Industrial Ethernet Article PDF
Industrial Ethernet Article PDF
Industrial Ethernet Article PDF
36 09/2005
their own proprietary networks for the long
term. Frequently forgotten in these discus-
sions is the fact that it’s not just technical
properties such as performance and trans-
fer rates that count, it’s also the soft facts
like easy implementation, openness, con-
formity, interoperability, long-term avail-
ability, and overall distribution. This article
will take a look at the four most well-known
systems with these points in mind.
PROFInet – Massively
Complex Industrial Ethernet promises a standardized networking environment from the control level down to the process
itself.
PROFInet is being managed and devel-
oped by the Profibus user organization
under the auspices of Siemens. As a transferred in real-time with microsecond 200 and ERTEC 400) were introduced ear-
direct evolution from PROFIbus, uniform precision. lier this year and are only available on test
Ethernet technology is supposed to be structures.
able to handle all company areas down PROFInet IRT transfers data cyclically and
to the sensor and actuator level. Three dif- reserves isochronous and asynchronous In order to be suitable for wide applica-
ferent PROFInet specifications have been communication channels. Communication tion, this technology must first get out of
designed to meet different demands: paths for time-critical data are freed up the teething phase. The complex switch-
■ PROFInet (previously V1) for applica- isochronously at exact predefined times ing between time- and address-controlled
tions with no real-time demands within the network. Information is then ex- communication doesn’t correspond to any
■ PROFInet RT (previously V2 or SRT) for changed ad-hoc according to address, as standardized procedures and requires
moderate real-time demands is the case with switched Ethernet. support from local software tools and
■ PROFInet IRT (previously V3) for high special test and analysis tools. For future
real-time demands in drive applica- Commercially available Ethernet switch and faster (gigabit) Ethernet variants, new
tions components are not familiar with the switch ASICs will need to be developed.
The first two variants can be implemented time-controlled communication for the iso- In addition to the costs of the special
using conventional Ethernet devices and chronous channel and must be replaced components in all of the end devices and
components. The IRT version demands by special PROFInet switch ASICs from switches, costs for software and licenses
new components that allow data to be Siemens. These components (the ERTEC must be taken into consideration as well.
Providers of price-sensitive sensors and
actuators will not be able to justify these
Openness
costs.
09/2005 37
area of highly dynamic drive systems, the devices alone. Because of this, the mini-
protocol variant CIPsync has been devel- mum possible cycle must be correspond-
oped. ingly higher.
To add to the confusion, this method is
CIPsync makes sure that all of the stations also integrated with the Beckhoff ADS,
in a network run synchronously and that CANopen, and SERCOS communication
real-time data is exchanged between them profiles. To guarantee that devices from
With EtherCAT, all devices are networked in a ring
as needed. This synchronization is handled formation. different manufactuers can work with one
by distributed clocks in each device as another, whether or not all devices sup-
well as by the IEEE 1588 synchronization port the same profile or the same physics
protocol. Extremely precise timing, such All devices are networked with the bus (E-bus or 100Base-T) must be cleared up
as that needed for drive applications, re- master in a ring formation. During each beforehand. For 100% interoperability, de-
quires special hardware clocks. The device cycle, relevant output data is extracted vice manufacturers would have to invest
clock stamps each time-critical message by the devices from the Ethernet data time and effort for all possible variations.
with the time. This makes it possible to packets sent by the bus master. Input In conclusion, EtherCAT is a new fieldbus
clearly specify when inputs were read and data is also stuffed into packets „on the that has some Ethernet properties. The
at which times commands should be ex- fly“; these packets arrive again at the bus technology itself has been partially made
ecuted. master upon reaching the end of the ring. public and protected by Beckhoff through
This system was designed for central- the use of several patents. Whether this
This IEEE 1588-based procedure, how- ized controller architectures with simple network gains wide acceptance will de-
ever, does not guarantee that data is field devices. EtherCAT is not suitable for pend primarily on its availability, price,
transmitted at the right time. Time-critical distributed intelligence. Individual devices and risk analysis when Beckhoff ASICs
messages are assigned a higher priority can only communicate with one another are taken into consideration.
and handled first on the network. All other by detouring over the bus master. The im-
messages are then ranked lower accord- plied ring structure means that star-formed
ingly. networks can only be implemented with ETHERNET Powerlink – With
limitations. Openness Comes Success
Although Ethernet/IP uses familiar IEEE ETHERNET Powerlink was introduced
standards, high precision in this case re- This technology can only be used with by B&R in 2001. Its goal was to provide
quires all devices and components to be custom-made ASICs from Beckhoff. De- standard Ethernet with real-time properties
equipped with hardware clocks that are spite several announcements, they are still and allow universal solutions all the way
closely integrated with the Ethernet chips. not available. The first device prototypes down to demanding motion applications.
These components are not yet available on that were used were expensive FPGAs. Since that time, the EPSG (ETHERNET
the market. The necessary prioritization of This is not an economical solution for Powerlink Standardization Group) has pro-
messages demands detailed knowledge manufacturers of price-sensitive sensors moted ETHERNET Powerlink and taken
of Ethernet mechanisms and the volume and actuators. responsibility for its openness, continuous
of traffic on the network. Automation improvement, and independence.
engineers will not be able to implement Although EtherCAT uses Ethernet packets,
this technology without first being trained it doesn’t have much else in common with ETHERNET Powerlink is a strictly cyclical
in the field of networks. Ethernet/IP is a the Ethernet standard beyond that. The protocol that organizes the access to a
switched network. Because of this, net- individual devices cannot be used on con- network as well as the synchronization of
work analysis is only possible on a limited ventional networks since MAC address- the devices. Its cyclic precision is under 1
basis. The important line-formed network ing isn’t used. Even IP-based protocols µs. The communication cycle is divided
structures of devices so important in au- have to go through the effort of being re- into an isochronous phase for time-critical
tomation cannot be implemented. Finally, packaged and virtually routed when sent. data as well as an asynchronous phase for
the overall performance of Ethernet/IP is Transferring asynchronous data for param- transferring ad-hoc data. All of the devices
strongly dependent on the total volume of eters and diagnostic purposes incidentally on the network can always directly read
network traffic. wasn’t covered at all in the performance all of the data from the other devices. De-
data. touring over a central bus master (as with
EtherCAT) is not necessary. This protocol
EtherCAT – New Fieldbus or In addition to 100Base-T Ethernet, an is equally suitable for local as well as re-
Industrial Ethernet? E-bus has also been designed to reduce mote control designs.
The EtherCAT network, a further develop- components and save approximately 1 µs
ment of Beckhoff’s Lightbus technology, at the expense of stability, electrical iso- Its electrical properties and all of its data
is purported by the company to be the lation, and compatibility. If the 100 axes packets correspond to the Ethernet stan-
fastest industrial Ethernet system available. described at the beginning of this section dard. For example, ETHERNET Powerlink
Being able to process 1,000 I/Os in 30 µs would be implemented with a standard transmits data in the asynchronous phase
or 100 axes in 100 µs is a bold claim that physical structure, then 100 µs would be using standard IP telegrams. Implemen-
needs to be closely analyzed. estimated just for the delay times of the tations are cost-effective because they
38 09/2005
A Guest Article by:
Andreas Pfeiffer
Member of the Board
EPSG, Winterthur
09/2005 39