Currently, many computer-based vision- and image-processing systems are developed around standard PCI- or VME-based systems. These standards allow developers of OEM frame-grabber, display-controller, and input/output boards to concentrate more on product development and less on porting their range of products to several buses.
While this has proved beneficial for developers and systems developers alike, integrating such systems into networks for machine-vision control is a complicated task. The reason results from the plethora of legacy control networks that consist of networked sensors, actuators, and controllers. These proprietary and public-domain controlled networks range from small networks embedded in machines to large networks with thousands of nodes controlling automation equipment.
Unfortunately, existing networks use a number of different topologies and physical media and support different numbers of devices at different distances. Worse, such networks cannot easily be integrated with plantwide communications systems that use standardized Ethernet-based networking for system modification, data collection, and program maintenance. Consequently, developers of control systems are turning to established Ethernet-based systems for new automation-control applications.
Despite the increased interest in Ethernet, developers have been hesitant to use such networks to transfer data on the plant floor because of the lack of interoperability among competing vendors' devices. This hesitancy comes about because Ethernet technology only provides a set of physical media definitions—a scheme for sharing the physical media and a simple frame format and addressing scheme for moving packets of data on a local-area network.
"To ensure that such communication is effective, a common application layer is required," says Nick Jones, chief technical officer with the Open DeviceNet Vendor Association (ODVA; Boca Raton, FL; www.odva.org). To overcome the Ethernet interoperability limitation, the ODVA has developed EtherNet/IP, a standard that applies the DeviceNet application layer over commercial off-the-shelf Ethernet (IEEE 802.3) products. This same application layer is found in the ControlNet network from ControlNet International (Boca Raton, FL; www.controlnet.org).
Unlike other Ethernet solutions that require gateways to and from the network, EtherNet/IP ports encapsulate the DeviceNet messages over Ethernet. Because the networks share a common application layer, Ethernet/IP uses the same object models and device profiles as DeviceNet. "By using the UDP/IP and TCP/IP protocols to encapsulate networked messages, both information and I/O messaging can be provided," says Frank Wood, executive director of the ODVA. "As a result, this is the first industrial Ethernet network to provide real-time I/O control," he adds.
Modbus from Schneider Electric (Palatine, IL; www.schneiderelectric. com) and Profibus International (Karlsruhe, Germany; www.profibus. com) are also moving in this direction. Combining a physical Ethernet network with a universal networking standard (TCP/IP) and a vendor-neutral data representation, the Modbus TCP specification embeds a Modbus frame into a TCP frame (www.modicon.com/openmbus). Profibus is also working on an Ethernet-based standard, the Profinet. Designed for transparent networking in automation systems, this approach will not implement an additional application layer 7 on TCP/ IP.
Some developers, however, remain critical of this "bolt-on" approach. "Aside from having an increase in data rate," says ODVA's Jones, "simple ports of existing protocols to TCP/IP will have the same inherent benefits and limitations as the original network." What has been lacking in control and information networking standards is an open application layer that can unite multiple networks.
