Siemens SIMATIC CPU 410 System Manual page 191

Rules for CiR
The new DP slave must be assigned a higher station number than all previously configured
DP Slaves.
Since the total made up of the station number of the added DP slave and the number of
slaves that can be added can be at most 125, we recommend that the station number for the
added DP slave be selected as follows:
Station number of the added DP slave = the highest station number of all previously
configured slaves + 1.
If you select a higher number for the added DP slave, then under unfavorable
circumstances, the guaranteed/maximum number of slaves that can still be added will be
decreased by more than 1. This is explained in the following example:
Let's assume that the highest station number of all previously configured slaves is 115 and
that the number of slaves that can be added is at most 10. If you assign station number 118
to the added slave, the maximum number of slaves that can still be added will be 7.
Adding PA slaves (field device) to an existing PA master system
In your configuration, the addition of a PA slave downstream of an existing DP/PA link
corresponds with the insertion of a module into a modular slave.
CPU 410 Process Automation/CPU 410 SMART
System Manual, 05/2017, A5E31622160-AC
11.10 Notes on Reconfiguration in RUN Mode Depending on the I/O
Plant changes in RUN - CiR
191