Siemens SIMATIC ET 200AL System Manual page 2146

Because the sub-cycles have to be within a send clock, oversampling needs an additional
send clock to copy the data to the IM, unlike the 3-cycle model of isochronous mode. The
result is a 5-cycle model.
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Figure 8-2
Due to the configured modules, the send clock of an IO device has a minimum possible
update time, which means the time interval within which an IO device/IO controller of the
PROFINET IO system is supplied with new data. Modules with oversampling functionality,
however, offer the option of further reducing the update time for their channels without
having to shorten the send clock for the entire S7 station at the same time. The subdivision
of the send clock into time-equidistant sub-cycles therefore enables an even faster signal
processing speed.
Example
In practice, the use of oversampling makes sense when the isochronous system works with
only one specific specific send clock (for example, 1 ms) due to the modules used and when
faster sampling of the process values is required. By using oversampling and a subdivision
of the send clock into 4 sub-cycles, for example, you can sample the process values in
intervals of 250 µs.
Configuring oversampling
Enable the option "Isochronous mode" in the IO device used and set the corresponding
parameters ("Send clock", etc.). You define the number of sub-cycles in the relevant
distributed analog module with the "Sampling rate" parameter for analog input modules and
the "Output rate" parameter for analog output modules. If, for example, you configure a
"Sampling rate" of 4 "Values/cycle" for a "Send clock" of 1 ms, the send clock is subdivided
into 4 sub-cycles and the process values are sampled in intervals of 250 µs.
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
Send clock is divided into sub-cycles which record the measured value
Measured values are copied to the IM
Measured values are processed and output values determined
Output values are copied by the CPU to the IM
Output values are written to the process
5-cycle model
High-speed analog modules
8.1 Basics
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