DeviceNet vs Ethernet
If only the solution were that simple. The question is a bit like asking if I should exercise or eat well to stay healthy. In most cases the answer is both, and the same applies to Ethernet and DeviceNet.
A DeviceNet network is an open, low-cost industrial network used to connect industrial devices such as limit switches, photoelectric cells, valve manifolds, motor starters, drives, and operator displays to PLC processors and PCs.
Ethernet, on the other hand, is a higher level of network cabling and signalling specifications developed by Xerox in the late 1970s. Ethernet enables file and printer sharing among machines across a local area network (LAN). The networking of millions of PCs in offices and the proliferation of the internet across the world has made Ethernet a universal networking standard.
However, a study by the ‘Big Three’ automotive manufacturers showed that Ethernet could potentially only serve up to 70% of plant floor networking applications. So how do they fit together?
Ethernet uses a communication concept called datagrams to transfer messages across the network. Datagrams take the form of self-contained packets of information comprising fields that contain information about their data, origin, destination and type. The data field in each packet can contain up to 1500 bytes. Take mailing as a metaphor. An Ethernet packet is not just a letter. It also contains the sender address, receiver address and the stamp indicating the exact contents of that packet.
A drawing of the first Ethernet system by Bob Metcalfe
Installation of Ethernet is easier and less expensive than other network protocols. It offers efficient ways to connect across Mac, PC, DEC/VAX, UNIX, HP workstation, IBM mainframe, and a number of other computer systems. It is for this reason alone that Ethernet is more popular than other network protocols.
Ethernet is not only less expensive than other types of network, it is also widely available. Macs have a built-in Ethernet port and today more and more PCs offer this feature too. Compare this to the Token Ring user who has to purchase another network card before spending time installing and configuring it prior to use.
Connectivity to backbone – As previously mentioned, Ethernet is a local area network (LAN) protocol for different sub-networks within a company or campus. For the successful networking of an entire company or campus Ethernet requires a backbone network protocol. A backbone network can be likened to a fish's backbone, to which each smaller bone (sub-network) is connected.
Ethernet has a distinct advantage in connectivity to backbone because other LAN protocols lag behind in backbone innovations. The only means of upgrading Token Ring backbone, for example, is to move to Asynchronous Transfer Mode (ATM). In contrast, Ethernet users have several choices including Gigabit Ethernet, ATM, and routing switches.
DeviceNet is one of the world's leading device-level networks for industrial automation. In fact, more than 40 percent of end users surveyed by independent industry analysts report choosing the DeviceNet network over other networks. The network helps eliminate expensive hard-wiring while providing device-level diagnostics.
As its name suggests, DeviceNet is a fieldbus standard for device-level communication. Much more than a sensor network, DeviceNet accommodates a range of devices from drives and pushbutton stations to PLC controllers and pneumatics. Achieving this level of versatility generally requires two differing fieldbuses, each with a different protocol.
DeviceNet increases overall system performance by providing both event-based and timer-based options. Take change-of-state, for example. Using COS prevents your network from being slowed down by unnecessary message traffic. Peak network efficiency is maintained.
DeviceNet allows you to configure many products in real time over your network. You an store these configuration parameters on a PC and quickly download them if a replacement device is installed. You can even replace devices on a live network.
A major advantage of a DeviceNet network is that it can support very small devices, down to switches and pushbuttons, in the hot, electrically noisy environment of a heavy-production, discrete manufacturing plant.
Communication between DeviceNet and Ethernet
As the DeviceNet network is an open, flexible network based on proven, well-known CAN technology, this helps to ensure both end user and industry-wide understanding, acceptance and inter-operability with Ethernet.
Messages from a node on either a ControlNet link or an Ethernet link can be routed through a ControlLogix to a node on a DeviceNet link.
The Challenges of Moving Ethernet to the Plant Floor
There are at least four major issues, which must be addressed satisfactorily for Ethernet to become a viable, popular, plant-floor Fieldbus.
1. A common “Application Layer” must be established. When your device receives a packet of data, what format is that data in? Is it a string of I/O values, a text document or a spreadsheet? Is it a series of parameters for a Variable Frequency Drive ? How is that data arranged? There are several competing standards. More about this below.
2. Industrial grade connectors will be necessary for many applications. Cheap plastic “Telephone Connectors” don’t cut it on the plant floor and RJ45 connectors aren’t up to the task. An industrial strength connector will be a great benefit.
3. Many users desire 24-volt power on the Bus. This is advantageous from a practical standpoint – it reduces wiring and power supply problems – but it adds cost and introduces noise and other technical problems.
4. Some applications require determinism. Ethernet as it is typically used is not deterministic or repeatable; in other words, throughput rates are not guaranteed. However, methods exist for architecting deterministic Ethernet systems. Note that, in reality, most people who think they need determinism really just need speed.
Ethernet Doesn’t Guarantee Interoperability
Ethernet is just a physical layer standard in much the same way that an RS232 or, for that matter, a telephone line is. Having a physical connection means that messages can be transmitted, but it does not assure successful communication. Just because you can make a telephone ring in Shanghai doesn’t mean you can speak Mandarin.
TCP/IP doesn’t guarantee interoperability either.
There are many transmission protocols that can be used on Ethernet; the most popular, and the one used on the World Wide Web, is TCP/IP or Transmission Control Protocol/Internet Protocol.
