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Ethernet's worldwide acceptance in individual and office environments has created an eagerness to expand its responsibilities on the plant floor.
By Andreas Somogyi, Rockwell Automation

Ethernet - The Situation Today

A typical Ethernet TCP/IP network today may extend plant-wide and be connected to a corporation's worldwide network via the Internet. It is generally used to conduct program maintenance, send data to and from MIS and MES systems, perform supervisory control, provide connectivity for operator interfaces, and log events and alarms. These functions require the high throughput and widespread accessibility that Ethernet offers. Response time is secondary to overall data capacity. Some customers currently use Ethernet for limited control purposes, such as processor interlocking, but the applications where this has been successful require Ethernet's high speed, but not a high level of determinism or repeatability of message response time.

Ethernet's worldwide acceptance in individual and office environments has created an eagerness to expand its responsibilities on the plant floor. This article will explore some of the ways this will occur in the near future. In particular it will consider the opportunities offered by Ethernet as a network for control in its own right and as a backbone for other control networks, such as ControlNet.

Ethernet as a Control Network?

Ethernet TCP/IP is the most popular LAN because the protocol, in addition to being used by the Internet, is fairly simple and is supported by most computer vendors and control suppliers. Making maximum use of existing Ethernet wiring in the plant is a tempting alternative to installing separate wiring to support a separate control network. However, running messaging and control information simultaneously over the same Ethernet network may not be appropriate for some applications where high-speed determinism and repeatability is absolutely required. As we will see later though, there are opportunities to combine these two requirements by running ControlNet services over Ethernet wires.

Today, Ethernet is used primarily as an information network. However, there has been much discussion lately in the industrial automation industry about whether or not it can be used for real-time control. Manufacturers that supply both Ethernet and other networking choices, such as Rockwell Automation, are working closely with end users to determine whether their application would benefit from using Ethernet for real-time control.

One of the most common arguments that has traditionally been used against the use of Ethernet for control is that Ethernet is non-deterministic. Determinism enables users to accurately predict data transmission and guarantee its arrival at the same time every time. Recent improvements in Ethernet technology have improved the determinism and performance of Ethernet to a great extent. However, these technologies are largely untried in high-speed control applications.

The Evolution of Ethernet Performance

More recent developments in Ethernet technology include Fast Ethernet and Gigabit Ethernet. Fast Ethernet is basically Ethernet running at 100 Mbits/sec. Fast Ethernet and its predecessor 10 Mbit Ethernet use the same frame structure, addressing scheme, and link access method (CSMA/CD). However, all network timing parameters must be scaled by a factor of 10 when configuring a Fast Ethernet network. This tends to reduce the distances between nodes in some configurations when compared to a 10 Mbit network.

Although Fast Ethernet provides a wire speed that is 10 times as fast as traditional Ethernet most applications will not enjoy a substantial performance increase. In particular, a plant floor network of small microprocessor-based intelligent I/O blocks, sensors, actuators, drives and other device interfaces are likely to consume and produce small amounts of data encapsulated in 64 byte Ethernet frames (the smallest frame size supported by Ethernet). The performance of these devices is more likely to be limited by the speed of their microprocessor and embedded firmware than the wire speed. It is unlikely a network of such devices would fully utilize the full 100 Mbit/sec (or even 10 Mbit/sec) Ethernet bandwidth today. This opens up the debate on the need for more than one type of network in industrial automation today.

The Need for More than One Network

Do these improvements in the capabilities of Ethernet eliminate the need for other control networks? Not according to Rockwell Automation. Rockwell Automation is taking the approach of utilizing the similarity in network architectures to ensure that choice of wire doesn't become the deciding factor in which network is used for which level of control.

Because of the different tasks in most control system, there are typically three basic network levels: Information, Control, and Device. Typically, Ethernet is the key choice of network at the Information level today as discussed already. At the Control level, PLCs and PCs typically control I/O racks and I/O devices such as variable speed drives and dedicated HMI. For this layer, the open network ControlNet is widely found in industrial applications where time critical interlocking between PLCs and guaranteed I/O updates rates are extremely important functions. At the Device level, where reduction of wiring is a key benefit, there are two types of networks. The first type handles communication to and from primarily discrete devices. DeviceNet is a popular example in this area. The other type handles communication to and from primarily process devices. And Foundation Fieldbus is a popular example in this area. As time goes on, new commercial technologies will continue to emerge and may eventually find a home in manufacturing applications. The bottom line is that there won't be — at least in the near future — a single network that can serve all purposes.

Producer/Consumer Functionality Provides a Common Language Over Any Wire

In the past, network communication models were based on what was referred to as Source/Destination or point-to-point models. Using this model, data that needed to be delivered to more than one destination had to be sent one at a time. Many end users determined that this procedure wasted bandwidth and made synchronization between devices very difficult (as data arrives at each device at a different time). An object-orientated communications model providing Producer/Consumer functionality was introduced to make data handling more efficient. On Producer/Consumer-based networks, messages are identified by content rather than as source to destination.

If a node needs data, it will accept that identifier and consume it. With Producer/Consumer networks, multiple nodes can consume the same data at the same time from a single producer, resulting in more efficient use of the bandwidth. In other words, the node with the data just needs to send it once, via multi-cast, and other devices on the network that need the data can consume it.

Separating the Network Services from the Physical Media

NetLinx^TM is a term introduced by Rockwell Automation that describes a particular strategy for the implementation of object-orientated Producer/Consumer services over any wire. While there are dozens of networks on the market today, it would be safe to assume that in the future some will be eliminated, and new ones will be added. With the uncertainty about which "wire" will be favoured at any given time, it only makes good business sense that users adopt networks that have a flexible migration path. NetLinx provides this migration path with its common Producer/Consumer language and a set of services that can be designed into the specification of any network.

This universal set of services is technically called the Control and Information Protocol (CIP). CIP is an object-orientated protocol that has been optimized for solving industrial automation needs and problems (configuration, control and collection of data). Ethernet, which is based on the same 7 layer OSI model network, along with DeviceNet and ControlNet contain CIP and hence are NetLinx enabled networks. By utilizing the similarities in network model architecture, Rockwell Automation is able to offer NetLinx Control Services over Ethernet (wire) as shown in figure 1. This offers industrial automation users the capability for deterministic high speed networking on the same wire already installed in their plants today.

Some Benefits of NetLinx

With an increased number of wire choices, product suppliers can start to look at the potential of deploying Producer/Consumer services over many wires, as they become appropriate for manufacturing environments. This may include ATM, USB and FireWire in the future.

NetLinx provides users a vast array of choices. They can begin by choosing the wire that works best for their application. This might include twisted pair, RG-6 coax, fiber optic, wireless or some other type, depending on distances needed, environmental conditions, etc. The PC might be next - users can choose where they want to connect into their architecture and access data - the device-level, control-level, information-level or corporate-level. This might include a laptop to plug in for maintenance, a workstation for MMI or maybe even a desktop for uploading and downloading files. And finally, they can choose when they want to access data - continuously, periodically, or on-demand.

This article was supplied by Rockwell Automation.