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Rosemount Model 8800C Vortex Flowmeter with FOUNDATION
TM
Fieldbus
A-6
Figure A-3. Scheduled Data Transfer.
Unscheduled Transfers Figure A-4 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.
Figure A-4. Unscheduled Data
Tran sfe r.
Schedule
X
Y
Z
CD(X,A)
DT(A)
Device X Device Y Device Z
ACDAB
PS PS P
A
S
LAS = Link Active
Scheduler
P = Publisher
S = Subscriber
CD = Compel Data
DT = Data Transfer Packet
LAS
FIELDBUS_0013
Schedule
X
Y
Z
PT(Z)
Device X Device Y Device Z
ACDAB
PS PS P
A
S
LAS = Link Active
Scheduler
P = Publisher
S = Subscriber
PT = Pass Token
M = Message
LAS
DT(M)
MM
FIELDBUS_0015