Industrial Communication Whitepaper Ethernet as a Distributed I/O Network 1
Contents Executive summary...3 Introduction...4 The need for open networks...5 Ethernet capabilities today...6 Ethernet architectures...7 Ethernet now ready for field bus applications...9 Conclusion...11 2
Executive summary Ethernet Communications has evolved from a network that connected Computer Systems for data exchanges to a true Industrial Communication Network that is used to connect all the major system components such as Distributed I/O systems. Today, Ethernet has the capabilities and hardware to meet all the real time and harsh environment requirements for todays industrial environment while maintaining its easy to use capabiliites and being an Open Standard. The architectures for control systems has evolved from large centralized systems to highly distributed systems, so has the has the selection of communication networks that are used to connect all of the major system components as well as the connection to the facility computer network. The early control systems were large centralized systems with all the control hardware and the I/O system all mounted in large cabinets located in the control room that required long wire runs to connect all of the field devices. As the systems became distributed with the I/O system being located near the field devices, communication networks were used to connect the I/O system to the controller. The early communication networks were supplier specific and had limited selection of devices, tools, and accessories. As Ethernet has evolved to meet the needs of the industrial automation market, its existing capabilities such high communications speeds, large data packets within a message, and standardized accessories and tools provide signifacent advantages to a user compares to the older propritory networks This paper will outline many of the enhanced features of Ethernet Communication for use as a communication network for distributed I/O system along with several of the network architectures that can be used. Making permanent savings through Active Energy Efficiency 3
Introduction In the mid 1990 s, I/O modules for many automation systems were moved out of large, centralized racks and instead were placed in smaller cabinets close to field devices. This change to Distributed I/O systems offered automation users many benefits, including: Eliminating the need for the large control cabinets Reducing the length of wire connecting field devices to I/O modules Simplifying maintenance and troubleshooting, as the I/O modules are close to the end devices. The result was a significant savings in the total cost of the control system when compared with systems where the automation controllers and I/O systems were mounted in a large centralized cabinet. 4
The need for open networks However, the introduction of distributed I/O did not resolve one major issue that had been a concern to many automation customers the need for open systems. Control suppliers have historically developed unique communication networks for connecting their I/O modules and other control devices with their controllers. Usually these were propriety networks or a network managed by an association that was dominated by a single vendor. Once a network was selected, customers were tied to a specific control supplier or to a limited group of suppliers. Some issues with proprietary networks included: The degree of openness of the network was limited to the size of the catalog of devices that supported that particular protocol. The need for special adapters to connect computers or HMI to the network Specialized training and support tools to maintain the networks Slow evolution of the protocol to respond to new technologies or customer requirements When evaluating total system cost, control engineers utilize a multi-network model to define all the levels of communication from the plant floor to the office. In the multinetwork model, one network is used for Distributed I/O, a second for connecting to the enterprise network, and sometimes a third network is needed to connect the automation systems to the local operator interfaces. Each network requires its own interfaces, cabling, software, support tools, spare parts and maintenance/operator training. Also, the automation system requires additional programming to pass on data from the I/O system to the enterprise network. A solution that utilizes different network protocols between applications also adds to the overall equipment and support cost of the solution parts. 5
Ethernet capabilities today Since its original introduction, Ethernet features and services have continued to expand and it now offers significant advantages over many vendor-specific protocols throughout many automation applications, including: High speed communications, 10 Mbit, 100 Mbit, and 1 Gbit options Large data packet size coupled with higher speeds improve communications to large I/O drops and intelligent field devices More predictable communications with the introduction of Ethernet I/O scanning in automation systems. No need to specially train personnel on proprietary networks. Knowledge of Ethernet is taught in universities and understood by personnel in other industries. Simplified configuration and troubleshooting by allowing management of the entire network from one central location. Also, access to a wide range of existing Ethernet diagnostic tools. The I/O data can easily be communicated through all the other levels of the plant system - from field through enterprise levels without needing conversion gateways. Fibre cable offers noise immunity and increased distance capabilities not available in copper-based standard field bus systems Web technology that includes browser and web pages allows access to information using standard software tools. Component costs are dropping as Ethernet leverages high economies of scale. 6
Ethernet architectures In the world of automation, Ethernet has long been viewed as a protocol designed for the Enterprise and Plant levels of the solution (refer to Figure 1 below). Vendors argued that Ethernet lacked the capabilities to operate as a field bus for a variety of reasons, including: Many suppliers felt that Ethernet lacked the real time capabilities and robustness to operate in the harsh environment of a plant floor. Figure 1: Industrial automation networking subdivisions 7
The infrastructure for the network consisted of a series of hubs, switches, and routers that were not available in robust enclosures The physical layout of the Ethernet Network was optimized for IT applications. The typical configuration was a Star (refer to Figure 2) which could result in long cable runs for the system. The configuration could also be difficult to troubleshoot. In comparison, many of the vendor specific networks used a multi-drop bus or a daisy chain configuration that allowed a single network cable to easily connect to all of the I/O drops as depicted in Figure 3. These configurations enable easier expansions as well as a lower installation and troubleshooting costs. In many cases, the savings in the cabling often offset the added cost to implement a vendorspecific Distributed I/O network. HMI Ethernet Automation Controller Ethernet Switch Distributed I/O Figure 2: Star network HMI Ethernet Automation Controller Proprietary Network Distributed I/O Other Control Devices Figure 3: Daisy chain 8
