Digital vs. analog signals

Digital or Discrete signals behave as switches, yielding simply an On or Off signal (1 or
0, True or False, respectively). Pushbuttons, limit switches, and photo-eyes are examples
of devices providing a discrete signal. Discrete signals are judged using either voltage or
current, where a specific range is denominated as On and another as Off. A PLC might
use 24 V DC I/O, with values above 22 V DC representing On and values below 2VDC
representing Off. Initially, PLCs had only discrete I/O.



Analog signals are like volume controls, with a range of values between zero and fullscale.
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. Pressure, temperature and weight are often analog signals. Analog signals can use
voltage or current, but do not have discrete ranges for On or Off. Instead they work in a
defined range of values that are reliable for a particular device. Typically an analog 4-20
mA or 0 - 10 V signal would be converted into an integer value of 0 - 32767. Current
inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than
voltage inputs.


Example


As an example, say the facility needs to store water in a tank. The water is drawn from
the tank by another system, as needed and our example system must manage the water
level in the tank.


Using only digital signals, the PLC has two digital inputs from float switches (tank empty
and tank full). The PLC uses a digital output to open and close the inlet valve into the
tank.


If both float switches are off (down) or only the ‘tank empty’ switch is on, the PLC will
open the valve to let more water in. If only the ‘tank full’ switch is on, the valve turns off.
Both switches being on would signal that something is wrong with one of the switches, as
the tank cannot be both full and empty at the same time. Two float switches are used to
prevent a ‘flutter’ condition where any water usage activates the pump for a very short
time causing the system to wear out faster.