Abstract: This article comprehensively describes the system architecture, system specifications, communication protocols, and communication conventions of the INTERBUS bus. The INTERBUS bus employs innovative lumped-frame isochronous transmission technology and has accumulated rich practical experience in system power supply, grounding and lightning protection, and rapid diagnostics, thus achieving an application record of 7.5 million installed nodes. Furthermore, this article also discusses in depth the current status and development of INTERBUS technology standards.
In May 2005, INTERBUS fieldbus officially became part of China's industry standard JB/TIO308.8, "Fieldbus Type 8: INTERBUS for Measurement and Control Digital Data Communication Industrial Control Systems." INTERBUS is one of the world's earliest developed fieldbuses, first developed in 1984 by Phoenix Contact in Germany, with support from the Interbus Club. Due to its rapid development and widespread use, INTERBUS has successively become the German standard DIN19258, the European standard EN50254, and the international standard IEC61158. INTERBUS has over 1,000 bus device manufacturers worldwide, offering up to 2,500 products. To date, INTERBUS fieldbus has been installed in over 7.5 million nodes worldwide, ranking second among various fieldbuses.
1. INTERBUS System Structure and Specifications
INTERBUS is a digital serial communication system used for communication between control systems (such as programmable logic controllers) and field devices such as industrial sensors and actuators. The INTERBUS bus uses a central master-slave access method and a tree topology for data exchange between connected master system applications and slave applications, as illustrated in Figure 1. The INTERBUS protocol provides users with two data transmission channels: a process data channel and a parameter channel. Combining these two channels forms a hybrid network communication structure. The network segment starting from the master station is the first segment (a group of slave stations), and this segment can be extended to include more segments through a summing coupler. Slave stations and bus couplers do not have addresses; their addresses are determined by their position in the ring.
Figure 1. INTERBUS System Architecture
For an INTERBUS system, the entire system consists of interconnected bus segments. The INTERBUS bus can be divided into three different types of bus segments: remote bus segments, local bus segments, and Interbus loop segments. Each remote bus segment begins with a remote bus terminal module, and the maximum length of a remote bus is 400 meters (copper cable). The total length of the entire INTERBUS system can reach 12.8 kilometers. If the remote bus requires power, it is called an installed remote bus, which transmits both data and power. Each remote bus terminal module leads out a local bus segment composed of local bus modules. The local bus is mainly used in control cabinets and provides additional power to transmitters and actuators. The Interbus loop segment is a local bus segment that can be directly applied to IP65 field applications. It uses two-core unshielded wire and is powered by the bus. One loop can support 63 modules and has a total length of 200 meters. Depending on the user's requirements, these different Interbus bus segments can be used to construct a field cable network structure that meets various practical needs. The INTERBUS system specifications are shown in Table 1.
Table 1 Typical INTERBUS System Specifications
The table shows that the security of INTERBUS data is fully protected.
2. INTERBUS protocol
The INTERBUS communication protocol fully complies with the ISO/OSI Open Systems Interconnection model and conforms to the IEC 61158 Fieldbus three-layer reference model. The INTERBUS reference model includes a physical layer, a data link layer, and an application layer. To improve transmission efficiency, the data link layer (DLL) consists of three sublayers: Media Access Control (MAC), Basic Link Layer (BLL), and Peripheral Device Data Link (PDL). This data link layer generates isochrones and summation frame transmission protocols, as shown in Figure 2.
Figure 2. Clustered Frame Transmission Protocol
The INTERBUS bus system is a data ring structure. The master station initiates the first network segment, and slave stations are connected to the segment. Data is transmitted from the master station to all slave stations. Slave stations receive data from preceding devices and transmit it to the next. A slave station retrieves the input data from the lumped frame, inserts its outgoing data at the corresponding location, and bypasses other data. For each slave station on the bus, a fixed time slice is determined based on its data width within the frame. Data packets are transmitted in the physical order of the connected slave stations. Periodic data (process data) and aperiodic data (parameter data) are transmitted simultaneously. Slave parameter data uses fixed-length 2, 4, and octets within the lumped frame. Longer PDUs are segmented by the data link layer; this is the INTERBUS bus-specific PCP (Peripherals Communication Protocol) peripheral device communication protocol. PCP then combines all the information fragments to form a complete message.
As can be seen from the frame format in Figure 2, a transmission cycle is initiated by the master station and begins with a specific data sequence, including the LBW echo word following the output data. After the lumped frame is output, all output data is correctly sent to the corresponding devices. The 32-bit Frame Check Sequence (FCS) following the process data is used to verify the transmitted data; it is implemented using a 16-bit CRC polynomial.
