Ethernet or DeviceNet - The choice of a new generation

If only it was 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 type of network cabling and signalling specifications developed by Xerox in the late 1970s that 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 only serve up to potentially 70 per cent of plant floor networking applications. So how do they fit together?

Ethernet uses a communication concept called datagrams to get messages across the network. These datagrams take the form of self-contained packets of information which have fields containing information about the data, their origin, their destination and the type of data. The data field in each package can contain up to 1500 bytes. Take mailing as a metaphor. An ethernet package is not just a letter. It is also provided with the sender address, the receiver address, and the stamp indicating what the package's contents are.

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 many other kinds of computer systems. Therefore, ethernet is more popular than other network protocols.

"Ethernet uses a communication concept called datagrams to get messages across the network. These datagrams take the form of self-contained packets of information which have fields containing information about the data, their origin, their destination and the type of data."

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. On the contrary, a Token Ring user has to buy another network card, spend time to install and configure it.

Connectivity to backbone. As mentioned before, ethernet is a local area network protocol for different subnetworks of a company or a campus. For networking the entire company or campus, it needs a backbone network protocol. A backbone network is like a fish's backbone, to where every smaller bone (sub-network) is connected. Ethernet has an advantage in connectivity to backbone because other LAN protocols lag behind in backbone innovations. For instance, the only way to upgrade Token Ring backbone is to move to asynchronous transfer mode (ATM). In contrast, ethernet have several choices such as 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 per cent 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 describes, DeviceNet is a fieldbus standard for device-level communication. Much more than a sensor network, DeviceNet accommodates a range of devices from drives to pushbutton stations, PLC controllers to pneumatics. Achieving this level of versatility would usually require two different 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 can 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 factory.

Communication between DeviceNet and ethernet

Because 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 three major issues, which must be addressed satisfactorily for ethernet to become a viable, popular, plant-floor Fieldbus:

• 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.
• Industrial grade connectors will be necessary for many applications. Cheap plastic 'telephone connectors' don't cut it on the plant floor, and the RJ45 connectors aren't up to the task. An industrial strength connector will be a great benefit.
• Many users desire 24 V 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.
• 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 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.

TCP/IP also doesn't guarantee interoperability.

There are many transmission protocols that can be used on ethernet; the most popular, and the one used on the web, is TCP/IP, which stands for transmission control protocol/internet protocol.

" DeviceNet is one of the world's leading device-level networks for industrial automation. In fact, more than 40 per cent 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."

When you download a file from the web, you can see the speed of the 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 at the other side. TCP/IP was developed at Stanford University in the 1970s as a 'handshaking' mechanism that would assure that 'the message would always eventually get through.'

Practical applications and the future of ethernet on the plant floor

It will be difficult and expensive to get ethernet to the sensor level. Using ethernet to turn a valve on or off, or to connect a node to a photo eye or prox switch, is like putting five-foot monster tires on a Holden Barina. However, it will do well for 'racks' and clusters of I/O tying into a single node.

• Ethernet will not necessarily be 'dirt cheap' in industrial applications, and overall may prove to be more expensive.
• Ethernet PC cards cost 1/10th 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, certainly not for professional, industrial use. (This is 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 drive the cost up as well.
• 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 10 Mbits may not be as fast as it sounds.
• The promise of fibre optic, 100 and 1000 Mbit 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.

A 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' 18 mm and 'Micro' 12 mm waterproof quick-disconnect plugs and receptacles, and 5 pin phoenix terminal block.
• Distance: 100 to 500 m
• Baud rate: 125, 250 and 500 Kbps
• 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 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: 100 m (10 Base T) to 50 km (Mono mode, fibre with switches)
• Baudrate: 10 to 100 Mbps
• 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: Nearly universal in office/business local area networks. Widely used also in PC to PC, PLC to PLC and supervisory control applications. Gradually working its way toward the 'sensor level' in plant floor applications.
• Advantages: Ethernet is the most widely accepted international networking standard. Nearly 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.