TechnologyJanuary 5, 2007
Could daisy-chained Ethernet become the new fieldbus?
Most older fieldbus automation networks already use a form of daisy-chain topology. Could daisy-chained Ethernet for Plant and Process do the same job? Schneider's Michael B. Roche thinks the answer is yes!
Ethernet requires infrastructure equipment like hubs and switches. It also imposes distance constraints which require careful planning for efficient (lowest cable use vs the number of switches to connect everything that needs to be connected) network infrastructure. End-devices must be located within 100 metres of the switch port.
An example of the difficulty of using Ethernet in bus applications is illustrated by a conveying application in the top drawing. This setup uses a star topology. There may be relatively few ports used within that 100m radius, and a separate media segment is required to each device, even if the devices are just a metre apart. This adds to the overall installation costs. A daisy-chain Ethernet topology addresses all of these issues, offering many of the benefits found in the earlier fieldbus.
What is daisy-chain Ethernet?
In the drawing below, a daisy-chain device has two embedded Ethernet ports which function as an Ethernet switch, as well as an interface to the local device. This allows information to flow to the device, or flow through the ports to other devices in the daisy-chain. Devices in the chain are cabled together in series using standard Ethernet cable, similar to legacy fieldbus �C but without the need for additional Ethernet switches. While daisy-chain networks can be used in a straight line bus topology, having a redundant ring daisy-chain connection offers fast recovery in the event of a cable break. Simply adding a single switch with redundant capability connects the daisy- chain to the overall network and affords robust redundancy.
Serial latency: how many devices?
Up to 32 devices in a daisy-chain can be sensibly deployed in an industrial setting. Theoretically there could be dozens of device deployed in a chain through use of store and forward technology �C used by both daisy-chain devices and standard Ethernet switches. This assures that each packet is checked for accuracy before being forwarded on the network. The cost of that accuracy is 20-40��s latency for the average automation protocol message passing through each device. This means that the total time it takes for a message to travel from the first device in a chain of 32 to the last device would be less than 1.3ms absolute worst case.