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Reference Manual
00809-0100-4697, Rev EA
October 2011
C-7
Rosemount 848T
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.”
Figure C-4. Scheduled Data
Transfer
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
Schedule
X, Y, Z
CD(X,A)
DT(A)
Device X
Device Y Device Z
ACDAB
PS PS P
A
S
LAS
P = Publisher S = Subscriber PT = Pass Token M = Message
Schedule
X, Y, Z
PT(Z)
Device X Device Y Device Z
ACDAB
PS PS P
A
S
LAS
DT(M)
MM