E-Zine August 2012
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FOUNDATION™ Fieldbus must maintain tight scheduling required of any process control system on a distributed Field Control network. In other control systems, control is scheduled and calculated within a device in the control system, such as a multifunction controller. With Field Control, the control is distributed around the network into many devices, so function block initialization is an issue.
Each node of a FOUNDATION™ Fieldbus network operates independently, but they share a common sense of time to within about 62.5 microseconds and can compensate for network transit time delays. The scheduling of the network is the responsibility of the Link Active Scheduler (LAS) that must exist for each network segment. The LAS is so important that most segments have more than one device capable of taking over the LAS function should the primary LAS fail. Most FOUNDATION™ Fieldbus devices are capable of being the primary or the backup LAS. Failover of the primary LAS to the backup is transparent to the remainder of the network, and is not usually an event worth noting unless the device itself has failed.
The general scheduling mechanism of FOUNDATION™ Fieldbus is called real-time publish and subscribe. When any device needs data from another device on the network, it issues a subscription request for that data by tag name and attribute. For example, if the output of the steam pressure controller (PC101) were required, subscription requests for PC101.OUT would be issued on the network as a broadcast request. This format is called object notation, where OUT is the attribute of the PC101 object. OUT is defined by the device description (DD) standard. When the device containing the control function block configured as PC101 receives the subscription request, it creates a buffer containing the required data and places a schedule for the publication of that data into the devices publication schedule queue. Thereafter, the required data will be published as scheduled with a data set identifier until told to quit. Transmission timing is under the control of the LAS for its own network segment.
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Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control