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

Wiring
5.2 Additional rules and regulations for operation of the S7-1500/ET 200MP with fail-safe modules
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 (for example, 230 V DC), safe
electrical isolation must be ensured between the outputs of a fail-safe output module and
the components carrying a higher voltage (in accordance with EN 60664-1:2010).
This is generally the case for relays and contactors. Particular attention must be paid to this
issue for semiconductor switching devices.
Technical specifications of s ensors and actuators
Refer to the manuals of the fail-safe modules for technical specifications to assist you in
selecting sensors and actuators.
5.2.3

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.
Caus e: Capacitive c rosstalk
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 F-DI modules, F-DQ modules or 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.
Caus e: magnetic c rosstalk
Note that an inductive load connected to the F-DQ channels can induce capacitive coupling
of a strong magnetic field.
S olution:
● Spatially disconnect the inductive loads or shield against the magnetic field.
● Configure the "Max. readback time dark test" to 50 ms or higher.
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Automation system
System Manual, 09/2016, A5E03461182-AD