUs in the form defined by the DCOM protocol. Connection object activity control (ACCO) ensures the establishment of communication relationships and data exchange between configured interconnected devices. The transmission itself is controlled by events, and ACCO is also responsible for recovery after failure, including the transmission of quality codes and time stamps, monitoring of connections, re-establishment after connection loss, and testing and diagnosis of interconnectivity.

Figure 10 Type 10 Profinet fieldbus architecture

Profibus can be easily integrated with other fieldbus systems through a proxy server. In this solution, the general Profibus network is connected to the industrial Ethernet through a proxy server; access to Profibus devices through Ethernet TCP/IP is handled by the Proxy using remote procedure calls and Microsoft DCOM.

Profinet provides engineering design tools and manufacturer-specific programming and configuration software. This tool can be used to create COM-based automation objects from devices developed by controller programming software. This tool will also be used to configure Profinet-based automation systems. Using this manufacturer-independent object and connection editor can reduce development time by 15%.

Conclusion

The 10 types of fieldbus use completely different communication protocols. Type 1 uses LAS and Publisher/Subscriber mode; Type 2 ControlNet uses CTDMA and Producer/Consumer mode, and EtherNet/IP uses Ethernet TCP/IP protocol; Type 3 is a token ring and master/slave mode; Type 4 communication uses virtual token passing; Type 5 uses CSMA/CD and Ethernet TCP/IP protocol; Type 6 uses TDMA multiple access; Type 7 uses bus arbitration; Type 8 uses an integral frame protocol; Type 9 uses LAS and Publisher/Subscriber mode; Type 10 uses Ethernet TCP/IP protocol.

From the above discussion, it can be seen that the ten types of fieldbus architectures and communication protocols are different, but there is a basic consensus on the use of Ethernet TCP/IP industrial Ethernet technology for H2 high-speed fieldbus. EtherNet/IP, FF HSE and Profinet industrial Ethernet technologies have in fact become international standards.

It is understood that the maintenance period of IEC61158 standard is December 31, 2007, which means that no new fieldbus will be added before this date, and no changes will be made to the fieldbus in the current standard. It can be seen that the architecture and pattern of the fieldbus system have been basically determined, and the positioning of industrial Ethernet and fieldbus in the system has been clarified. The relationship between the two is shown in Table 1.

Table 1 Relationship between Industrial Ethernet and Fieldbus

The table lists the four main competitors of the current industrial Ethernet technology. FF HSE, EtherNet/IP and Profinet all have their own fieldbuses at the field level, and this pattern will not change in the future. The IDA industrial Ethernet technology adopted by Phoenix Contacts' Type8 INTERBUS has developed rapidly and extended the industrial Ethernet to the field I/O level. At the same time, the Modbus TCP/IP protocol has been submitted to the International Internet Engineering Task Force (IETF), hoping that this protocol can become the industrial Internet protocol standard. In order to accelerate this work, the Modbus organization and the IDA organization announced a merger at the end of October 2003. The development of industrial Ethernet is also being carried out in China. The EPA industrial Ethernet developed by Zhejiang University Zhongkong Technology Co., Ltd. has been extended to the field level and has been successfully tried in chemical plants.

The long-term debate on fieldbus has made people gradually realize that the unification of digital technology is completely different from analog technology. The path to the unification of fieldbus control systems should fully absorb the successful experience of achieving unification with Internet technology, face the market, continuously innovate, establish an interoperable industrial Ethernet network, and ultimately create a unified information platform for industrial automation.