Emerson Rosemount 848T, Reference Manual, Page: 105 / 136

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Reference Manual
00809-0100-4697, Rev GA
Appendix C: F
OUNDATION
Fieldbus Technology
November 2014
F
OUNDATION
Fieldbus Technology
Figure C-4. Scheduled Data Transfer
C.5.4 Unscheduled transfers
Figure C-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 F
OUNDATION
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.
Figure C-5. Unscheduled Data Transfer
LAS = Link Active Scheduler
P = Publisher
S = Subscriber
CD = Compel Data
DT = Data Transfer Packet
P = Publisher S = Subscriber PT = Pass Token M = Message
Schedule
X, Y, Z
CD(X,A)
DT(A)
Device X
Device Y Device Z
A C D A B
P S P S P
A
S
LAS
Schedule
X, Y, Z
PT(Z)
Device X Device Y Device Z
A C D A B
P S P S P
A
S
LAS
DT(M)
M M