Siemens Simatic PCS 7 Commissioning Manual page 8

Basics of the FOUNDATION Fieldbus
2.1 Basic knowledge of FOUNDATION Fieldbus
● Communication types
● Schedule and LAS (Link Active Scheduler)
● Macrocycle
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Two types of communication are used with FF:
– cyclic communication
Cyclic communication is defined in the Schedule. It includes tasks such as controlling
process variables (control functions) and operating and monitoring functions.
– acyclic communication
Acyclic communication is used for transmitting unscheduled information. Examples
include the following information:
* Maintenance/diagnostic data
* Configuration data
* Parameterization data
With FF, the Schedule defines when an FF device sends or reads cyclic data. Use the
Schedule to prevent communication conflicts. An FF Link assumes the LAS function in
normal operation. If there is no FF Link online on the FF segment, suitable FF devices (Link
Master) can assume the LAS function.
The macrocycle is a time period which must be defined for each system. The following
tasks must be performed during this time period:
– All FF devices must be processed.
– Information must be transmitted by means of acyclic communication.
– The FF specifications recommend that at least 50% of the bus time is kept free for acyclic
communication.
Commissioning Manual, 03/2013, A5E03880935-02
FOUNDATION Fieldbus