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

Introduction
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
Fisher-Rosemount 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 will use its programmed address. Each
of the other devices will be given one of four temporary addresses
between 248 and 251. If a temporary address is not available, the device
will be unavailable until a temporary address becomes available.
Scheduled Transfers Information is transferred between devices over the fieldbus using
three different types of reporting.
• Publisher/Subscriber: This type of reporting is used to transfer
critical process loop data, such as the process variable. The data
producers (publishers) post the data in a buffer that is
transmitted to the subscriber (S), when the publisher receives the
Compel data. The buffer contains only one copy of the data. New
data completely overwrites previous data. Updates to published
data are transferred simultaneously to all subscribers in a single
broadcast. Transfers of this type can be scheduled on a precisely
periodic basis.
• Report Distribution: This type of reporting is used to broadcast
and multicast event and trend reports. The destination address
may be predefined so that all reports are sent to the same
address, or it may be provided separately with each report.
Transfers of this type are queued. They are delivered to the
receivers in the order transmitted, although there may be gaps
due to corrupted transfers. These transfers are unscheduled and
occur in between scheduled transfers at a given priority.
• Client/Server: This type of reporting is used for
request/response exchanges between pairs of devices. Like Report
Distribution reporting, the transfers are queued, unscheduled,
and prioritized. Queued means the messages are sent and
received in the order submitted for transmission, according to
their priority, without overwriting previous messages. However,
unlike Report Distribution, these transfers are flow controlled
and employ a retransmission procedure to recover from
corrupted transfers.
Figure 1-3 diagrams the method of scheduled data transfer. Scheduled
data transfers are typically used for the regular cyclic transfer of
process loop data between devices on the fieldbus. Scheduled transfers
use publisher/subscriber type of reporting for data transfer. The Link
Active Scheduler maintains a list of transmit times for all publishers in
all devices that need to be cyclically transmitted. When it is time for a
device to publish data, the LAS issues a Compel Data (CD) message to
the device. Upon receipt of the CD, the device broadcasts or “publishes”
the data to all devices on the fieldbus. Any device that is configured to
receive the data is called a “subscriber.”