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Digital signals behave as switches, yielding simply an On or Off signal (logical 1 or 0, respectively). These are interpreted as boolean values by the PLC. Pushbuttons, limit switches, and photo-eyes are examples of devices providing a digital signal. Analog signals behave as volume controls, yielding a range of values between On and Off. These are typically interpreted as floating-point values by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. Pressure transducers, scales and gas leak detectors can provide analog signals.
Digital signals generally use either [[voltage]] or [[current]], where a specific range is denominated as On (logical 1) and another as Off (logical 0). A typical PLC might use 24VDC I/O (with 24V representing On and 0V representing Off). Analog signals generally use [[voltage]] or [[current]] as well, but do not have discrete ranges for On or Off. They define a range of valid values, typically the range in which the I/O device operates reliably. Other methods of signal I/O include [[serial communications]] (typically [[RS-232]]), and proprietary networks like Allen-Bradley's Data Highway and Siemens' Profibus.
PLCs have a limited number of connections built in for signals such as digital inputs, digital outputs, analog inputs and analog outputs. Typically, expansions are available if the base model does not have sufficient I/O.
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