The quest to bring lockstep efficiency to labor intensive factory production at first relied on mechanical ingenuity. Over time, as reliable and cost-effective microprocessor technology became available, assembly lines were retooled to use the new electronics to achieve greater automation and productivity. Today, pervasive network technology, including the Internet, is bringing about another evolutionary change in industrial automation. No longer is it sufficient for individual machines to perform their specific tasks independently. Instead, individual parts of a larger process must be aware of each other; they must exchange data in realtime, and adapt to changes in the process or environment. Additionally, it is increasingly necessary to exchange data outside of the traditional industrial network and beyond the factory floor, and to include enterprise LANs and the Internet. Up-tothe- minute information on manufacturing processes needs to be available to analysts, inventory managers, and others within the office environment.

Figure 1. Example of a real-time publish-subscribe system
To achieve this, individual machines need more than the intelligence to drive a particular part of the process; they need instant access to data across the network. This means distributing data, often in real-time, across dozens, hundreds, or even thousands of nodes on a broad network encompassing controllers, sensors, factory-floor computers, desktop PCs and servers.

The key to real-time data distribution is the publish-subscribe approach. Simply put, a data source, such as a programmable logic controller, can publish data to a listener. Other devices, such as an embedded computer-based device, a PC, or an Internet server, can then subscribe to that data source and obtain the data as soon as it is available. The publish-subscribe model can be made to perform in real-time by optimizing the availability and transport of data, so that it can be analyzed and acted upon at almost the same moment in time. This model is illustrated in Figure 1.

In the past, industrial automation applications were not always able to take advantage of real-time publish-subscribe architectures; however, advances in data networking technology are changing this. Manufacturing applications typically have not supported real-time data exchange, except for specific point-to-point data transfers. In many cases, data has been used for record-keeping, rather than to provide real-time analysis and feedback to multiple devices on an industrial network.

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