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Reference Manual
00809-0100-4697, Rev GA
Appendix C: F
OUNDATION
Fieldbus Technology
November 2014
F
OUNDATION
Fieldbus Technology
Figure C-2. Simple, Single-Link Fieldbus Network
A. Link Active Scheduler (LAS)
B. Fieldbus Link
C. Link Master
D. Basic Device and/or Link Master Devices
C.5.1 LAS
All links have one LAS that 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 Time (DL) and Link Scheduling Time (LS) on the link
– DL is a network-wide time periodically distributed by the LAS to synchronize all
device clocks on the bus.
– LS time is a link-specific time represented as an offset from DL. 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. The devices that are capable of becoming the LAS
are called Link Master devices (LM). 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.
There may be many LM devices on a segment but only the LAS is actively controlling
communication traffic. The remaining LM devices on the segment are in a stand-by state, ready
to take over if the primary LAS fails. This is achieved by constantly monitoring the
communication traffic on the bus and determining if activity is not present. Since there can be
multiple LM devices on the segment when the primary LAS fails, the device with the lowest node
address will become the primary LAS and take control of the bus. Using this strategy, multiple
LAS failures can be handled with no loss of the LAS capability of the communications bus.
A
B
C
D