Siemens S7-1200 System Manual page 586

Communication
10.4 AS-i
In the "Set AS-i address" field, you can change the AS-i slave address. A new slave that has
not been assigned an address always has address 0. It is detected by the master as a new
slave without an address assignment and is not included in normal communication until
assigned an address.
Configuration error
When the yellow "CER" LED is ON, there is an error in the AS-i slave device configuration.
Select the "ACTUAL > EXPECTED" button to overwrite the AS-i master CM1243-2 module
slave device configuration with the AS-i field network slave device configuration.
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S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG