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Motion-control engineers, and control-systems managers and operators want control systems with high throughput, low installation and operating costs, and multi-vendor hardware and software compatibility. Control networks based on the technology of the traditional source-destination model cannot satisfy these ever-increasing demands for higher productivity. Increasing the baud rates and protocol efficiency helps, but is not enough to meet the challenge.

A newer approach to network technology, the producer-consumer model, promises to fulfill these control system goals. Producer-consumer networks provide more functions, use bandwidth more efficiently, increase information flow, and reduce network traffic.

Unlike source-destination communication systems, producer-consumer networks permit all nodes on the network to simultaneously access the same data from a single source. One network can handle both real-time control messages plus other types of messages required for programming, device configuration, trending, and diagnostics. These networks also support traditional master-slave, multimaster, and peer-to-peer communication systems.

Network capabilities

The traditional sourcedestination network model was developed for the computer and data processing industry. This model is well suited for a variety of applications that don’t require complex coordination and sharing of data.

A source-destination network is based on point-to-point messaging between nodes. Its operation is analogous to one person (the source) telling each person in a large group (the destinations) the time of day (the data), one at a time. Some people may listen, others may ignore the data, thus wasting communications effort. Because it takes time to communicate on a one-on-one basis, the data being communicated is not accurate after the first-person exchange. Therefore, to synchronize all destinations, time-delay adjustments need to be made by the source or by each of the destinations.

Put another way, the major drawbacks of source-destination networks are that they require considerable bandwidth to send the same data to multiple nodes, and synchronized action between nodes is difficult because data arrives at each node at a different time.

With a producer-consumer network, all nodes on the network can simultaneously access the same data from a single source. Thus, data are produced only once, regardless of the number of destinations. This is called multicast communication, Figure 1. Producers identify messages by their content. A node reads the message identity, and if it needs the data, it “consumes” them. True multicast communication is not possible with source-destination networks, although limited attempts are made with such mechanisms as global addressing.

Data transfer is also synchronized because the data arrive at each node at the same time, and no adjustment is required by either the producer or consumers. This method provides an optimal solution for applications that require synchronization, such as controlling several axes from a single signal in a packaging machine labeling application.

A producer-consumer network is also highly deterministic because delivery time is constant regardless of how many nodes are added to, or leave, the network.

Message types. A producer-consumer network can send both I/O data for control (implicit messages), and explicit messages that are used for such tasks as uploading and downloading programs, modifying device configurations, trending, and diagnostics. An explicit message contains protocol information, instructions for the service to be performed, and an address the service is to be applied to. The receiving device (node) must interpret the message, perform the requested task, and generate a response. These messages vary widely in both size and frequency.

I/O data messages send real-time control information. This implicit type of message only contains data, no protocol information. The meaning of the data is predefined, therefore processing time in the node is minimal because there is no protocol information to interpret. The device simply uses the dataandreacts accordingly. Implicit messages are short, vary frequently, and require fast throughput.

In the past, separate networks dealt with the different requirements of these two message types. A network designed for I/O control could not tolerate the variability introduced by explicit messages. However, because of its ability to carry both types of messages on the same wire, a producer-consumer network can simultaneously exchange I/O data, upload and download programs, configure devices, and provide device diagnostic data.

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