Rosemount OCX 8800, Instruction Manual

Page: 105 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-5
Figure 7-2. Single Link Fieldbus Network
Basic Devices and/or LinkMaster Devices
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.
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 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.
LAS
(Link Active Scheduler)
Fieldbus Link
Link Master
3
9
9
3
0
0
1
7