Siemens SIMATIC S7 Manual page 26

Missing or Wrong
Licensing
!
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
When the registration key is missing or a wrong one is detected, the SF LED
(S7-300 and IM151-8) or the INTF LED (S7-400) of the CPU is flashing. A
cyclic error message regarding the missing license is displayed in the
diagnostic buffer too. The error number of the missing license is W#16#A090.
The entry in the diagnostic buffer is carried out by means of OB121
"Programming Error".
_____________________________________________________________
Warning
The CPU will turn to STOP mode, if the OB121 is not available.
_____________________________________________________________
Modbus communication is carried out even with a missing or wrong
registration key, but the output STATUS_MODBUS is set to W#16#A090 "no
valid license".
If this error code is shown although the registration key has been inserted,
please check if the FC "EQ_STRING" is copied into the project.
Licensing
5-4