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Rules for Failsafe Operation of an S5-95F
Call DQ Test While System is in a Non-Critical State
FB252 allows you to invoke the DQ test at a defined point in time. Select the instant of the call so
that the test will take place while the system is in a non-critical state (for instance because the
outputs are "0" or because the brief actuator drop-out has no effect). Note that the DQ test must be
invoked in the user program at least once every 50 minutes, as the S5-95F will otherwise invoke it
automatically (at a point in time not known to the user).
Connecting Actuators to Diodes and/or RC Networks
By connecting them to diodes or RC networks, you can delay actuator drop-out. The dimensioning
of the required components is dependent on the actuator used. If you have any special questions,
your contact at the Siemens branch office nearest you would be glad to help.
18.10 Response to I/O Errors
In order to increase its availability, the S5-95F does not always respond to I/O errors by going to
STOP. If the inspector is agreeable, you may determine the system response to I/O errors yourself.
Signal group numbers were instated to enable differentiation between I/O errors. These group
numbers make it possible to combine process-related I/Os into a signal group.
One of the responses listed in Table 18-7 is assigned to each signal group in DB1.
Programmable
Response
to I/O Errors
STOP
Passivation
Formation of a
standard value via AND
operation
Formation of a
standard value via OR
operation
Formation of a
standard value through
reading OLD value
18-16
Table 18-7. System Response to I/O Errors
S5-95F Response
System STOP
User program is no longer scanned
S5-95F reads all inputs belonging to the
signal group as having a "0" signal. S5-95F
outputs "0" signal to all outputs belonging to
the signal group (outputs are reset).
S5-95F reads the input signal as "0" and
continues the user program scan with this
value.
S5-95F reads the input signal as "1" and
continues the user program scan with this
value.
S5-95F reads the input signal in its last valid
state and continues the user program scan
with this value.
S5-95F
Failsafe Response
Failsafe shutdown
I/O shutdown
Error report and safety-
oriented response in
user program.
Error report and safety-
oriented response in
user program.
Error report and safety-
oriented response in
user program.
EWA 4NEB 812 6210-02
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