Siemens SIMATIC PCS 7 Operating Manual page 57

Maximum dark test readback time
Dark periods arise during deactivation tests and the complete bit pattern test. In this process,
a test signal is connected to the output from the fail-safe output module while the output is
active ("1" output signal). This deactivates the output briefly (= "dark period"). A sufficiently
inactive actuator does not respond to this and remains activated. Each output channel has
its own configurable maximum readback time for the dark test.
The maximum readback time should be set sufficiently long enough if the affected channel
activates large capacitive loads. If the maximum readback time is set too short for a
connected capacitive load, the output channel is passivated, because discharge does not
occur within the parameterized time.
Configure these parameters so that the module reads back correctly and your actuator still
does not react to the switch-off pulse.
Set readback times:
1. If an output channel is continuously being passivated with an intact F-SM, this can be in
contact with a capacitive component of the controlled load that is too large.
In such an instance, configure the readback time to the maximum value of 400 ms. If
passivation of the channel continues to occur, either an external fault is present or the
connected capacity lies outside of the permitted range.
2. Since the fault response time is extended by the readback time, the readback time should
be set as short as possible when testing (but long enough so the output channel is not
passivated).
3. If you have set a readback time which leads to the output channel becoming passivated
sporadically, set the next highest value of the maximum dark test readback time. Dark
periods arise during deactivation tests and the complete bit pattern test. In this process, a
test signal is connected to the output from the fail-safe output module while the output is
active ("1" output signal).
Redundant interconnection
With this parameter you set whether this channel is operated redundantly.
Note
If you do not activate this channel with a redundant interconnection the "Short circuit of
output to L+ or output driver defect" diagnosis is reported.
Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB
Configuring S7F/FH hardware
6.4 Setting system parameters for F-signal modules
57