Emerson FIELDVUE DVC6200f, Instruction Manual

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F
OUNDATION
Fieldbus Communication
November 2010 269
group addressing and for use by the data link layer. 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. Commission devices use addresses 20
−
35,
and standby devices use addresses 232-247.
Link Active Scheduler (LAS)
There is only one active Link Active Scheduler (LAS)
for the entire fieldbus control system. The digital valve
controller includes an LAS. The Link Active Scheduler
operates as the bus arbiter for the link, and
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.
The DVC6200f can be designated to act as the
backup Link Active Scheduler (LAS) in the event that
the LAS is disconnected from the segment. As the
backup LAS, the DVC6200f will take over the
management of communications until the host is
restored. The host system may provide a configuration
tool specifically designed to designate a particular
device as a backup LAS. Otherwise, this can be
configured manually as follows:
1. Access the Management Information Base (MIB)
for the DVC6200f.
2. To activate the LAS capability, write 0x02 to the
BOOT_OPERAT_FUNCTIONAL_CLASS object
(Index 605). To deactivate, write 0x01.
Restart the processor.
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 are a member
of 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 numerical order according to address.
Device Communication
Scheduled Transfers
Information is transferred between devices over the
fieldbus using three different types of communication:
Publisher/Subscriber: This type of
communication 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 is
issued the Compel Data (CD) message from the LAS.
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 are scheduled on a precisely periodic basis.
Figure D-4 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”.
Unscheduled Transfers
Figure D-5 diagrams an unscheduled transfer.
Unscheduled transfers are used for things like
user-initiated changes, including set point changes,
mode changes, tuning changes, and upload/download.
Unscheduled transfers use either report distribution or
client/server type of reporting for transferring data.
All of the devices on the fieldbus are given a chance to
send unscheduled messages between transmissions
of scheduled data. The LAS grants permission to a
device to use the fieldbus by issuing a pass token (PT)
message to the device. When the device receives the
PT, it is allowed to send messages until it has finished
or until the “maximum token hold time” has expired,
whichever is the shorter time. The message may be
sent to a single destination or to multiple destinations.
D