Industrial Ethernet
January 2006
Anthony J. Calati Jr.
CALATI
FRABA Incorporated
14 Washington Blvd. Building 6
Princeton Junction, NJ 08550
Tel: 1-609-750-8705
Fax: 1-609-750-8703
[email protected]
www.fraba.com
Abstract
The use of Industrial Ethernet in the manufacturing environment is increasingly becoming a viable
way of interconnecting the machines on the plant floor with users in the office, as well as with each
other. This enables control systems to take in data from new and different sources in order to better
control all aspects of production. When deciding to implement Industrial Ethernet, many decisions
have to be made that are different from the implementation in the office environment. The available
options range from the physical network configuration to network protocol. An understanding of the
plant’s requirements and capabilities are needed before selecting a system.
Industrial Ethernet
Ethernet - History
Ethernet is a communication system that has been widely adopted for use within office and home
networks. A high bandwidth, which allows for the sending of large data packets in soft real-time,
coupled with the increased amount of information needed for interoffice communications, i.e.
streaming video, video conferencing and presentations, makes this type of network infrastructure ideal
for the office environment. In addition, the World Wide Web’s exponential growth has increased the
availability and amount of information making faster downloading a must have for gathering data.
Ethernet is used to connect PCs to the local area network (LAN) where it can connect to the Internet,
printers, mainframes/servers, and other PCs.
Because of Ethernet’s popularity in the office and its perceived advancement in speed in LANs, plants
have begun implementing Industrial Ethernet in the hopes of being able to interconnect the operations
of the entire plant. The problem is that the needs of the office environment are completely different
than the needs on the plant floor. As mentioned earlier, large amounts of data are needed in an office,
and time is not a critical factor. The plant environment needs many data packets sent at specific times
for use in control systems.
Traditional Ethernet protocols are unable to meet the deterministic requirements needed for closed
loop applications. This is because of latency and “jitter”. Latency is the amount of time between
when information is requested and when it is received. Jitter is the variation in time between packets
arriving and is caused by the collision of data when two sources are trying to send in the same interval.
When this happens, the devices both wait a few moments and then try to resend. This is not critical in
the office environment because jitter usually only lasts 1ms and is barely noticeable to the user. In the
plant environment, applications run at much faster speeds requiring synchronization to be within 1s.
While current office protocols, i.e. HTML and http, are unable to achieve this, new protocols have
been and are being designed for use on the shop floor.
Ethernet – Physical Layer
The OSI (Open System Interconnection) reference model for communication systems divides the
communication network into seven different layers (as shown in Fig. 1). Layers 1 through 4 are the
lower layers and are oriented towards the network. Layers 5 through 7 are the upper, application
layers.
(Fig. 1)
Data starts at layer 7 of the sending device and gathers addressing information as it moves down to
layer 1 to become a standard data frame (Fig. 2). Because Ethernet is a logical bus, the information is
sent out to all devices on the network, but the information is only picked up by the device whose
destination address matches the one in the message or to all when the information is addressed to all
(comment: addressing is done on Layer 3, not on the Ethernet layers (1+2)). The receiving device then
brings the frame up through the layers, which removes the addressing information, so it can be used in
the Application Layer.
Preamble
8 Byte
(Fig. 2)
Destination
6 Byte
Source
6 Byte
Type
2 Byte
Field Data
Field Check
4-1,500 Byte 8 Byte
Ethernet, as defined by IEEE standard 802.3, is made up of the Physical Layer (layer 1) and part of the
Data Link Layer (layer 2), which means that it can be combined with various higher-level protocols.
The Network Layer (layer 3) implements the Internet protocol (IP) to manage routing of data packets
through different networks. The Transport Layer (layer 4) utilizes TCP and UDP to ensure error free
transmission of data. The Session Layer (layer 5) establishes and closes down communication
between devices on the network, so it is unnecessary for unconnected devices. In the Presentation
Layer (layer 6) data is transformed into a common syntax to be translated by the receiver in the
Application Layer (layer 7). The Application Layer is the interface layer for the user. It allows access
to files and operations of devices.
Why Industrial Ethernet?
With the rapid advance of new technology, product life cycles are continually decreasing. Along with
this, the deterring high cost of the next big innovation readily becomes affordable as the next
generation of products comes out. Because of these changes, consumers are becoming accustomed to
having the choices of new models more often.
The impact of this change has forced plants to seek a new way of doing business. The cost of capital
equipment will not allow companies the ability to retool every time there is a model change. Also,
because of the increasing product line-ups, high changeover times are no longer an option. Plants need
to be more flexible in the way they design their production processes. Industrial Ethernet will bring
this flexibility to the plant. Machines and devices will be set up to run in multiple configurations and,
if linked to the office network, can be controlled or troubleshot from anywhere.
