Siemens SINAMICS S120 Function Manual page 722

Communication
11.3 Communication via PROFINET IO
Update cycles and send cycles for RT classes
Definition of the update time / send cycle:
If we take a single IO device in the PROFINET IO system as an example, this device has
been supplied with new data (outputs) by the IO controller and has transferred new data
(inputs) to the IO controller within the update time. The send cycle is the shortest possible
update cycle.
All cyclic data is transferred within the send cycle. The actual send cycle that can be set
depends on various factors:
● Bus load
● Type of devices used
● Computing capacity available in the IO controller
● Supported send clocks in the participating PROFINET devices of a synchronization
domain. A typical send cycle is 1 ms.
The table below specifies the reduction ratios which can be set between the send cycle and
the update times for IRT "high performance", IRT "high flexibility", and RT.
Table 11- 14 Settable send cycles and update cycles
Send cycle
Range 250, 500,
"even" 1000 µs
1)
2000 µs
4000 µs
Range 375, 625, 750, 875, 1125,
"odd" 1250 µs ...
3)
3875 µs
(increment
125 µs)
Explanations for the above table:
It is only possible to set send cycles from the "even" range when IO devices with real-time class "RT" are assigned to a
1)
synchronization domain. Likewise, only the reduction ratios from the "even" range can be set for a send cycle setting
from the "even" range.
It is generally only possible to set a reduction ratio of 1:1 between the update time and send cycle for IO devices
2)
(ET200S IM151-3 PN HS, SINAMICS S) which are operated in isochronous mode. In this case, the update cycle mode
must always be set to "fixed factor" (under IO device properties, "IO cycle" tab, "Mode" pulldown menu). This means
that STEP 7 will not automatically adjust the update cycle and thus the update cycle will always correspond to the send
cycle.
The send cycles from the "odd" range can be set only if a synchronization domain does not include any IO devices with
3)
real-time class "RT". Likewise, only the reduction ratios from the "odd" range can be set for a send cycle setting from the
"odd" range.
Isochronous operation is not compatible with IRT "high flexibility".
4)
Odd send cycles can be used only if the IO systems assigned to the synchronization domain do not include any RT or
5)
IRT "high flexibility" devices.
722
Reduction ratio between update and send cycles
RT
IRT "high flexibility"
4)
1, 2, 4, 8, 16, 32, 64, 128, 256, 512
1, 2, 4, 8, 16, 32, 64, 128, 256
1, 2, 4, 8, 16, 32, 64, 128
Not supported
5)
IRT "high performance"
1, 2, 4, 8, 16
2)
1, 2, 4, 8, 16
2)
1, 2, 4, 8, 16
2)
1
Function Manual, (FH1), 04/2014, 6SL3097-4AB00-0BP4
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