Ethernet - the future of industrial control networks
The prevalence of ethernet in the industrial environment is growing at an exponential rate. The expectation is that this form of industrial communication will develop throughout the industrial environment from the exchange of information for business services and systems, to supervisory plant control and eventually down to the actual sensor actuator level. While this final stage is some way off it is important for decision-makers to seriously consider the advantages of ethernet as a means of communication for any industrial application.
In industrial environments there are many alternatives for intercommunication between controllers, devices and supervisory equipment. The decision for selection of a protocol is based on an existing company standard or the experience and comfort of key people making the decisions.
At the plant supervisory and controller level this will be typically be a high-speed, high throughput industrial network such as an ethernet-based Modbus TCP/IP. At the device and sensor or actuator level it is more likely to be one of the many fieldbuses, such as Profibus DP, Interbus, Modbus Plus, DeviceNet, CANopen or ASi.
In the future, however, even these devices will have ethernet connectivity so that there will be potential for one form of communication network throughout the whole enterprise from the boardroom down the floor sensor.
Comparing the numbers of installed nodes on a worldwide basis, there are significantly more (almost five times) ethernet devices than the total number of fieldbus and other specific industrial network devices. This is represented in the following graph. In fact, the number of ethernet devices according to ARC Advisory Group is growing at an exponential rate.
This is mainly due to the extensive use of ethernet in the office, commercial, domestic and industrial environments. A benefit of such extensive use is the wide availability of the expertise, infrastructure and support available for ethernet based protocols.
The term 'ethernet' was coined by a group of researchers at the University of Hawaii to describe their inter island radio based communication system, known as 'Aloha net'. Xerox developed an ethernet standard based on this, which in turn was the basis of the IEEE 802.3 standard for Carrier Sense Multiple Access/Collision Detect communications which ran over unshielded twisted pair cable (UTP).
Ethernet itself defines only two levels of a communication protocol, these are the physical level and the network access level. When two devices support ethernet, this means that we can connect both devices to the same network, but it does not mean that they will be able to understand what the other is saying. There are other layers of protocol which exist on top of these basic levels, each additional protocol offers a unique service which the user or developer can incorporate into their own industrial version of an ethernet protocol.
In recent times, organisations and suppliers have realised the benefit of using an ethernet over UTP for industrial communications.
Hence the development of:
- Ethernet I/P administered by ODVA;
- PROFInet developed by the Profibus Foundation;
- High-speed ethernet developed by the Fieldbus Foundation;
- Mitsubishi Electric supports ethernet with its Melsec range of PLCs;
- GE Fanuc ethernet;
- CANopen profiles on Ethernet Powerlink.
In a similar fashion there are other standards evolving which use ethernet for specific applications such as:
- IEC 61850 substation automation;
- IEEE 802.11b,g wireless LANs with industrial applications;
- IEEE 802.15.3, 4 high rate wireless personal area networks with low power battery consumption;
- EEE 802.3af-2003 Power over IP.
These particular standards are worthy of note as they illustrate how robust and extensive the use of ethernet has become.
In the case of substation automation the standard uses ethernet as a means of communication of fault detection information and responding action which requires completion of an action within 6 ms.
Advantages of ethernet
The advantage of encapsulating the overall protocol into an ethernet format such as Modbus TCP/IP or PROFInet is the availability of protocol transparency. Through a simple bridging device the protocol is simply transformed from a fieldbus to an ethernet format.
This means that existing fieldbuses can be left in place and the new ethernet implementation installed and expanded over time. As the benefits of ethernet become apparent the fieldbuses and associated devices can be replaced in a staged fashion.
Ethernet is the basis for a whole range of different protocols, services and tools, which can be easily adapted and used in the industrial environment. Again, due to the wide use of ethernet, the number of suppliers and availability of these products exceeds those on offer for any other specific network or bus protocol.
Some examples of these protocols include:
- Simple network management protocol (SNMP) enables users to interrogate and monitor the status of all devices on the network. Through specific device profiles (MIB descriptions) an SNMP manager can monitor specific information relevant to that device.
- The use of hypertext transmission protocol (HTTP), the basis for access to information via web pages. A simple web page server built into the device such as a PLC, a drive or motor starter, enables a user to interrogate the status of the controlling device, as well as monitor and control the process from any PC using a web browser such as Internet Explorer. A low-cost but simple way of extracting information from a device.
