Figure 8-2: Single Link Fieldbus Network
A. LAS (link active scheduler)
B. Link master
C. Fieldbus link
D. Basic devices and/or LinkMaster devices
8.2.1 Link active scheduler (LAS)
All links have one and only one link active scheduler (LAS). The LAS operates as the
us 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 blocks 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 an 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.
F
OUNDATION
™
Fieldbus Manual
December 2022 00809-0500-4880
118 Emerson.com/Rosemount
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