Siemens SIMATIC ET 200AL System Manual page 40

Connecting
5.2 Additional rules and regulations for the operation of the ET 200AL with fail-safe modules
5.2.3 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
Note that an inductive load connected to the F-DQ channels can induce coupling of a strong
magnetic field.
Remedy:
• Separate the inductive loads spatially or shield against the magnetic field.
• Configure the readback time to 50 ms or higher.
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Distributed I/O system
System Manual, 08/2021, A5E31861578-AJ