3.3 Ethernet Cabling Systems
3.5.1 Simple Network Management Protocol
Simple Network Management Protocol (SNMP) is an industry standard protocol supported by virtually all infrastructure component manufacturers. Through the SNMP protocol, engineers can set configuration parameters (e.g., assign IP addresses) and receive diagnostics (i.e., remote monitoring data). This information, however, only relates to infrastructure components, not to Ethernet embedded controllers, devices, or I/O. EtherNet/IP is used to access Ethernet control device data.
SNMP drivers are available as standard in virtually all infrastructure-related (network management) software packages. SNMP allows a software package from vendor X to display and set parameters in vendor A, B, and C’s infrastructure components. Managed infrastructure components provide added diagnostics, such as port status (cable connected/unconnected, communications active/inactive), packet size, and statistics on the packet type transmitted (unicast, multicast, broadcast), power supply status, etc. These devices contain a data table called a Management Information Base (MIB) that contains
diagnostic and other information.
The SNMP defines the format and standard types of remote monitoring (RMON) information. This standard allows multiple vendor diagnostics to be accessed by vendor-independent software. The standard also allows vendor-specific information to be added. Within a point-to-point tree structure containing hundreds of connected devices, the ability to quickly locate cabling or other faults in the Ethernet infrastructure can be key to expediting start-ups. For more information, see 5 Deploying the Network.
3.5.2
Traps
Managed infrastructure devices allow a network engineer to set up SNMP traps. If it is important that multiple devices receive diagnostic information, all the extra network traffic could significantly affect the response time of the control network. Traps concentrate, or “trap,” the diagnostics from a quantity of infrastructure devices. Without traps, PCs, etc. would have to constantly poll the MIB data from all the infrastructure devices to determine if errors occurred. Infrastructure components can be configured with the IP addresses of one or more networked PCs and devices.
When an alarm or error condition happens (some infrastructure diagnostics can determine if a parameter exceeds a high or low limit), the infrastructure component sends an error message to the trap. This
concept is similar to the “Change of State” messaging found in CIP. An event, in this case an error, triggers a message to a specific device, in this case a trap. This “report by exception” approach limits Ethernet network management traffic, allowing greater network bandwidth to be used for control.
3.5.3
Network Management and Web-based Management Software
Many vendors offer network management software that uses SNMP, Bootstrap Protocol (BootP), and accessed MIB data to allow an operator to view the status of the entire network. It also allows the IP addresses to be loaded into the devices. This overview of the installation can be used to identify problem areas during startup or on an ongoing basis. Detailed information about the devices may also be accessed. Many devices have an integrated Web-server that allows SNMP data to be accessed using standard Internet browsers. The combination allows problem areas to be identified using network management software with the Internet browser to “zoom in” on the device’s web page(s) for access to detailed information or settings. Some devices also include embedded data sheets or user manuals that can be accessed.3.6
Isolated vs. Integrated Networks
Historically, device-level networks have been “isolated” from one another. A single controller, such as a PLC or PC, is the “master” of a small network of nodes, such as I/O blocks, variable frequency drives (VFDs), barcode readers, servo drives, and so on. This small control network does not have a physical connection to any other network other than through the master PLC or PC. As such, communications between these device-level networks require a separate network port on the PLC or PC and also require programming in the controller to send and receive data. In this way, the network communications from machine to machine, from cell to cell, and throughout a manufacturing campus have been isolated to only what was needed for the specific connected devices.
These isolated networks, while very effective at providing distributed control, do not provide the
“seamless” connectivity that industrial Ethernet offers. The ability to connect and communicate between the business system networks and the plant-floor networks via industrial Ethernet provides a huge opportunity to reduce work-in-process inventory, improve throughput, find manufacturing bottlenecks, increase uptime, and improve customer service. The “integrated network,” while providing these manufacturing improvement opportunities, demands a conscientious effort at an overall network design and layout for an entire facility. The benefit of Ethernet is that it can be scaled, from small original equipment manufacturer (OEM) machine control applications, to integrated manufacturing lines. As the applications become more complex, the quantity, type, and functionality of the required infrastructure components also grows. (See 6 Infrastructure Application Scenarios for more information.)
Typically, manufacturing and OEM control engineers can achieve the goals of a good industrial Ethernet network design by using robust industrial Ethernet components. First, the industrial Ethernet electronics and cabling systems should implement full-duplex switching technology, follow proper Category 5e (or higher) twisted-pair and fiber installation techniques, and address the environmental conditions present around the equipment. Next, using the proper equipment and building industrial Ethernet control networks in a “managed network” scheme, manufacturing and OEM control engineers can achieve machine control objectives while adding the benefits of total plant network integration. Finally, when integrated networks with industrial Ethernet are used, new requirements such as security and network traffic routing can arise. Additional “higher-level” network equipment and networking techniques may be required to meet these needs.