7.2.1 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.
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.
7.2.2
Device addressing
Fieldbus uses addresses between 0 and 255.
Addresses 0 through 15 are reserved for group addressing and for use by the data link
layer. For all Emerson Fieldbus devices, addresses 20 through 35 are available to the
device. If there are two or more devices with the same address, the first device to start
uses its programmed address. Each of the other devices is given one of four temporary
addresses between 248 and 251. If a temporary address is not available, the device is
unavailable until a temporary address becomes available.
Foundation Fieldbus
Reference Manual 99
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