When you download a file from the web you can see the rate of transmission speed up and slow down as network traffic levels change. TCP/IP is the mechanism that breaks the downloaded file into any number of bits and pieces and re-assembles them on the other side. TCP/IP was developed at Stanford University in the 1970’s as a “handshaking” mechanism that would assure that ‘the message would always eventually get through’. In view of this, it does seem miraculous that those files usually come across perfectly, with every single piece intact.
To carry the Web example further, we’ve all had the experience of downloading a large file only to discover that our PC “cannot find an associated application for this file type”. So you end up downloading a plug-in like Shockwave or RealAudio or Winamp or Adobe Acrobat Reader just to open the file.
The same problem applies to industrial controls. You can send any file or piece of process data over Ethernet or the Internet, but the receiving end has to know what to do with that data. TCP/IP doesn’t assure you of opening the file; it just guarantees that it will arrive.
Practical Applications and the Future of Ethernet on the Plant Floor
• It will be difficult and expensive to expand Ethernet to sensor level. UsingEthernet to turn a valve on or off, or to connect a node to a photo eye or prox switch is like putting truck tyres on to a Ford Escort. However, it will do well for “Racks” and clusters of I/O tying into a single node.
• Ethernet will not necessarily be cost effective in industrial applications, and overall may prove to be more expensive.
• Ethernet PC cards cost one tenth as much as, for example, DeviceNet cards, for several reasons. Obviously, they’re made by the millions. They don’t have a processor: they’re passive, meaning that the PC does most of the work. DeviceNet cards usually have a processor on board that handles all communication. Also, long term availability is a real problem with consumer computer products. Product life cycles are measured in days, not years! Finally, the quality of cards you buy at an office supply store may be lacking and are certainly not for professional, industrial use. (This is the reason why Grid Connect makes industrial grade Ethernet NIC cards for PC/ISA and PC/104.)
• Ethernet in embedded applications is much more expensive than CAN. CAN chips cost $1 or less; Ethernet chips are much more. So devices themselves with Ethernet built in will definitely have a cost factor. We expect Ethernet to have costs similar to Profibus, which has more expensive ASICs.
• Industrial grade cables and connectors will also drive the cost up.
• Not only are Ethernet ASICs more expensive than CAN chips, running a TCP/IP stack takes more horsepower than your usual 8051 can supply. The processors will cost more, too.
• A TCP/IP packet has 68 bytes of overhead. For short messages, corresponding to typical industrial I/O products, that’s a lot of overhead. So 10Mbits may not be as fast as it sounds.
• The promise of fibre optic, 100M and 1000Mbit Ethernet is certainly exciting, and potentially overcomes most people’s speed issues. However, the cost associated with these enhancements may be substantial.
The Bottom Line
Ethernet will establish itself among the popular fieldbuses as a legitimate and attractive option. It will not replace them, but for some applications it will be the clear winner.
Quick Comparison
DeviceNet: The Do-All FieldBus for Low and Mid-Level Factory Networking
Origin: Allen-Bradley, 1994
Maximum number of nodes: 64
Connectors: Popular ‘Mini’ 18mm and ‘Micro’ 12mm waterproof quick-disconnect plugs and receptacles, and 5 pin phoenix terminal block
Distance: 100M to 500M
Baud rate: 125, 250 and 500 Kbits/sec
Maximum message size: 8 bytes of data per node per message
Messaging formats: polling, strobing, change-of-state, cyclic, and others; producer/consumer-based model
Supporting trade organisation: Open DeviceNet Vendor Association, www.odva.org
Typical applications: Most commonly found in assembly, welding and material handling machines. Single-cable wiring of multi-input sensor blocks, smart sensors, pneumatic valves, barcode readers, drives and operator interfaces
Advantages: Low cost, widespread acceptance, high reliability and efficient use of network bandwidth, power available on the network
Disadvantages: Limited bandwidth, limited message size and maximum length
Ethernet: The Worldwide Defacto Standard for Business and PC Networking
Origin: Digital Equipment Corporation, Intel and Xerox, 1976
Implemented on multitudes of chips produced by many vendors. Based on IEEE 802.3
Formats: 10 Base 2, 10 Base T and 100 Base T, 100 Base FX, 1 Gigabit; copper (twisted pair / thin coax) and fibre
Connectors: RJ45 or coaxial
Maximum number of nodes: 1024, expandable with routers
Distance: 100M (10 Base T) to 50 KM (mono mode, fibre with switches)
Baud rate: 10M to 100M Bit/sec
Message size: 46 to 1500 bytes
Messaging format: peer-to-peer
Supporting trade organisation: Industrial Ethernet Association (www.IndustrialEthernet.com) and IAONA (www.iaona.com).
Typical applications: Almost universal in office / business LANs. Widely used also in PC to PC, PLC to PLC and supervisory control applications. Gradually working its way toward “sensor level” in plant floor applications
Advantages: Ethernet is the most widely accepted international networking standard. Almost universal worldwide. Ethernet can handle large amounts of data at high speed and serve the needs of large installations
Disadvantages: High overhead to message ratio for small amounts of data; no power on the bus; physically vulnerable connectors and greater susceptibility to EMI/RFI than most fieldbuses; confusion based on multiple open and proprietary standards for process data.
Machinery Automation & Robotics
1/101 Derby Street
Silverwater NSW 2128
Phone: (61) 2 9748 7001
http://www.machineryautomation.com.au