Foundation Fieldbus Link Active Scheduler Algorithm

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Thus far we have described the three basic types of communications that take place on a fieldbus H1 segment and the role that the link active scheduler or LAS plays in these fieldbus communications. Weve also indicated that the LAS has other responsibilities for data link maintenance as well. This may have left you a little foggy on just what these responsibilities are and how and when they are carried out. This information is presented here and includes a logic flow diagram used by the link active scheduler as it executes its foundation fieldbus scheduling algorithm.

Here is a partial list of the tasks performed by the link active scheduler. As you already know, this includes granting permission for scheduled communications, and unscheduled communications on the fieldbus. The details of these data transfers were presented previously in this fieldbus tutorial. Next we have the data link time synchronization. This must be done to ensure the internal clock of each field and host device is synchronized to within a one-millisecond accuracy. The clock in each device is not only used to sync data communications over the fieldbus, but is also key in scheduling activities internal to each device. This includes, for example, the scheduling of function block execution, which well discuss later in this fieldbus tutorial. Next, the link active scheduler must be capable of dynamically identifying new devices as they are placed onto the bus. Once a new device is identified, a configuration tool can be used for automatic address assignment and control configuration. This is done without hardware dipswitch or jumper settings. All hardware and software settings are made over the fieldbus, using client server type data communications. Thus, we have a true plug-and-play type operation.

So, lets take a look at how the LAS determines what is done on the bus and when. It must satisfy all needs for deterministic execution of process control algorithms, and at the same time achieve some semblance of order and sharing of fieldbus resources among all devices. This is as much a mouth full, as it is a tall order for the link active scheduler.

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Category:

Science & Technology

Tags:

• Foundation
• Fieldbus
• Process
• Control
• Engineering
• System
• Interoperability
• Link
• Active
• Scheduler
• Bus
• Communications
• High
• Speed
• Ethernet
• DCS