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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
NetLinxTM 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.
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Source: Industrial Ethernet Book Issue 1:8

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