System planning
3
3.1 Nodes in the FOUNDATION Fieldbus
An FF segment contains all nodes communicating via the FOUNDATION Fieldbus. The
following information is used for the system-specific optimization of FF segments.
Size of the process image
An FF segment is restricted to the size of the process image of the FF Link (max. 244 bytes
I/O in each case).
● The 244 bytes for inputs can be divided among the FF devices:
– Digital inputs (2 bytes per value, however maximum 40 DI)
– Analog inputs (5 bytes per value, however maximum 40 AI)
● The 244 bytes for outputs can be divided among the FF devices:
– Digital outputs (2 bytes per value, however maximum 40 DO)
– Analog outputs (5 bytes per value, however maximum 40 AO)
● A maximum of 64 bytes can be used for outputs and 64 can be used for inputs for each
FF device.
Nodes in an FF segment
The maximum number of nodes in an FF segment depends on the system requirements with
regard to update times. Typically, 3 to 7 FF devices are integrated in an FF segment (maximum
31).
The time for a macrocycle is influenced primarily by the share of cyclic communication. This
share depends on the following factors:
● Properties of the FF devices which are used on the FF segment.
● Number of input values and output values of all components connected to the FF segment.
You may indicate a target value for the macrocycle as well as the ratio between cyclical and
acyclic share. The schedule is calculated for these conditions.
If a mean time of approximately 30 ms is assumed for the output of a value from FF devices,
and the block run time is considered as acyclic bus time, then the minimum macrocycle for 15
transmitted values will be approximately 1 second. The time for output of a value depends on
the device in question.
FOUNDATION Fieldbus
Commissioning Manual, 03/2013, A5E03880935-02 17
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