Siemens SIMATIC ET 200M Operating Instructions Manual page 170

Functions
8.6 Isochrone mode
8.6 Isochrone mode
8.6.1 What is Isochronous?
Basics
Reproducible (e.g. equidistant) response times will be realized in the SIMATIC with an
equidistant DP bus cycle, the synchronization of the user program to the DP bus cycle and
the isochrone transmission of the input / output data to the I/O modules. The isochrone parts
of the user program will be worked off via the synchronous cycle interrupts (OB 61 to OB 64)
to the DP bus cycle. The input / output data will be transferred at defined and isochronic
intervals via the backplane bus of the DP slave and to the I/O modules and then switched
through isochronically up to the "terminal".
This means that in the isochrone mode, the synchronization of all up until now freely running
individual cycles is carried out, from the user program in the CPU via the DP cycle on the
PROFIBUS subnet, the cycle in the DP slave right up to the cycle in the I/O modules of the
DP slaves.
Properties
● In equidistant mode, during start up the transition in the data exchange can be delayed by
● The maximum jitter amounts to 5 μs, if the equidistance cycle of the DP master (clock on
● In the case of asynchronous events (e.g. inserting and removing, diagnoses and
● Nothing can be gained from switching off the response monitoring in isochrone mode.
Note
The output of the IM 153-2 to the I/O modules or the input of the I/O modules to the
IM 153-2 takes place in isochrone mode only without active bus modules.
With active bus modules, there is a time offset depending on the slot used by the I/O
module in the ET 200M.
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up to 150 DP cycles compared with operation without the isochrone mode.
the PROFIBUS DP) does not exceed a jitter of 1.3 μs.
The jitter of the I/O modules of the ET 200M cannot be taken into consideration due to
the existing variety.
interrupts of the I/O modules) during a cycle the response of the IM 153-2 to the I/O can
fail or a greater jitter can occur. It is possible to eliminate the influence of asynchronous
events on the jitter by configuring "Back-up periods".
In order to work with diagnoses and interrupts without cycle failure, the equidistance cycle
Tdp must be increased by at least 0.5 ms. Every further increase of the equidistance
cycle improves the performance of asynchronous events. As a general guideline for each
asynchronous event that is to be processed simultaneously (diagnosis, interrupt, data
record) the equidistance cycle must be increased by in each case 0.5 ms.
After cycle failure, the isochrone mode is automatically reset.
Operating Instructions, 12/2008, EWA-4NEB780600602-08
ET 200M