As we all know, SNMP (Simple Network Management Protocol) provides a method for collecting network management information from various devices on the network. It is a standard protocol for managing IP network nodes and is a field device. Provides a way to report problems and errors to the network management station. At present, almost all network equipment manufacturers have implemented support for SNMP. With the continuous development of industrial Ethernet technology in the field of automation, SNMP communication technology has also been widely used in automation control equipment. It can easily transfer some information on the industrial site to the factory management system quickly and conveniently.
2 SNMP overview
The SNMP management network is mainly composed of three parts: a network management system (NMS --- Network-management systems), a managed device (Managed device), and an agent (Agent). The network management system is used to monitor and control the managed equipment and provide a large amount of computing and memory resources required for network management. The managed device is also called a network node, and can collect and store management information through the management information base (MIB) for the network management system to read. The agent is a network management software module existing in the managed device. The agent controls and transmits the management information of the local device.
The management information base (MIB-Management InformaTIon Base) is a collection of objects. Each object is basically a data variable that contains data about configuration and performance. It can be set by SNMP through the SNMP management agent of the managed device and represents the network. Manageable resources and equipment.
The management information structure (SMI-Structure of Management InformaTIon) is used to define the rules of the objects accessible through the network management protocol. SMI defines the data types used in the MIB and the names or representations of network resources in the MIB.
SNMP Trap is an important function of SNMP. When an unexpected event occurs on the managed device, such as a system problem or some problems set by the user, the agent will actively send an alarm message to the management station, which can be handled by a predefined method.
3 SNMP in Industrial Ethernet Controller
3.1 Introduction to WAGO Industrial Ethernet System
WAGO-I / O-SYSTEM industrial Ethernet control system adopts modular structure, plug and play, compact size, flexible installation, in addition to supporting various industrial Ethernet protocols such as Modbus / TCP, Ethernet / IP, ProfiNet, SERCOS, etc. At the same time support BootP, DHCP, DNS, FTP, SNTP, SNMP and other network management protocols.
The 750-841 programmable fieldbus controller (PFC) is a classic industrial Ethernet product launched by WAGO in 2003. It supports SNMPV1 / V2c and SNMPV3, and integrates a general MIB that conforms to RFC1213 (MIBâ…¡) and a dedicated WAGO-MIB. The SNMP protocol gives full play to its advantages in this product, and plays an important role in realizing the information management of industrial field control.
3.2 Composition of MIB in PFC 750-841
The accessible network devices and their attributes are defined in the management information base (MIB), which is uniquely specified by the object identifier (OID: Object IdenTIfier). The management information base is a tree structure, and the SNMP protocol messages access the devices in the network by traversing the nodes in the SNMP MIB tree directory?
The following figure shows the structure of the WAGO-MIB object recognition tree in the SNMP-accessible 750-841 device in the network management system:
iso.org.dod.internet.provate.enterprise.wago (1.3.6.1.4.1.13576)
The grouping composition of general MIBâ…¡ in 750-841 is shown in the table below:
The objects in the WAGO-MIB for 750-841 can be read through the SNMP protocol. These objects contain information such as the status and current settings of the 750-841 device. The detailed grouping is shown in the following table:
In Table 2, the wioPlcDataWriteArea (1.3.6.1.4.1.13576.10.1.100.1.1.3) in the Plc Data Group can display and store 255 DWORD-type data, all of which come from the 750-841 fieldbus node. The internal control program determines the data distribution of each unit. These data can be the data collected directly from the industrial site through the IO module or the intermediate variables generated inside the controller. wioPlcDataReadArea (1.3.6.1.4.1.13576.10.1.100.1.1.2) can also store 255 DWORD type data, but these data are set by the network management system and transferred to the 750-841 fieldbus controller, which can be used for control Field equipment or for internal use of the 750-841 controller.
Here, two other groups that need special introduction are Snmp Trap String Group and Snmp User Trap String Group. The Snmp Trap String Group contains 740-841 field node system information, such as PLC start, PLC stop, KBUS bus error, PLC online modification, etc. The group Snmp User Trap String Group contains 10 user-definable Trap information, which is set in the 750-841 controller.
WAGO-MIB management information database files 750-917 can be obtained free of charge from the WAGO website:
3.3 Information transmission between PFC 750-841 and network management system
SNMP is an application layer protocol, and its basic functions include: obtaining, setting, and receiving unexpected information sent by agents. "Acquisition" means that the management station sends a request, and the agent sends back the corresponding data according to the request; "Setting" means the management station sends the setting (ie, agent) value to the managed device; "Receiving the unexpected information sent by the agent" means the agent It is possible to report unexpected situations to the management station without requesting the management station.
The above functions are realized by a dedicated library file WagoLibSnmp.lib in the 750-841 controller. In the CoDeSys programming environment, the function SNMP_SET_PLCDATA_WRITEAREA is used to set some data in 750-841 to the specified OID-memory according to the DWORD type, ranging from 1.3.6.1.4.1.13576.10.1.100.1.1.3.1 to 1.3.6.1 .4.1.13576.10.1.100.1.1.3.100, in other words, transfer the data in CoDeSys to the wioPlcDataWriterArea of ​​the WAGO-MIB management information base. The function SNMP_GET_PLCDATA_READAREA is used to read the data set by the user in the wioPlcDataReadArea of ​​the WAGO-MIB management information base and display the data in a specified array in CoDeSys, that is, to transfer the data in wioProcessImage to CoDeSys.
The function SNMP_SEND_TRAP_SIMPLE is used to transfer a String type data from CoDeSys to wioUserTrapMessages (1.3.6.1.4.1.13576.10.1.40.4.5.1) in WAGO-MIB as a simple Trap message to the management station. The function SNMP_SET_USERTRAP_MSG can be used to set data for all 10 user-defined messages (1.3.6.1.4.1.13576.10.1.40.4.5.1-10) in wioUserTrapMessages. These set Trap messages can be sent to WAGO-MIB using the SNMP_SEND_TRAP_ADVANCED function in.
3.4 Example analysis
The network management system uses HiVision software from HIRSCHMANN. The software can automatically scan to WAGO Ethernet 750-841 node, as shown below:
Managed equipment selects WAGO 750-841 programmable fieldbus controller, 750-430 switch input module, 750-530 switch output module, 750-461 RTD temperature detection module and 750-600 terminal module, WAGO-I / O -PRO CAA programming software. Use SNMP_SET_PLCDATA_WRITEAREA function in the programming software to input data "8", "215", "215" to WAGO-MIB 1.3.6.1.4.1.13576.10.1.100.1.1.3.1 ~ 3, where the data "215" is 750 The ambient temperature value detected by the -461 module in real time, as shown below:
The corresponding data in WAGO-MIB can be clearly observed through the Snmp Browser in the network management system. As shown below:
4 Conclusion
SNMP network management technology can be fully applied to WAGO industrial Ethernet control products, so that some real-time data on the industrial site can be directly transmitted to the factory management system, which is convenient for the equipment management of the entire factory, improves management efficiency, and reduces management costs. In addition, SNMPV3 uses DES (Data EncrypTIon Standard) algorithm to encrypt data communication and uses MD5 and SHA (Secure Hash Algorithm) technology to verify the node identifier, thereby preventing attackers from impersonating the identity of the management node to operate the network, ensuring the security of the SNMP system .
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