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Data transfer via tunneling
February 11, 2011
Dear Control Engineering: Is it possible to transport data from a different type of network over Ethernet? For example, can we send data from fieldbus segments?
There is a technique called tunneling that sends information from one kind of network over another and converts it back at the other end. For a more thorough explanation, here are some thoughts from Corey McAtee, technical marketing manager for Beckhoff Automation. (He cites options available using Beckhoff equipment and software, but similar functionality can be accomplished with other platforms.)
EtherCAT permits IP-based protocol communication through the network using a “tunneling” concept. Examples of this include communications to remote HMI panels via Web thin client, or to a standard printer or other TCP/UDP-based components. Beckhoff calls this tunneling functionality EoE, or Ethernet over EtherCAT. With our hardware, this is accomplished using the EL6601 or EL6614 Ethernet switch port terminals which provide standard RJ45 connector(s) in order to connect network-based devices to EtherCAT. On the Beckhoff software side, TwinCAT I/O (EtherCAT master) hands these packets up to the OS layer (Windows) where they are processed like any other standard network packet.
Tunneling is not unlike the layering of protocols that is a key feature already implemented in Internet technologies used in Ethernet, such as layering TCP under IP (TCP/IP) or HTTP under TCP under IP, which is all part of our everyday computer use. By tunneling the non-time critical Internet protocols for these devices inside the EtherCAT frames, the EtherCAT network can ensure that its highly deterministic, high scan rate frames are unaffected by the communication of these comparatively slow IP-based devices.
In addition to IP-based protocol tunneling, Beckhoff has also implemented MDP (modular device profile) into our fieldbus design, providing a standard way to pass data from legacy fieldbuses over EtherCAT. As a result, Beckhoff users can implement fieldbus gateways like the EL6752 DeviceNet master terminal on the network. We treat all DeviceNet data as standard process data on the EtherCAT master side, and the terminal simply provides a translation to and from the DeviceNet network. Benefits of an implementation like this are:
1. The secondary fieldbus data is transported synchronously across the fieldbus and is not managed on the service channel; and
2. It results in reduced CPU use on the EtherCAT master due to the removal of the traditional cyclic object mapping.
EtherCAT gateway terminals are also available for Profibus, Profinet, CANopen, and more, providing exceptional flexibility in establishing communication from an EtherCAT-enabled system to non-EtherCAT devices and to older plant equipment.