Configuration Options with the ET 200M
3-15
Distributed I/O Device ET 200M
EWA-4NEB780600602-06
3.7
Clock Synchronism
Reproducible response times (i.e. response times with identical length) are
implemented in the SIMATIC using the equidistant DP bus cycle, synchronization
of the user program with the DP bus cycle, and clock-synchronized transmission of
the I/O data to the I/O modules. The clock-synchronized components of the user
program are processed using the clock-synchronized interrupts (OB 61 to OB 64)
synchronously to the DP bus cycle. The I/O data is transferred to the I/O modules
via the backplane bus of the DP slave at defined and constant
(clock-synchronized) intervals and switched through to the ”terminal” with clock
synchronism.
Clock synchronism therefore results in the synchronization of all free-running single
cycles (from the user program in the CPU, through the DP cycle on the
PROFIBUS subnetwork and right up to the cycle in the I/O modules of the DP
slaves).
The maximum jitter is 10
µ
s.
Features
In equidistance mode, the transition into data exchange at startup can be
delayed by up to 150 DP cycles compared to the mode without clock
synchronism.
With asynchronous events (e.g. removal and insertion, diagnoses and interrupts
of I/O modules), the response of the IM 153-2 to the I/Os can fail for one clock
cycle.
To be able to process diagnoses and interrupts without clock cycle failure, the
interval between Ti and To must be increased to at least 500
µ
s. Increasing the
interval beyond this minimum further improves the performance of
asynchronous events. As a general guideline, you should increase the interval
by 500
µ
s for every asynchronous event which is be processed simultaneously
(diagnosis, interrupt, data record).
The clock synchronism adjusts automatically following a failure.
Deactivating the watchdog has no effect with clock synchronism.
Note
Output of the IM 153-2 at the I/O modules and input of the I/O modules at the
IM 153-2 are only performed with clock synchronism if no bus modules are active.
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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