Siemens SIMATIC ET 200AL, System Manual

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Functions
5.1 Isochronous mode
PROFIBUS with STEP 7 V13
Function Manual, 12/2014, A5E03775446-AC
65
5.1.4 Sequence of synchronization
From reading of input data to output of output data
The sequence of all components involved in the synchronization is explained in the
paragraphs below:
●
Isochronous reading of input data
● Transport of input data by means of the PROFIBUS subnet to the DP master (CPU)
● Further processing in the isochronous application of the CPU
● Transport of output data by means of the PROFIBUS subnet to the output DP slave
● Isochronous output of output data
T_DC Date cycle
T
I
Time for reading the input data
T
O
Time for output of output data
Figure 5-3 Time sequence of synchronization
To ensure that all input data is ready for transportation via the PROFIBUS DP line when the
next PROFIBUS DP cycle begins, the I/O read cycle has a lead time T
I
so that it starts
earlier. The T
I
is the "flash gun" for all inputs. This T
I
is necessary to compensate for analog
to digital conversion, backplane bus times, etc. The lead time T
I
can be configured by
STEP 7 or by the user. We recommend that you have T
I
assigned automatically by STEP 7 .
The PROFIBUS DP line transports the input data to the DP master. The synchronous cycle
interrupt OB SynchronousCycle is called. The user program in the synchronous cycle
interrupt OB decides the process reaction and provides the output data in time for the start of
the next data cycle. The length of the data cycle is always configured by the user.