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Reference Manual
00809-0100-4697, Rev GA
Appendix C: F
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Fieldbus Technology
November 2014
F
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Fieldbus Technology
F
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fieldbus uses addresses between 0 and 255. Addresses 0 through 15 are reserved for
group addressing and for use by the data link layer.
If there are two or more devices on a segment with the same address, the first device to start up
will use the assigned address. Each of the other devices will be given one of the four temporary
addresses. If a temporary address is not available, the device will be unavailable until a
temporary address is available.
Use the host system documentation to commission a device and assign a permanent address.
C.5.3 Scheduled transfers
Information is transferred between devices over the F
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fieldbus using three different
types of reporting.
Publisher/subscriber
This type of reporting 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,
when the publisher receives the Compel Data (CD). 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 can be scheduled
on a precisely periodic basis.
Report distribution
This type of reporting is used to broadcast and multi-cast event and trend reports. The
destination address may be predefined so that all reports are sent to the same address, or it may
be provided separately with each report. Transfers of this type are queued. They are delivered to
the receivers in the order transmitted, although there may be gaps due to corrupted transfers.
These transfers are unscheduled and occur between scheduled transfers at a given priority.
Client/server
This type of reporting is used for request/response exchanges between pairs of devices. Like
Report Distribution reporting, the transfers are queued, unscheduled, and prioritized. Queued
means the messages are sent and received in the order submitted for transmission, according to
their priority, without overwriting previous messages. However, unlike Report Distribution,
these transfers are flow controlled and employ a retransmission procedure to recover from
corrupted transfers.
Figure C-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
LAS 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 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.”