Which analog signal input can a PLC accept?

A PLC, or Programmable Logic Controller, is designed to interface with various types of input signals, including analog signals. The correct option, which is 4-20 mA, is commonly used in industrial applications for several reasons. This current loop standard offers a clear advantage in terms of noise immunity and the ability to transmit signals over long distances.

The 4-20 mA range is particularly favored because the 4 mA level represents a zero or minimal level of measurement, while the 20 mA indicates the maximum measurement. This allows for an intrinsic failure detection since if the signal drops to 0 mA, it indicates a problem in the signal transmission. Additionally, current signals are less susceptible to voltage drops across long runs of wire, making this option reliable for many PLC applications.

In contrast, the other options present various forms of inputs that are less suitable for direct PLC analog input under standard conditions. For instance, 4-10 mV (AC) and 0-10 mV (DC) both represent very low voltage ranges that are prone to noise interference and may not provide adequate resolution for analog signal processing. Similarly, while 0-10V (AC) could theoretically work in certain conditions, it is typically