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A-7
FOUNDATION
TM
fieldbus Technology and Fieldbus Function Blocks
Function Block Scheduling Figure A-5 shows an example of a link schedule. A single iteration of
the link-wide schedule is called the macrocycle. When the system is
configured and the function blocks are linked, a master link-wide
schedule is created for the LAS. Each device maintains its portion of the
link-wide schedule, known as the Function Block Schedule. The
Function Block Schedule indicates when the function blocks for the
device are to be executed. The scheduled execution time for each
function block is represented as an offset from the beginning of the
macrocycle start time.
Figure A-5. Example Link Schedule
Showing scheduled and Unscheduled
Communication.
To support synchronization of schedules, periodically Link Scheduling
(LS) time is distributed. The beginning of the macrocycle represents a
common starting time for all Function Block schedules on a link and for
the LAS link-wide schedule. This permits function block executions and
their corresponding data transfers to be synchronized in time.
Macrocycle Start Time
Offset frommacrocycle
start time = 0 for AI
Device 1
Scheduled
Communication
Sequence
Macrocycle
Offset from macrocycle
start
Unscheduled
Communication
Device 2
Offset from macrocycle
start
Offset from macrocycle
start
AI
AI
PI
AO
PI
AO
FIELDBUS_0016