1 What is the bus protocol?
Fieldbus (Fieldbus) is the late 1980s, early 1990s international development of the formation of fieldbus technology for process automation, manufacturing automation, building automation and other areas of the field of intelligent equipment interconnection communication network. It serves as the foundation of the factory digital communication network, communicating between the production process sites and control devices and their connection with higher control and management levels. It is not only a grass-roots network, but also an open, new fully distributed control system. This intelligent sensing, control, computer, digital communications and other technologies as the main content of the integrated technology, has received worldwide attention, become a hot spot for the development of automation technology, and will lead to a profound change in the structure of the automation system and equipment. The working environment of fieldbus equipment is at the bottom of process equipment, as the basic communication network of factory equipment level, it is required to have the characteristics of simple protocol, strong fault tolerance, good security and low cost: it has a certain degree of time certainty and high real-time requirements, and it also has the characteristics of stable network load, most of which are short frame transmission and frequent information exchange. Because of the above characteristics, fieldbus system from the network structure to communication technology, have different upper level high-speed data communication network characteristics.
At present, there are more than 40 kinds of field buses in the international arena, but there is no one field bus that can cover all application surfaces, according to the size of its transmission data can be divided into 3 categories: sensor bus (sensorbus), belonging to the bit transmission; device bus (devicebus), belonging to the byte transmission; field bus, belonging to the data stream transmission.
Fieldbus technology to realize a key issue, is to form a manufacturer in the automation industry to comply with the technical standards of fieldbus communication protocols, manufacturers can be born in accordance with the standard products, system integrators can be in accordance with the standards of different products to form a system. This raises the issue of fieldbus standards.
Internationally renowned automation products and field equipment manufacturers, aware of the fieldbus technology is the future direction of development, have formed an alliance of enterprises, the introduction of their own bus standards and products in the market to cultivate users, expanding the impact, and actively support the international standards organization to develop fieldbus international standards. Whether they can make their own bus technology standards in the future international standards have a large proportion of components, the relationship between the company's related products, the future of the user's trust and corporate reputation. And historical experience has proved that international standards are based on one or several successful technologies in the market. Therefore, the major international companies in the development of fieldbus international standards in the competition, reflecting the company's leading position in the technology competition, but ultimately comes down to the market strength of the competition. Fieldbus consider the starting point:
Field communication network, field device inter, interoperability, decentralized function block, communication line power supply, open interconnection network
2 Characteristics of fieldbus
1, enhanced field-level information integration capabilities
Fieldbus can obtain a large amount of rich information from the field equipment, to better meet the requirements of factory automation and CIMS system information integration. Fieldbus is a digital communication network, it does not simply replace the 4-20mA signal, but also to achieve the status of equipment, faults, parameter information transfer. In addition to completing remote control, the system can also complete remote parameterization work. The system can also accomplish remote parameterization work in addition to remote control.
2, open, interoperability, interchangeability, integrability.
As long as the products of different manufacturers use the same bus standard, they are interoperable and interchangeable, so the equipment has good integrability. The system is open, allowing other manufacturers to their own expertise in control technology, such as control algorithms, processes, formulations, etc. integrated into a common system, therefore, the market will have many industry-oriented features of the monitoring system.
3, the system is highly reliable and maintainable
Fieldbus-based automation monitoring system uses a bus connection instead of one-to-one I/O wiring, for large-scale I/O systems, reducing the unreliability caused by the wiring points. At the same time, the system has the online fault diagnosis, alarm and record function of field-level equipment, and can complete the remote parameter setting and modification of field equipment and other parameterization work, which also enhances the maintainability of the system.
4, reduced system and engineering costs
For large-scale, large-scale I/O distributed system, eliminating a large number of cables, I/O modules and cable laying engineering costs, reducing the system and engineering costs.
3 General fieldbus network structure
The fieldbus protocol is based on the ISO Reference Module (OSI-RM) for open system connectivity. However, only layers 1, 2 and 7 are used, and layers 3 to 6 are not used due to real-time considerations. Meanwhile, the newly defined user layer is placed above the application layer.
The layers are defined as follows
Layer 1: Physical layer defines the communication medium of the fieldbus, i.e., alternative to the 4-20mA standard.
Layer 2: The data link layer is used to monitor the communication that takes place between the devices and to detect communication errors.
Layer 3: The application layer standardizes the communication data format in order to form information that can be interpreted by all networked devices for process control and to provide an interface to the user layer.
Layer 4: The user layer is associated with a separate plant area and provides the environment for further applications. It has an interface to higher-level control functions.
