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Manual
00809-0500-4881
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.
Two types of tokens are used by the LAS. The LAS sends a time-critical token, compel data
(CD) according to a schedule. The LAS also sends a non-time critical token, pass token (PT)
to each device in ascending numerical order according to address.
8.2.2
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.
8.3
Rosemount OCX8800 function blocks
8.3.1
Implemented function blocks
Table 8-1
Rosemount OCX8800C
shows the Rosemount OCX8800 implemented function blocks.
F
Fieldbus
™
OUNDATION
April 2023
99
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