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

4-2
The 375 Field Communicator draws approximately 17 mA from the fieldbus segment. Be sure
the power supply or barrier on the fieldbus segment has the capacity to provide this additional
current. If a heavily loaded fieldbus segment is drawing near the capacity of the segment's
power supply, connecting the 375 Field Communicator may result in loss of communication.
BASIC
FEATURES
AND
FUNCTIONS
Link Active
Scheduler
(LAS)
WARNING
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.
Although the 375 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 Live Device List" on page 4-9.
Fieldbus Functionality