4 Profinet
The communication between the PROFINET network and external devices is realized by means of PROFINET
PROFINET IO defines the communication functions of the field-connected external devices and is based on the real-time concept of cascade. PROFINET IO defines the complete data exchange, parameterization and diagnostic functions between controllers (devices with "master function") and other devices (devices with "slave function") PROFINET IO is designed to provide fast data transfer between Ethernet-connected devices and supports the provider-consumer model. Devices that support the PROFIBUS communication protocol can be seamlessly connected to the PROFINET network without the need for an IO-Proxy. Device developers can utilize commercially available Ethernet controllers to develop PROFINET-IO devices.
PROFINET IO is suitable for systems with network cycle times of several ms.
PROFINET IO systems include the following types of devices:
IO controllers, which control the task work of the automation.
IO devices, generally field devices, are controlled and monitored by the IO controller. An IO device may consist of several modules or sub-modules.
An IO monitor is a piece of PC software that can set parameters and diagnose the status of individual modules.
PROFINET IO establishes an Application Relation (AR) between the IO Controller and the IO device, which defines Communication Relations (CR) with different characteristics such as parameter transfer, cyclic data exchange and warning handling.
The characteristics of an IO device are described by the device manufacturer in the GSD (General Station Description) file in GSDML (GSD Markup Language), which provides the PC monitoring software with the basic information needed for planning PROFINET configurations.
5 CCLInk
Cclink has three different buses: cclink, cclink ie, CC-Link IE Field Basic, we only support CC-Link IE Field Basic. cclink is similar to 485 twisted-pair, ie is Gigabit Ethernet, can't go to normal tcp, ie basic is the standard Ethernet.
CC-Link (Control &Communication Link, control and communication link system), is Mitsubishi Electric's newly launched open fieldbus, its data capacity, communication speed multi-level selectable, and it is a device layer-based network, but also can cover the higher level of the control layer and the lower level of the sensing layer. In general, CC-Link whole layer network can be composed of 1 master and 64 slaves. The master in the network is acted by PLC, and the slaves can be remote I/O modules, special function modules, local stations with CPU and PLC, human-machine interfaces, frequency converters, and all kinds of measuring instruments, valves and other field instrumentation devices.
The slave station can be remote I/O module, special function module, local station with CPU and PLC, HMI, inverter and various measuring instruments, valves and other field instrumentation devices. CC-Link has a high speed data transfer rate of up to 10 Mb/s. The underlying communication protocol of CC-Link is a high speed data transfer rate. The underlying communication protocol of CC-Link follows RS 485, and in general, CC-Link mainly adopts the broadcast-polling method for communication, CC-Link also supports instantaneous communication between master station, local station and intelligent device station.
In July 2005, CC-Link was approved by China National Standards Committee as the guiding technical document of China National Standards.
CC-Link network has perfect RAS (Reliability, Availability, Serviceability) function. CC-Link network has perfect RAS (Reliability, Availability, Serviceability) function, i.e. automatic return, cut off slave, error detection by link relay/register.
6 EthernetIP
Ethernet/IP is an industrial application layer protocol for industrial automation applications. It is built on top of the standard TCP/IP protocol and utilizes fixed Ethernet hardware and software to define an application layer protocol for configuring, accessing and controlling industrial automation devices.
Ethernet/IP divides Ethernet nodes into predefined device types in a special way. the Ethernet/IP application layer protocol is based on the Control and Information Protocol (CIP) layer and provides a seamlessly integrated system from the industrial floor to the enterprise network. ethernet/IP uses all the traditional ethernet protocols and is built on top of the standard ethernet technology, this This means that Ethernet/IP works transparently with all current standard Ethernet devices. More importantly, the fact that Ethernet/IP is built on a standard Ethernet technology platform ensures that the former will evolve as the latter develops. The pro-Ethernet/IP community is working to produce a comprehensive and robust standard, and work on Ethernet/IP is being done by multiple distributors, including customized specifications and comprehensive testing in accredited test labs.
7 EtherCAT
EtherCAT (Ethernet Control Automation Technology) is an open-architecture, Ethernet-based fieldbus system, whose name CAT is an acronym for Control Automation Technology. EtherCAT is a deterministic industrial Ethernet, first developed by Beckhoff in Germany.
EtherCAT supports all device connection topologies such as linear, tree, ring and star, and the physical media can be either 100Base-TX standard Ethernet cables or fiber-optic cables. With 100Base-TX cables, the distance between stations can be up to 100 m. Up to 65,535 devices can be connected to the entire network. The entire network can connect up to 65,535 devices. A master-slave ring structure is formed using Fast Ethernet full-duplex communication technology.