24VDC digital input is widely used in PLCs (Programmable Logic Controllers) for several reasons, primarily related to safety, compatibility, and industrial standards. Here are the main reasons why 24VDC digital input is prevalent in PLC applications:

1. Safety : 24VDC is considered a safe voltage level for industrial applications. It provides enough voltage to power sensors, switches, and other input devices without posing significant electrical hazards to personnel. Higher voltages could be dangerous if accidental contact occurs.

2. Compatibility : Many industrial sensors and switches are designed to operate with 24VDC power supply. Using 24VDC digital input ensures compatibility with a wide range of standard industrial devices, reducing the need for additional voltage conversion or customization.

3. Industrial Standards : The use of 24VDC digital input has become an industry standard over time. It allows for consistency and compatibility between different manufacturers' products and different PLC systems.

4. Noise Immunity : 24VDC digital input signals are relatively immune to electrical noise and interference commonly found in industrial environments. This makes the signals more stable and reliable, improving the accuracy of the PLC system.

5. Cost-Effectiveness : Implementing 24VDC digital input is often more cost-effective compared to higher voltage levels. The components required for 24VDC systems, such as sensors and input modules, are readily available and competitively priced.

6. Low Power Consumption : Operating at 24VDC results in lower power consumption compared to higher voltage systems, which can be beneficial in energy-efficient applications.

7. Control Logic Compatibility : PLC programming languages and control logic are typically designed with 24VDC digital input in mind. Using 24VDC aligns well with the binary logic used in PLC programming.

8. Fieldbus Compatibility : Many fieldbus protocols commonly used in industrial automation, such as Profibus, Profinet, and DeviceNet, are designed to work with 24VDC digital signals.

9. Scalability : 24VDC digital input allows for easy expansion and addition of input devices, as it can handle a large number of inputs with appropriate expansion modules.

Overall, the use of 24VDC digital input in PLCs offers a balanced combination of safety, compatibility, noise immunity, and cost-effectiveness, making it a popular choice in industrial automation and control applications.