Siemens SIMATIC ET 200M Operating Instructions Manual page 138

Functions
8.2 Time stamping the input signals with IM 153-2
Time stamping of signal changes in an S7-400H
Both IM 153-2 save the messages of the time-stamped signals. This way, after a changeover
process from active to passive IM 153-2 the "new" active IM 153-2 prepares the current
messages for further processing.
Note
During the changeover between the two IM 153-2 the signal changes are not time stamped.
This time is communicated by the special message "Changeover with redundant BEGIN /
END".
You will find more information on time stamping in redundant systems in the "PCS 7
documentation (http://support.automation.siemens.com/WW/view/en/10806846/130000)"
documentation.
Figure 8-7
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138
Example structure with 2 x IM 153-2 for redundancy in an H system
WinCC Operator station (OS) for unit visualization
Industrial Ethernet
Time master (SICLOCK)
ET 200M distributed I/O with 2 x IM 153-2
redundant DP master system
S7-400H automation system
Optional: RS 485 repeater
Operating Instructions, 04/2022, EWA-4NEB7806006-09
ET 200M