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SI-PROFINET RT User Guide
Issue Number: 2
6
Additional features
This chapter provides information on the additional functions and features of the SI-
PROFINET RT option module.
6.1
Update time
In contrast to some other fieldbus networks, PROFINET has no single bus cycle where
the slowest device determines the update rate for all devices, PROFINET allows
different update rates for each device. SI-PROFINET RT can be configured with an
update rate of between 2 ms to the slowest rate of 512 ms; the possible update rates
are 2, 4, 8, 16, 32, 64, 128, 256 and 512 ms.
When selecting the update time, it is recommended that the slowest rate necessary is
used, this is to minimize the possibility of overloading the network.
6.2
PROFINET transmission cycle
The PROFINET transmission cycle is the time required to update all the devices on the
PROFINET network; the length of the PROFINET transmission cycle (send cycle) is
determined by the PROFINET device which has the slowest update rate.
The transmission cycle can be split into a number of phases where each phase contains
one or more devices with similar update rates, the duration of each phase will be equal
to the fastest update rate.
Example:
In the following example (Figure 6-1
PROFINET transmission cycle example
), a system
uses four PROFINET devices, device 1 has an update rate of 8 ms, device 2 has an
update rate of 4 ms, device 3 has an update rate of 2 ms and device 4 has an update
rate of 1 ms.
Figure 6-1 PROFINET transmission cycle example
The PROFINET transmission cycle is effectively the slowest update rate (8 ms in this
example), and the phase time is effectively the quickest update rate (1 ms in this
example). The following information is provided for use when designing a PROFINET
network to calculate the total and individual transmission cycle times.
1
Send cycle 8 ms
2
3
4
Phase 1
Phase 2
Phase 3
Phase 4
Phase 5
Phase 6
Phase 7
Phase 8
1
Device 1 (8 ms update rate)
Device 2
Device 3
Device 4
2
3
4
Each phase represents a time period in which a frame is actually sent.
The length of the phase is important as it indicates if the time is sufficient to
send all frames in the phase.
4
4
4
4
4
4
4
3
3
3
3
4
2
2
1
(4 ms update rate)
(2 ms update rate)
(1 ms update rate)
