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60
BASIC FEATURES AND
FUNCTIONS
Fieldbus Application
functionality
Link Active Scheduler
(LAS)
LAS hierarchy
The Fieldbus Application lets you communicate with connected
fieldbus devices, modify settings for the application, and run
diagnostics. Your 475 Field Communicator must be licensed for
F
Fieldbus to run this application. See your sales
OUNDATION
representative or the Online Licensing feature in the Field
Communicator Easy Upgrade Utility to purchase this license.
All segments have only one Link Active Scheduler (LAS). The LAS
operates as the bus arbiter for the segment. The 475 Field
Communicator is set up to always be the last node to become the LAS
on a segment.
The devices that are capable of becoming the LAS are called link
master devices. All other devices are referred to as basic devices. You
can set a device to be a link master or basic device using the 475 Field
Communicator. See "Setting the device to be a Link Master or Basic
device" on page 74 for more information.
When a segment first starts up, or upon failure of the existing LAS, the
link master devices on the segment bid to become the LAS. The link
master that wins the bid (the one with the lowest address) begins
operating as the LAS immediately upon completion of the bidding
process. Link masters that do not become the LAS can act as LAS
backups by monitoring the segment for failure of the LAS and then
bidding to become the LAS when a LAS failure is detected.
Although the 475 Field Communicator is capable of becoming the LAS,
it does not maintain the communication schedule and should not be
thought of as a backup LAS.
Only one device can communicate at a time. Permission to
communicate on the bus is controlled by a centralized token passed
between devices by the LAS. Only the device with the token can
communicate. The LAS maintains a list of all devices that need access
to the bus. This list is called the Live Device List; see "Displaying the
connected fieldbus devices" on page 65.
When a 475 Field Communicator is connected to a live fieldbus
segment, it joins the fieldbus segment but not as the LAS. This means
the 475 Field Communicator can view and edit device parameters, but
the control of the segment is still handled by the node that is
designated the LAS for the segment. You can also read and write if
there is another LAS. Prior to connecting to a host system, review the
475 Field Communicator Host System Interoperability on
www.fieldcommunicator.com.
Fieldbus functionality
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