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Until recently, a PLC would communicate with a slave machine using one of several possible open or proprietary protocols, such as Modbus , Profibus , DeviceNet or Foundation Fieldbus . However, there is now increasing interest in the use of Ethernet as the link-layer protocol, with one of the above protocols as the application-layer (see OSI Model ).

Some of the advantages are:
  • Increased speed, up from 9.6 kbit/s with RS232 to 1 Gbit/s with IEEE 802 over Cat5e / Cat6 cables or Optical Fiber

  • Increased overall performance

  • Increased distance

  • Ability to use standard Access Point s, Router s, Switch es, Hubs , Cable s and Optical Fiber , which are immensely cheaper than the equivalent serial-port devices

  • Ability to have more than two nodes on link, which was possible with RS485 but not with RS232

  • Peer-to-peer architectures may replace master-slave ones

  • Better interoperability


The difficulties of using industrial Ethernet are:
  • Migrating existing systems to a new protocol (however many adapters are available)

  • Real-time uses may suffer for protocols using TCP (but some use UDP and Layer 2 protocols for this reason)

  • Managing a whole TCP/IP stack is more complex than just receiving serial data


Main Industrial Ethernet Protocols


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Serial version Ethernet version Protocol Network Standards
Modbus-RTU Modbus-TCP TCP/IP IEC 61158 and IEC 61784
Profibus PROFINET IO Isochronous real time protocol ( IRT ),
Real time protocol (RT),
Real time over UDP protocol (RTU)		 Switches , Router and Wireless ,
from 100 Mbit/s up to 1 Gbit/s		 IEC 61158 and IEC 61784
DeviceNetCIP (EtherNet/IP) IP IEC 61158 and IEC 61784
Foundation Fieldbus H1Foundation Fieldbus High Speed Ethernet (HSE)
Ethernet Powerlink Ethernet 100Mbit/s by EPSG (Ethernet Powerlink Standardization Group)
EtherCAT time slicing Ethernet 100Mbit/s by EtherCAT Technology Group (trying to get in IEC 61158)

''(Note the highly ambiguous name given the Ethernet version of DeviceNet.)''


REFERENCES

  • Arndt Lüder, Kai Lorentz (Editor), IAONA Handbook Industrial Ethernet, Industrial Automation Open Networking Alliance e.V., 150 S., Magdeburg (Germany), 2005, ISBN 3-00-016934-2, free copy at handbook(at)iaona.org.



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