Siemens SIMATIC ET 200AL, System Manual

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Functions
5.7 Isochronous mode
PROFINET with STEP 7
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Function Manual, 05/2021, A5E03444486-AL
Setting the application cycle
The application cycle is a multiple of data cycle T_DC. If the runtime of the isochronous mode
interrupt OB is short, the application cycle can be identical to the data cycle (= send clock).
You can reduce the application cycle of the isochronous mode interrupt OB relative to the
send clock of an isochronous PROFINET IO system. Set an integer multiple of the send clock
as the reducing factor.
You use the factor to reduce the CPU utilization by executing the isochronous mode interrupt
OB less frequently. You set the application cycle in the properties of the isochronous mode
interrupt OB.
Setting the delay time
The delay time is the time between the start of the send clock and the start of the
isochronous mode interrupt OB. During this time, the IO controller performs cyclic data
exchange with the IO devices.
STEP 7 sets the default delay time in such a way that the isochronous update of the process
image partition automatically falls within the execution window of the application cycle.
You set the delay time in the properties of the isochronous mode interrupt OB. A shorter
delay time enables you to increase the processing time for your user program in the
isochronous mode interrupt OB.
Additional configurations for isochronous mode on PROFINET IO:
• Setting IRT as the RT class for the interconnected PROFINET interfaces
A precondition for isochronous operation on PROFINET IO is IRT communication
(Isochronous Real Time Communication). IRT means synchronized data exchange at
reserved time intervals.
• Configuring the topology of the configuration
A precondition for IRT communication is the topology configuration. In addition to the
reserved bandwidth, the exchange of frames on defined transmission paths is used for
further optimization of data communication. For this, the topological information of the
configuration is used for planning the communication.
• Using a sync domain , you assign the IO devices (sync slaves) to an IO controller (sync
master) for the isochronous data exchange.
A precondition for IRT communication is a synchronization cycle for all PROFINET devices
in a sync domain, for distribution of a common time base. With this basic synchronization,
synchronous operation of the transmission cycle of the PROFINET devices within a sync
domain is achieved.