What are the common input signal types for PLCs?

Common PLC input types: Digital - 24 VDC (most common), 120 VAC, 230 VAC, dry contact. Analog - 4-20 mA (standard process signal), 0-10 VDC, 0-5 VDC, thermocouple (various types), RTD (2, 3, 4 wire), resistance. Input modules are sourcing (provide current) or sinking (receive current), and must match field device type. Consider isolation requirements, high-speed inputs for counters/encoders, and appropriate resolution for analog signals. Intrinsically safe modules are used for hazardous areas.