Digital Signal:-
Discrete
(digital) signals behave as binary switches, yielding simply an ON or OFF signal
(1 or 0, True or False, respectively).
Examples of digital signals:-Push buttons, limit switches, and photoelectric
sensors are examples of devices providing a discrete signal.
Discrete
signals are sent using either voltage or current, where a specific range is designated as ON
and another as OFF.
For
example, a PLC might use 24 V DC I/O, with values above 22 V DC representing ON,
values below 2VDC representing OFF, and intermediate values
undefined. Initially, PLCs had only digital I/O.
Analog Signal:-
Analog
signals are like volume controls, with a range of values between zero and
full-scale.
These are
typically interpreted as integer values (counts) by the PLC, with various
ranges of accuracy depending on the device and the number of bits available to
store the data.
As PLCs
typically use 16-bit signed binary processors, the integer values are limited
between -32,768 and +32,767.
Examples of analog signal:-Pressure,
temperature, flow, and weight are often represented by analog signals.
Analog
signals can use voltage or current with a magnitude
proportional to the value of the process signal. For example, an analog 0 to
10 V or 4-20 mA input would be converted into an integer
value of 0 to 32767.
Current inputs are less sensitive to electrical noise (e.g.
from welders or electric motor starts) than voltage inputs.