The Department of Defense (DOD) developed Ethernet TCP/IP for the communication of devices
within and between networks. It is a widely used protocol that is perfect for the office environment
because it provides a robust system that is able to connect several computers on a single LAN. Also,
latency and jitter in TCP/IP systems can last up to a couple of seconds each, which is acceptable for an
office environment but unsuitable for and industrial environment.
Device Control in a plant environment depends on hard real time conditions, so Industrial Ethernet
proposes an interesting question in the design of new plant facilities and the redesign of others: can it
operate in hard real time? The simple answer is yes, but the existing range of technologies needs to be
extended before the office network will reach the device level.
Different Solutions
Due to commercial and competitive reasons different market players have proposed different solutions.
Former field bus organizations are offering a migration perspective and new multi-vendor
organizations have been established to promote specific solutions.
Below you will find a list of the current options available and a few details about each protocol.
Ethernet/IP
Ethernet/IP was developed by ControlNet International (CI), the Industrial Ethernet Association (IEA)
and the Open DeviceNet Vendor Association (ODVA) to be a common application-layer protocol for
control systems based on Ethernet technology. Like ControlNet and DeviceNet; it is based on the CIP
protocol that has become a widely accepted standard, e.g. in the automobile industry. For hard realtime applications the extended protocol CIP sync was specified. Ethernet/IP is a pure software based
solution that does not require any specific ASICS. Detailed information is available under
www.odva.org.
Ethernet Powerlink
Ethernet Powerlink is an open and completely software based solution that is promoted by the EPSG
(Ethernet Powerlink Standardization Group). This multi-vendor organization has worked with the
leading standardization bodies, such as; the CAN in Automation Group, the IAONA, the IEC and the
ISO, to ensure a balance between the different needs of the plant environment. In September 2004,
EPLsafety became the market’s first open and independent Ethernet protocol for safety-related
applications according to IEC 61508 SIL 3 and 4. Daisy chain cabling is possible. A first version
called V1 is proven in a wide range of application since 2001. Detailed information is available under
www.ethernet-powerlink.org.
Profinet
Profinet is an Industrial Ethernet protocol that is driven by Profibus International. It is designed to be a
cross-vendor communication’s system capable of communicating with different BUS systems through
a proxy server. There is a real-time solution “RT” with capabilities similar to Profibus-DP and an
isochronous real-time solution “IRT” with a jitter of 1 µs. The real-time solution is based on software
only, for IRT specific ASICS are required. Detailed information is available under www.profibus.com
EtherCAT
EtherCAT is a technology developed by Beckhoff with outstanding real-time capabilities. It provides
daisy chain cabling and the Ethernet frame is processed on the fly: each device reads the data
addressed to it while the telegram is forwarded to the next device. Telegrams are only delayed by
nanoseconds. This excellent performance is related to a specific ASIC that needs to be used in every
device. Detailed information is available under www.ethercat.org
Modbus TCP
Modbus TCP is an extension of the Modbus family into Ethernet environment. The Modbus
application layer is used on top of Ethernet TCP/IP. It is an open software based solution without a
specific real-time extension. Performance is highly dependent on the design of the Ethernet network
and the performance of the processors in the communication interfaces of the devices on that network.
Detailed information is available under www.modbus-ida.org .
The Future of Industrial Ethernet
Industrial Ethernet is the latest innovation in industrial networking. Unfortunately though, it will not
be the Universal network BUS to replace all current network systems. As shown above, there are
many different protocols that can be chosen, and the diversity in those networks is going to be a result
of mainly commercial reasons, not technical.
A major concern in the cost of implementing Industrial Ethernet is not only the cost of the equipment,
but also the training of engineers to properly design and diagnose those networks. The choice of an
appropriate network solution will depend on the control system used and on what the plant engineers
are comfortable working with.
Security is another major concern when installing these systems. Industrial Ethernet is capable of
connecting the plant floor to the office environment. While an office environment can continue to
function while viruses are removed from the system, the plant environment would have to be shut
down. Internal or external attacks could cost the company millions of dollars through downtime.
Even with these concerns, Industrial Ethernet is a viable option for networking most industrial
applications. Several companies, like Rockwell Automation, Siemens, B&R and Beckhoff have
already installed real-time Ethernet solutions successfully and the number of available components is
increasing very quickly.
References:
IAONA Handbook Industrial Ethernet, Third Edition
http://www.iaona.org/home/home.php
http://pclt.cis.yale.edu/pclt/COMM/TCPIP.HTM
http://www.ethernet-ip.org
http://www.odva.org/10_2/03_events/New-EtherNet/ethernet.pdf
http://www.ethernet-powerlink.org