Siemens S7-300 Installation And Operating Manual page 48

Wiring
5.4 Sensor and Actuator Requirements for F-SMs in Safety Mode
Additional requirements for sensors and NAMUR sensors
A "0" value is output to the F-CPU when faults are detected at the fail-safe input modules.
Ensure that the "0" state of the sensors triggers a reliable response in the safety program.
Example: The safety program of an EMERGENCY-OFF sensor must trigger "0" state at the
corresponding actuator (EMERGENCY-OFF button pressed).
The time interval between two signal transitions (pulse duration) must be greater than the
PROFIsafe monitoring time to allow reliable detection of the pulses.
Requirements for the duration of sensor signals for the SM 326; DI 24 x DC 24V
In order to guarantee accurate detection of sensor signals by the SM 326; DI 24 x DC 24V,
you must ensure that the sensor signals have a defined minimum duration.
Reliable detection with the SM 326; DI 24 x DC 24V
The minimum duration of sensor signals required for reliable detection by the
SM 326; DI 24 x DC 24V depends on the short-circuit test parameter assignment in
Table 5- 1
Reliable detection in the safety program of the F-CPU
For information in terms of the reliable detection of sensor signals in the safety program,
refer to the the
48
WARNING
WARNING
Minimum duration of sensor signals for proper detection by an SM 326; DI 24 x DC 24V
Short-circuit test parameter
Deactivated
Activated
Safety Engineering in SIMATIC S7
Minimum duration of sensor signals
25 ms
30 ms
system description.
Installation and Operating Manual, 01/2010, A5E00085586-10
STEP 7
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Fail-safe signal modules