Siemens SIMATIC S7-1500 Manual page 23

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Image 4-2
This parameter allows you to set the time for the readback.
If the expected signals could not be read back correctly after expiration of the readback time,
the output channel is passivated.
No new process values are switched to the output channels while a bit pattern is still active
(switch test is carried out). This means that a higher maximum readback time for the dark
test increases the response time of the F-module.
Through the configured readback time dark test, short-circuits (cross-circuits) to a signal
with a frequency greater than 1/(2 x configured readback time dark test) Hz cannot be
recognized (50:50 sampling ratio).
Short-circuits (cross-circuits) to an output of the same module are recognized.
The parameter also has an effect on the short-circuit detection (cross-circuit) with "1" signal
when the output signal is changed from "1" to "0" with the safety program.
Setting readback time dark test
Because the fault reaction time is extended by the length of the readback time dark test, we
recommend that you set the readback time dark test as low as possible, but high enough
that the output channel is not passivated.
In case of the "Connecting a load per digital output, PP switching" (Page 39) application, see
the warnings and notes in the section describing the application.
To determine the readback time required for your actuator, refer to the diagram in the section
Switching capacitive loads (Page 55).
Digital output module F-DQ 8x24VDC/2A PPM (6ES7526-2BF00-0AB0)
Manual, 01/2016, A5E03858037-AA
Readback (The two P-switches do not switch simultaneously.)
Functional principle of the dark test (PP switching)
WARNING
Parameters/address space
4.2 Explanation of parameters
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