System Planning; Nodes In The Foundation Fieldbus - Siemens SIMATIC PCS 7 FOUNDATION Fieldbus Commissioning Manual

System planning

4.1

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 bus 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 bytes for inputs for each FF device.
Nodes in an FF segment
The maximum number of nodes in an FF segment depends on the requirements of a plant
regarding update times.
Typical number of FF devices that are integrated in a plant in an FF segment: 8 to 10 (maximum
number 31)
The time for a macro cycle is influenced primarily by the share of cyclic communication. This
share depends on the following factors:
● Properties of the FF devices used in the FF segment
● Number of input values and output values of all components connected to the FF segment
You can specify a target value for the macro cycle as well as for the ratio between the cyclic
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 macro cycle for
15 transmitted values will be approximately one second. The time for output of a value depends
on the device in question.
FOUNDATION Fieldbus
Commissioning Manual, 02/2015, A5E32711026-AC
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