Crosstalk Of Digital Input/Output Signals - Siemens SIMATIC ET 200SP System Manual

6.2 Additional rules and regulations for the operation of the ET 200SP with fail-safe modules
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.
6.2.3

Crosstalk of digital input/output signals

When fail-safe digital output and input signals are in a single cable, F-DQ modules and F-
PM-E modules may experience readback errors.
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 F-DI modules, F-DQ modules, and F-PM-E modules or non-fail-safe
DQ modules
● Separate cables for F-DQ channel and F-DI channels for the F-PM-E module
● 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.
Distributed I/O system
System Manual, 09/2019, A5E03576849-AJ
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