Siemens SIMATIC NET CP 243-2 Manual page 66

Signaling Errors and Diagnostics in the User Program
4.1 Signaling
Error Signaling
If the CP 243-2 recognizes errors on the AS-Interface (AS-i slave failure, AS-i
Power Failure) during operation, it signals these errors by resetting the input data
of the affected slave and by setting the corresponding bit in the error register in the
SM area (Special Memory).
The SM byte addresses depend on the slot of the CP 243-2.
Caution
!
Note that the operating system of the S7-200 CPU does not always update the
error register in the SM area before a program cycle. For this reason, it is possible
that the input data of a binary AS-i slave are set to '0' although no error is signaled
in the error register.
If you require a consistent view of the input data, error bits and the delta list for
programming your system, you can achieve this using the "Read Data and Delta
List" command (see Section 5.2.32).
4-2
SIMATIC NET CP 243-2 AS-i Master
C79000-G8976-C142/02