Basic Features And Functions; Link Active Scheduler (Las) - Emerson 375 User Manual

4-2
BASIC
FEATURES
AND
FUNCTIONS
Link Active
Scheduler
(LAS)
All segments have one and only one Link Active
Scheduler (LAS). The LAS operates as the bus
arbiter for the segment. The 375 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
(i.e. 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.
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 List"; see "Displaying the Live
Device List" on page 4-11.
Fieldbus Functionality