Device Communications; Scheduled Transfers - Fisher FIELDVUE DVC6200f Instruction Manual

F fieldbus Communication
OUNDATION
July 2013
D recognizes and adds new devices to the link.
D removes non‐responsive devices from the link.
D 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.
D polls devices for process loop data at scheduled transmission times.
D 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:
D 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".
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Instruction Manual
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