Why 24VDC digital input is widely used in PLCs?

24VDC is just robust enough to overcome any long distance signal wire v-drop to carry a 4-20ma signal of accurate meaningful value. Likewise it is robust enough to power smart transmitters at a distance such as those that provide modbus communication. And there remains the option to insert a low watt resistor across the input card input terminals to convert the 4-20ma signal to a voltage signal such as 1-5 vdc if required. And the 24 vdc power is safe for live power work and is manageable for inserting IS barriers if needed for entering power loops into class 1 Zone 1 and 2 hazardous areas…then there is safety…finally there is IEC, IEEE, ISA standards and specs that take the lead on standardizing the 24 Vdc loops.

Another practical reason for using 24 VDC inputs over 120 VAC is for resolution of Sequence of Events Alarms. When a unit or process controlled by the PLC such as a boiler trips because of some process upset such as low water level or high fuel pressure, etc. , the 24 VDC inputs provide faster time resolution of which fault might have been the real initiator of the unit trip by avoiding the potential up to 16 millisecond delay associated with AC voltage inputs falling from peak voltage to zero

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