Siemens SIMATIC ET 200AL System Manual page 62

Connecting
8.2 Additional rules and regulations for the operation of the ET 200AL with fail-safe modules
The minimum duration of sensor signals for F-modules with inputs depends on the
configured input delay, the parameters of the short circuit test of the sensor supplies, and the
configured discrepancy behavior for 1oo2 evaluation. The signal must be greater than the
maximum response time of the configured application. Information on calculating the
maximum response time can be found in the section "Response times" of the relevant
F-module.
The maximum permitted switching frequency of the sensor signals results from the minimum
duration.
Additional requirements for actuators
The fail-safe output modules test the outputs at regular intervals. The F-module briefly
switches off the activated outputs and, if necessary, switches on the deactivated outputs. You
can assign the maximum duration of the test pulses (dark and light period) with parameters.
Fast reacting actuators may briefly drop out or be activated during the test. If your process
does not tolerate this, set the pulse duration of the light or dark test correspondingly or use
actuators that have sufficient lag.
If the actuators switch voltages greater than 24 V DC (e.g. 230 V AC), the outputs of a fail-
safe output module and the parts carrying a higher voltage must be electrically isolated (in
accordance with IEC 60664-1).
This is generally the case for relays and contactors. Particular attention must be paid to this
with semiconductor switching devices.
Technical specifications of sensors and actuators
Refer to the manuals of the fail-safe modules for technical specifications to assist you in
selecting sensors and actuators.
8.2.3 Crosstalk of digital input/output signals
Readback errors may occur on the F-DQ channels if the fail-safe digital output signals and fail-
safe digital input signals are routed through a single cable.
Cause: capacitive crosstalk
During the bit pattern test of the outputs or the sensor supply of the inputs, the steep
switching edge of the output drivers caused by the coupling capacitance of the line may
result in crosstalk to other non-activated output or input channels. This may then lead to a
response of the readback circuit in these channels. A cross circuit/short-circuit is detected,
which leads to safety-related tripping.
Remedy:
• Separate cables for fail-safe DQ modules and non-fail-safe DQ modules
• Separate cables for F-DQ channel and F-DI channels
• Coupling relay or diodes in the outputs
• Disable the sensor supply test if safety class requirements allow it.
Cause: magnetic crosstalk
62
WARNING
Distributed I/O system
System Manual, 12/2023, A5E31861578-AL