Siemens SIMATIC S7-1500 Function Manual page 24

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/DP master performs the
cyclic data exchange with the IO devices/DP slaves.
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 base
synchronization, a synchronous operation of the transmission cycle of the PROFINET
devices within a sync domain is achieved.
Isochronous mode
Function Manual, 10/2018, A5E43884988-AA
Configuring isochronous mode
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