- Other protocols such as dynamic host configuration protocol (DHCP), File transfer protocol (FTP) can be used to assist the maintenance personnel in automatic re-configuration of a new device when replacing a faulty unit, again another part of the Schneider Electric 'Transparent Ready' offer.
As ethernet is a common physical layer for all ethernet-based protocols, there is a large selection of suppliers for industrial ethernet products.
- Industrial ethernet switches can be sourced from CISCO, Hirschmann, Sixnet, Harting, Phoenix Contact, Moxa, Advantech, Schneider Electric;
- Industrial grade shielded twisted pair (STP) or fibre optic cables can be sourced from Lumberg, Siemens, Schneider Electric, Voltec, Comm Scope;
- Suitable in RJ45 or M12 connector types can be sourced from Lumberg, Siemens, Harting, Amphenol;
- Wireless ethernet from Locus, Data-Linc, RACOM.
Another advantage for the end user is that there is no longer a specific cable for each different protocol. PROFInet, Ethernet I/P and Modbus TCP/IP can all co-exist on the same network at the same time.
A PROFInet device may not be able to talk to a Modbus TCP/IP device, however it can talk to another PROFInet device at the same time as two Modbus TCP/IP devices communicate to each other.
At the same time you can connect your laptop which has an ethernet connection as standard or insert a low-cost ethernet network interface card into your desktop computer to talk to each device. No longer do you have to buy expensive proprietary network cards, or worry about the lack of serial ports on your laptop computer.
Ethernet typically runs at 10 to 100 MBaud, this is equal to or better than all other industrial networks and buses. Ethernet-based protocols are also able to transport higher volumes of data per message. For example, CANopen will transmit 8 bytes per message, Interbus will allow 512 bytes per message, ethernet messages can register in the 1000s bytes per message.
Popularity of the Internet means that users are more familiar with ethernet and are more comfortable and hence more likely to explore ethernet-based services and create potential security issues.
While it is not a major concern for most industrial environments, the need for security should be considered when designing a system.
Use firewalls to limit external intrusion or to segregate the office and the industrial control networks. Virtual LANs can be also used to limit internal access to specific switch or router ports which are connected to end devices such as PLCs or drives or displays. Finally, password access to individual devices such as PLCs should be implemented and regularly updated. If no password protection can be provided, implement audit trail capability such as 21CFR11 to monitor changes in your device so that a recovery can be performed.
Ethernet of the future
Ethernet communications-based protocols are used at many levels with the factory or process plant. It is expected (and this is confirmed by ARC) that this will extend to connect down to the lower device levels in the future, including sensors and actuators.
The result will be one or more ethernet-based networks can be used for all levels of communication. For sensor or actuator level communication a fieldbus or bit bus such as Asi is required. To enable ethernet-based protocols to reach this level there are several issues to address, such as:
- For a sensor or actuator to have ethernet connectivity the following components are required: an IP stack, an M12 or RJ45 connector, an LED, a physical converter and a low voltage transformer. These components add cost and additional size to the device.
- To connect the device to an ethernet network using twisted pair cabling requires the use of ethernet switches or hubs configured in a star topology. In addition to this power must also be provided to the switch and the device. This is more labour intensive and costly than using a traditional fieldbus topology.
- In addition to this the change of mindset that an ethernet network can be used instead of a proven and tested fieldbus will have to be overcome.
There are developments which will help establish ethernet as a viable alternative. The IEEE 802.3af-2003 power over ethernet standard, while intended for Voice over IP, will suit industrial applications as both the power and the communications signal can be incorporated into one connection. In a similar way IEEE 802.11b,g or IEEE 802.15.3,4 wireless communications will remove the need for an ethernet star topology and make the case of retro fitting systems far simpler.
Industrial ethernet is here and it is here to stay. While fieldbuses and other industrial networks are well tried and proven, there is also a place for industrial ethernet networks and this should be considered when making industrial network selections.
Ethernet competes favourably in terms of its speed, reliability and ability to transfer large or small volumes of information as well as providing a large range of additional services which fieldbuses do not.
The factory of the future may have ethernet networks from the plant floor sensor through to its business level communications. The advantages of having only one network structure throughout the enterprise would be a significant step to creating a fully collaborative manufacturing environment.
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