Siemens SIMATIC ET 200AL System Manual page 939

Maximum readback time
The maximum readback time is a user-configured parameter that assigns the maximum time
allowed for an output to reach the new state (ON or OFF) due to a process value change
without generating an error. Also, this is the maximum width of a diagnostic test pulse
applied during the bit pattern test to verify that an output can be turned OFF while it is in the
ON state. The time of the OFF pulse should be as long as possible, but short enough so that
the actuator cannot react.
The "Maximum readback time" parameter allows you to set the maximum time for
verification that the channel's output switch states are as expected after a process value
change.
If the expected output switch states (as reported by P-readback and M-readback signals) are
not reached upon expiration of the "Maximum readback time", the output channel is
passivated.
No new process values are switched to the output channels while a test pattern is applied to
the outputs. This means that a higher "Maximum readback time" increases the response time
of the F-module.
Short-circuit (cross-circuit) detection may not be possible for high frequency signals
Through the configured "Maximum readback time", short-circuits (cross-circuits) to a signal
with a frequency greater than 1/(2 x configured "Maximum readback time") Hz cannot be
detected (50:50 sampling ratio).
You must take this into consideration for your application and respective requirements of
your product standards.
Short-circuits (cross-circuits) to an output of the same module are recognized regardless of
the frequency of the output channel.
You can adjust the maximum readback time to change the frequency of the signal that can
be detected.
The parameter also effects the short-circuit detection (cross-circuit) with a "1" signal when
the output signal is changed from "1" to "0" with the safety program.
Setting the parameter "Maximum readback time"
Because the fault response time is extended by the configured maximum readback time, we
recommend that you set the "Maximum readback time" as low as possible, but high enough
that the output channel is not passivated.
You determine the readback time required for your actuator with the diagram in "Connecting
capacitive loads (Page 86)".
If the capacitance of the actuator is not known, it may be necessary for you to determine the
value for the "Maximum readback time" by trial and error. This may also be necessary due to
the part variances in the actuator or external influences.
Digital I/O module F-DI 4+F-DQ 2x24VDC/2A, 4xM12 (6ES7146-5FF00-0BA0)
Equipment Manual, V1.0, 05/2021, A5E51082342-AA
WARNING
Parameters/address space
4.3 Explanation of parameters
33