Emerson 475, User Manual

Page: 54 / 114

Fieldbus Functionality
4-2
BASIC FEATURES AND
FUNCTIONS
Fieldbus Application
Functionality
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
OUNDATION
Fieldbus to run this application. See your sales
representative or the Online Licensing feature in the Field Communicator Easy
Upgrade Utility to purchase this license.
Link Active Scheduler (LAS)
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. 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 4-7.
LAS Hierarchy
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.