An analog
input is an input signal that has a continuous signal. Typical analog inputs
may vary from 0 to 20 milliamps, 4 to 20 milliamps, or 0 to 10 volts. In the
following example, a level transmitter monitors the level of liquid in a tank.
Depending on the level transmitter, the signal to the PLC can either increase
or decrease as the level increases or decreases.
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A
discrete input, also referred to as a digital input, is an input that is either
in an ON or OFF condition. Pushbuttons, toggle switches, limit switches,
proximity switches, and contact closures are examples of discrete sensors which
are connected to the PLCs discrete or digital inputs. In the ON condition a
discrete input may be referred to as a logic 1 or a logic high. In the OFF
condition a discrete input may be referred to as a logic 0 or a logic low.
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