Because it uses lumped frames to transmit data, the INTERBUS fieldbus has a significantly higher communication efficiency than other fieldbuses. Table 2 lists a comparison of the transmission efficiency of various fieldbuses, using a system with 32 devices as an example, including 20 8-bit input devices and 12 8-bit output devices.
Table 2 Comparison of transmission efficiencies of various fieldbuses
As can be seen from the table, the communication efficiency of the INTERBUS fieldbus is over 52%, while that of the Ethernet network is only 0.77%. For this reason, the rapidly developing industrial Ethernet technology largely adopts the INTERBUS isochronous transmission protocol.
The INTERBUS Application Layer (AL) provides services responsible for both periodic data transmission (process data channel) and aperiodic data transmission (parameter channel). The process data channel allows for highly efficient and high-speed transmission of process-related data, such as setpoints and measured values between a master and slave station. Typically, the data transmitted via the process data channel has a simple structure but is highly dynamic and up-to-date; through process data transmission, applications directly access DLLs. For periodic data transmission, INTERBUS uses a promotional publisher/subscriber pattern for efficient data delivery, with the corresponding Application Relationship Protocol (ARPM) mechanism being the "Network Scheduled Intent Buffer (BNU)". The parallel parameter channel, operating in parallel with the process data channel, transmits complex data between two communication partners aperiodically. Data transmitted via the parameter channel has no requirements for dynamic response and the transmission volume is small. It is typically used for parameter initialization. Aperiodic transmission uses a client/server communication model, where both the master and slave stations can act as clients or servers. Two types of user ARPMs are provided for aperiodic transmission: "User Initiated Bidirectional Queuing with Flow Control" (QUB-FC) and "Transparent Mode User Initiated Bidirectional Queuing" (QUB-TM).
3. INTERBUS Communication Standards
Because fieldbus standards are conceptual technical standards, they are highly conceptual, abstract, and broad in scope. To facilitate users in designing fieldbus devices for industrial networks, the IEC/TC65/SC65C Data Communications Committee recently developed "IEC 61784 Fieldbus Communication Profiles for Continuous Discrete Manufacturing Industrial Control Systems," which specifies the selection criteria for communication protocol stacks for various communication devices. In this standard, the INTERBUS fieldbus is designated as Communication Profile Families (CPF) 6 (Category 6). JB/T 10308.8, "INTERBUS Specification," is the first fieldbus standard to incorporate communication profiles, thus facilitating use by a wide range of domestic users.
According to the INTERBUS specification, three predefined communication rules are established. Each communication rule provides a complete set of regulations and additional constraints to ensure interoperability of INTERBUS devices. For some unique devices, further selection of service, parameters, and parameter annual values is required. The three defined communication rules are:
Rule 6/1: Rule 6/1 defines the general standard INTERBUS rules, which include the selection of AL, DLL and PHL services and the definition of protocols that conform to INTERBUS application access.
Rule 6/2: Rule 6/2 expands the aperiodic data exchange capabilities of Rule 6/1. It specifies transparent access to INERBUS devices via AR-Send-Data-Acknowledge, suitable for devices with other protocol stacks such as TCP/IP-based applications. This protocol stack uses AR-Send-Data-Acknowledge without affecting the rule definition.
Standard 6/3: Standard 6/3 uses a simplified service group for non-periodic data exchange, suitable for INTERBUS devices with limited resources. Master or slave devices conforming to the communication standard can be further subdivided based on CP identifiers, as shown in Table 3. For detailed specifications of the CPF6 communication standard, please refer to "JB/TI0308.8 INTERBUS Specification".
Table 3 Device CP Identifier Allocation
4. New Developments in INTERBUS Technology
With the development of IT technology, INERBUS fieldbus technology has been continuously improved and developed in practice through expanding its applications.
4.1 The INTERBUS bus will be organically integrated with PROFINET industrial Ethernet.
Starting in early April 2004, the INERBUS Club decided to collaborate with PROFIBUS International (PI) to develop the PROFINET real-time Ethernet standard. PROFINET is an Ethernet-based automation standard that forms the architecture of component-based distributed automation systems from the I/O level to the management level. The system architecture combining INTERBUS and PROFINET is shown in Figure 3. As shown in the figure, INTERBUS connects to PROFINET in real-time via Ethernet through a proxy server. After adopting the PROFINET solution, users can still use the existing INERBUS high-efficiency communication protocol as a supplement to PROFINET. Therefore, the combination of INTERBUS and PROFINET will provide an ideal solution for many customers.
Figure 3. System consisting of INTERBUS and PROFINET
4.2 Fieldsafe Bus Combining INTERBUS and Safety Technology
With the development of fieldbus technology, the integration of safety signals into the field to form an integrated safety instrumented system has become an urgent task. To meet this need, Phoenix Contact has developed an economical and reliable solution: INTERBUS-Safety. This safety bus employs the addition of a Safety Control module, enabling conventional controllers to become compatible systems for controlling and processing safety data, as detailed in Figure 4. The Safety Control module operates within the INTERBUS bus, utilizing the bus network to control the transmission of safety data and information between the fieldbus and the safety module.
