Siemens SIMATIC S7 F Manual page 208

Fail-Safe Blocks
Substitute Value/Keep Last Value
In the case of an invalid non-linearized value as a result of a communication error
(PROFIsafe), channel fault, overflow/underflow or violation of channel fault limits
and in the case of passivation, depending on the parameter assignment (input
parameter SUBS_ON), a substitute value or the last valid value is output, and the
output QBAD = 1 is set. During a startup (cold or warm restart), there is no last
valid value yet available, and, regardless of the parameter assignment, the
substitute value configured at the input SUBS_V is output.
If the output of the substitute value or the last valid value is not caused by
passivation, the output PASS_OUT = 1 is set additionally to passivate other
channels.
When the input parameter SUBS_ON = 0, the last valid value of V is output with
the quality code (QUALITY) 16#44.
When the input parameter SUBS_ON = 1, the substitute value SUBS_V is output
with the quality code (QUALITY) 16#48, and the output QSUBS = 1 is set.
Startup Characteristics
After a startup (cold restart or warm restart), communication must first be
established between the F module driver and the analog input module. In this time,
regardless of the parameter assignment at the input SUBS_ON, the substitute
value SUBS_V is output with the quality code (QUALITY) 16#48, and the outputs
QBAD = 1, QSUBS = 1 and PASS_OUT = 1 are set.
Error Handling
If the value for measurement range and measurement type (MODE) is invalid, an
invalid non-linearized value is assumed.
In the event of an error that is critical to safety, the system function SFC F_CTRL
is called. This records the event in the Diagnostic Buffer and requests a switch to
the reserve CPU if the error occurred only on the master CPU. For non-redundant
systems or a common-cause error occurring in both CPUs, the shutdown logic can
be configured to either disable the erred F-run-time group or the entire Safety
Program.
Error Information in Diagnostic Buffer
Error Code (W#16#...)
75DAH
75D9H
8-20
Description
Error in the safety data format (error due to online modification
of the Safety Program or internal CPU fault)
Invalid REAL number (DATA component)
Fail-Safe Systems
A5E00085588-03