Are the DI cards of PLCs/DCSs by default passive contacts for input?
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Are the DI cards of PLCs/DCSs by default passive contacts for input? There’s no need to connect an external 24V power supply, right?
The DI cards of both PLCs and DCSs use passive contacts as the default input. This means they can receive external switch signals and convert them into digital signals for processing, without the need to connect an external 24V power supply. .
The DI cards of DCS are generally set to passive input by default; no external 24VDC supply is required, as the DI cards themselves provide the 24VDC power for the DI circuits. Some DI inputs of the PLC require an additional 24VDC supply.
Not necessarily; there are also active signals!
I would like to ask under what circumstances an active signal appears
The signals of the DI module can be active or passive; they can be connected to the corresponding terminals of the module as needed. As for whether they are active or passive, that isn’t important