Siemens SINAMICS S120 Function Manual page 526

Communication PROFIBUS DP/PROFINET IO
10.3 Communication via PROFINET IO
IRT "high flexibility"
The telegrams are sent cyclically in a deterministic cycle (Isochronous Real Time). The
telegrams are exchanged in a bandwidth reserved by the hardware. One IRT time interval
and one standard Ethernet time interval are created for each cycle.
Note
IRT "high flexibility" cannot be used for isochronous applications.
IRT "high performance"
In addition to the bandwidth reservation, the telegram traffic can be further optimized by
configuring the topology. This enhances the performance during data exchange and the
deterministic behavior. The IRT time interval can thus be further optimized or minimized with
respect to IRT "high flexibility".
In addition to the isochronous data transfer provided by IRT, even the application itself
(position control cycle, IPO cycle) can be isochronous in the devices. This is an essential
requirement for closed-loop axis control and synchronization via the bus. Isochronous data
transfer with cycle times well below one millisecond and with a deviation in the cycle start
(jitter) of less than a microsecond provide sufficient performance reserves for demanding
motion control applications.
The RT classes IRT "high flexibility" and IRT "high performance" can be selected as options
in the synchronization settings configuration area of HW Config. In the description below,
both these classes are simply referred to as "IRT".
In contrast to standard Ethernet and PROFINET IO with RT, the telegrams for PROFINET IO
with IRT are transmitted according to a schedule.
IRT* (formerly IRTtop)
Equivalent to the IRT "high performance" function, but a different synchronization method is
used. This is incompatible with the IRT synchronization method (IRT "high flexibility", IRT
"high performance").
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Function Manual, (FH1), 10/2008, 6SL3097-2AB00-0BP5
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