The INTERBUS Club began researching the INTERBUS safety bus in 1999. In 2001, it obtained a license from the German BIA organization for the EN954-ILKAT4 standard and conforms to the SIL3 level of the IEC61508 standard, "Functional safety of electrical/electronic/programmable electronic safety systems." This solution is superior to other solutions because it separates the functions of the control system from the safety bus, resulting in a clear and simple control system structure that does not affect future expansion and modification, and significantly reduces costs. It represents a development direction for fieldbuses.
4.3 The IEC61918 standard adopts extensive practical experience in INTERBUS installation and maintenance.
INTERBUS bus installation and maintenance are very simple, and there is extensive practical experience in system configuration planning, power supply selection, grounding and lightning protection, fiber optic technology, quick field connections, and online diagnostics and rapid troubleshooting. To promote and guide engineering practices in this area, the International Electrotechnical Commission (IEC) SC65C/WG10 working group is developing IEC 61918, "Installation Practice Specification for Fieldbus Systems." Only three types of fieldbuses are included in this international standard: INTERBUS, PROFIBUS, and Control NET. The main contents of INTERBUS technical practice include:
(1) Select a highly reliable power supply
The first issue to address in designing the entire control system is the power supply. The selected power supply should be comprehensive in variety, highly reliable, and capable of handling transient power outages and long-term phase loss. It should have a very wide input voltage range to ensure safe and reliable output even under significant grid fluctuations. The power supply should be able to operate in ambient temperatures up to 70°C. It should also have a high power density at the output to ensure reliable startup under heavy inrush current loads. Furthermore, it should feature triple output voltage status alarm functions, including LED display and dry contact output. Phoenix Contact's QUINT system switching power supply meets these requirements in terms of performance and safety. Phoenix Contact power supplies are shock-resistant, impact-resistant, and corrosion-resistant. They can be connected in parallel without isolation diodes, offering simplicity, convenience, and high reliability.
(2) High-level grounding and lightning protection technology
Currently, grounding and lightning protection technology for fieldbus systems has reached a new level. Based on in-depth research into the causes and fundamental principles of lightning electromagnetic pulses and surge impacts, the principles of grounding have undergone significant changes, leading to new solutions for lightning and surge protection. Simultaneously, a new series of lightning and surge protectors has been developed. In this field, Phoenix Contact's lightning protection technology and its TRABTECH series products are world-leading. Phoenix Contact's TRABTECH lightning and surge protectors can be used in power systems, process control systems, data networks and standard interface products, wireless transceiver systems, and telecommunications systems. Regarding system grounding, Phoenix Contact proposes that equipotential bonding and its grounding system connection are crucial components of surge protection systems. Equipotential bonding should be as close as possible to the equipotential bonding network. Therefore, it is recommended to install cross-shaped conductive connectors between system components directly connected to the main equipotential bonding system. For all grounding cables and equipotential bonding cables, equipotential busbars should be installed as central connection points. Equipotential bonding should be considered in advance during the planning stage to ensure the system's technical efficiency and economy.
(3) INTERBUS has extremely strong diagnostic capabilities.
The INTERBUS bus boasts powerful monitoring and diagnostic capabilities. Its bus monitoring function oversees the entire network system's operational status and also provides network reconfiguration capabilities, allowing for the immediate disconnection and connection of specific sub-bus segments according to design requirements. Monitoring is a powerful tool for field installation, commissioning, diagnosis, and maintenance. Specifically, it identifies and determines installation and component errors, while its diagnostic function not only pinpoints faults but also eliminates them without the need for specialized tools. INTERBUS modules feature input/output status displays, allowing for output status settings during commissioning and the saving of parameters for certain intelligent devices. INTERBUS provides hardware and software tools for on-site diagnostics. Its DSC controller hardware features a central diagnostic display that accurately shows fault location and type, enabling operators to quickly diagnose and resolve issues, significantly improving work efficiency. INTERBUS's CMD and Diag+ software are powerful tools for accelerating on-site commissioning and rapid diagnosis.
Due to its innovative technologies, the INTERBUS fieldbus enables vertical integration from the field level to the management level, as well as horizontal integration at each level, thus forming a distributed, fully integrated automation control system. For this reason, the INTERBUS fieldbus system has been widely used in China in fields such as automotive manufacturing, machinery, papermaking, metallurgy, light industry, food, tobacco, and logistics, and has become one of the preferred automation systems for many manufacturers.
