By MorethanIP GmbH, National Semiconductor Corp., Altera Corp.April 18, 2008 -- industrialcontroldesignline.com
One of the biggest growth technologies in the industrial market is Ethernet based networking. Most industrial Ethernet standards use the IEEE 802.3 standard Ethernet protocol giving them the ability to carry standard network traffic as well as real time data. Each standard uses a different technique to deliver real-time performance " some use custom hardware, others use custom software and some use a completely standard Ethernet/TCP/IP implementation. The result is a set of operationally incompatible standards with different levels of performance and cost.
One increasingly popular way around the non-deterministic communication times of the Ethernet protocol is to implement a local clock in every device; as most devices contain a microprocessor and a (relatively) high speed clock, this is usually easy to implement. If accurate clock synchronization across the network can be implemented and maintained, while controlling the accurate timing of operations across the whole system, the only limitations are the latency of communication and accuracy of the system wide clock synchronization.
This type of system control is not suitable for applications like accurate motion control (for example precision speed control of a motor under varying load) as these require low latency communication between the controller and device, but it is very useful for accurate control of whole systems (for example a large printing press or long automated production line) where highly synchronous system wide control (e.g. a speed change) is required. If there is enough time to issue a command to every device, the only limitation to the accuracy of clock based control is the accuracy of the clock synchronization across the system.
Several industrial networking standards (and not just Ethernet based standards) are implementing the IEEE 1588 standard to provide this kind of control capability. IEEE 1588 provides a high accuracy master clock and proven mechanisms for clock synchronization that can be used to create and maintain a very precise system wide synchronization of all local clocks with the master clock.
Ethernet based networks are attractive due to the low cost and ease of implementation of Ethernet. Ethernet switches are a key component that helps deliver these benefits, and corporate systems rely heavily on them to implement high performance and easy to maintain infrastructures. The huge corporate market for switches means that they are readily available and low cost, however most of the switches presently available on the market have not been designed to deliver low latency performance or deterministic routing times, making them difficult to use in the industrial environment.
The IEEE 1588 system synchronizes the master and slave clocks by measuring the communication delay time between the master and slave. Having a switch placed between the master and slave clock introduces an additional latency as the switch has to analyze the packets and route them. This additional latency is not good, but it is not the major issue as it can be factored into the delay correction. The problem is when traffic increases the time taken to route the packet gets significantly longer.
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