Ethernet now ready for field bus applications The concerns stated in the previous section were a roadblock to the wide acceptance of Ethernet as a field bus for many years. However, Industrial Ethernet solutions today are available that address the original limitations expressed by end users. Real Time Capabilities In the 1990s, vendors recognized that the inherent advantages of Ethernet would make it an attractive field bus network, so they began to build application protocols based on Ethernet. The protocols used layers 1 & 2 of the Ethernet stack and a new application layer optimized for automation applications. The resulting protocol needed to have the flexibility to meet a range of requirements while being easy to use for non-it personnel within the plant. An additional requirement was for these protocols to use standard Ethernet hardware technology (users could utilize common off the shelf network components). Today, the most widely accepted Ethernet protocols used for distributed I/O communication and other industrial automation applications are Modbus TCP/IP and EtherNet/IP. These protocols are managed by independent organizations they are not controlled by a single vendor. The advantage of an open protocol is the wide selection of devices from vendors that can be used in an application. Large device catalogs allow customers to pick best in class devices for their system, and be assured that these devices will work together. System Response Time The system response time is defined as the time from the receipt of an input signal by the end device to receipt of response from the controller by the end device. Figure no. 4, shows the components that contribute to the system response time. For a Distributed I/O network that uses Ethernet communications, the response times for most devices in a system will be equivalent to systems using other field buses. As an example, the processor updates the I/O (read inputs and write outputs) at the end of the logic scan. With an Ethernet Distributed I/O network that has multiple I/O drops, an Ethernet switch is needed, either as a stand alone device or built into an I/O drop adapter. The typical switch adds about a 50 microsecond delay to the response time, but the switch provides capabilities such as Broadcast Limits, Priority Services, and Unicast/Multicast traffic types enabling fast response times necessary for industrial applications. Controller processor Ethernet I/O Scanner Ethernet TCP/IP Switch Adapter Response Time Input Module Device Adapter Output Module Figure 4: System Components for determining response time 9
Robust, flexible, cost-effective networks Ethernet infrastructure devices such as hubs, switches, and cables are now available that meet the same environmental requirements as the I/O systems for operating temperature, shock, vibration, and noise immunity. Ethernet components also support the same agency approvals as traditional field bus solutions. Ethernet I/O products, infrastructure devices, and cables are now available with an IP67 watertight rating with operation temperature ranges from -20 to + 70 C. In addition, the new generation of Ethernet Interfaces for Distributed I/O systems includes built-in Ethernet switches that allow Daisy Chain connection of I/O drops without the need for an external switch, as shown in Figure 5. This further reduces the installed cost of the I/O system and improves the ease of maintenance by reducing the needed hardware. Fault Tolerant Ethernet networks include both single and dual ring configurations (refer to Figure 6) to increase the distributed I/O network availability for applications that require continuous operation, In the event of a cable break or interface failure, the network will automatically reconfigure to re-route the data traffic, bypassing the failure to continue communication with the I/O system. Figure 5: Ethernet Daisy chain architecture Figure 6: Ethernet ring architecture 10
Conclusion Ethernet has evolved into an Industrial Communication Protocol that is capable of being used at all levels of an Industrial Automation application. The performance offered by Ethernet distributed I/O networks has surpassed many of the older vendor specific networks. While the easiest specification to quote is always the communication speed, automation controllers have the ability to communicate 4,000 I/O messages per second, with each message containing up to 125 words of data, with the capability to scan an Ethernet device in under 0.25 msec. With this type of performance, concerns about Ethernet s ability to be used in automation applications where system response is a critical requirement, has virtually been eliminated. Some of the benefits and capabilities of Ethernet include the following: A single network for all levels of an automation system reduces many of the costs associated with life of a system and simplifies data flow between all levels of the system (removing the need for gateways). Ethernet is an open communication protocol that uses standard hardware, leveraging IT standards, know-how, and tools resulting in a lower total cost of ownership. Open Protocols supported by multiple suppliers allows the user to select the Best in Class I/O products to meet the requirements of their application. High speed communication and data handling at the I/O devices provide the system response times needed for industrial automation applications. Predictable communications with the introduction of new capabilities, message prioritization, and traffic flow control reduce message collisions and improve data response times. Hardened Ethernet devices that meet industrial automation environmental and application requirements. Flexibility of network configurations (star, ring, bus, mesh) allows the users to design their I/O systems to best meet the requirements of their applications. Industrial Ethernet uses many of the same tools used in other industries (like telecom) such as web pages and diagnostic tools. Using common tools simplifies engineering, and reduces the training requirements for plant personnel, which lowers the cost of implementing and maintaining a network. The list of options available to adapt the Ethernet I/O system to a wide range of environmental and operational requirements continues to grow. Ethernet is a high growth technology used globally in a variety of industries. This insures continuous product development by multiple vendors insures long term support and product evolution to better meet future requirements. Wireless connectivity simplifies device connections in areas where traditional cabling is not the best option. 11
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