Capacitive Crosstalk Of Digital Input/Output Signals - Siemens SIMATIC S7-1500 System Manual

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.
High-speed actuators may briefly drop out or be activated during this 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
requirement for 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

Capacitive crosstalk of digital input/output signals

Readback errors may occur on the F-DQ modules 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. The module detects a cross circuit/short
circuit and performs a safety-related shutdown.
Remedy:
• Separate cables for fail-safe DI modules and fail-safe DQ modules / non-fail-safe DQ
modules
• Coupling relay or diodes in the outputs
• Disable the short-circuit test of the sensor supply 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.
Solution:
• Spatially disconnect the inductive loads or shield against the magnetic field.
• Configure the "Max. readback time dark test" to 50 ms or higher.
SIMATIC Drive Controller
System Manual, 11/2023, A5E46600094-AD
WARNING
8.2 Additional rules and regulations for operation
Connecting
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