Discrete Input

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.




A Normally Open (NO) pushbutton is used in the following
example. One side of the pushbutton is connected to the first
PLC input. The other side of the pushbutton is connected to an
internal 24 VDC power supply. Many PLCs require a separate
power supply to power the inputs. In the open state, no voltage
is present at the PLC input. This is the OFF condition. When the
pushbutton is depressed, 24 VDC is applied to the PLC input.
This is the ON condition.