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Rosemount Model 8800C Vortex Flowmeter with FOUNDATION
TM
Fieldbus
A-4
Figure A-2. Simple, Single-Link
Fieldbus Network.
Link Active Scheduler
(LAS)
All links have one and only one Link Active Scheduler (LAS). The LAS
operates as the bus arbiter for the link. The LAS does the following:
• recognizes and adds new devices to the link.
• removes non-responsive devices from the link.
• distributes Data Link (DL) and Link Scheduling (LS) time on the
link. Data Link Time is a network-wide time periodically
distributed by the LAS to synchronize all device clocks on the
bus. Link Scheduling time is a link-specific time represented as
an offset from Data Link Time. It is used to indicate when the
LAS on each link begins and repeats its schedule. It is used by
system management to synchronize function block execution
with the data transfers scheduled by the LAS.
• polls devices for process loop data at scheduled transmission
times.
• distributes a priority-driven token to devices between scheduled
transmissions.
Any device on the link may become the LAS, as long as it is capable.
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 begins operating as the LAS immediately upon
completion of the bidding process. Link masters that do not become the
LAS act as basic devices. However, the link masters can act as LAS
backups by monitoring the link 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.”
Two types of tokens are used by the LAS. A time-critical token, compel
data (CD), is sent by the LAS according to a schedule. A non-time
critical token, pass token (PT), is sent by the LAS to each device in
ascending numerical order according to address.
LAS = Link Active Scheduler
LAS
Basic Devices and/or link
master devices
Link Master
Fieldbus Link
FIELDBUS_0013