Emerson Rosemount OCX 8800, Instruction Manual

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Instruction Manual
IM-106-880C, OI
Section 7: Foundation
Fieldbus
April 2017
Network
Communication
93
7.5 Network Communication
Figure 7-3 illustrates a simple Fieldbus network consisting of a single segment (link).
Figure 7-3 Single Link Fieldbus Network
7.5.1 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 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.