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There are a lot of
different types of signals that PLCs can read and write. The two most
basic signals are discrete (digital) and analog. Discrete means on or
off, 1 or zero, high or low, etc. Two possible states. Using discrete
signals you could have a switch that when pressed starts a motor
running. Both the switch “input” and the motor start “output” would
be discrete signals. Analog signals are continuous signals varying
between two limits. Analog signals can have a multitude of different
values between those two limits such as pressure, temperature, level,
etc.
Early PLCs were
designed primarily as relay replacements. Those PLCs were
designed completely around discrete applications. They were a
fantastic improvement over the rooms of relays that were used to
operate what would be considered now simple sequences. You can
imagine the improvement it would be to be able to add a new discrete
input and output into your program as opposed to having to wire a new
relay into an existing system.
A single discrete
signal is also referred to as a "bit". Although they sound very
simple, bits can be used in a lot of different ways. For example, by
turning a bit on and off you can generate pulses. By using a bit going
into a counter you can count the number of times the bit turns on. For
example, if you have a motor and each revolution of the motor moves
the crane one foot, and you need to move the crane six feet, then you
would simply count six pulses. You can also have a bit go into a
timer. So if the crane moves one foot per second, then the "crane is
currently moving" bit starts a timer that in six seconds tells the PLC
that the crane moved six feet. Typically however, discrete signals
are used for simple applications. Starting a motor or opening and
closing valves. Not glamorous, but very useful
A single bit can
have two states – one or zero. Two bits can have four states (00, 01,
10, and 11). Eight bits is known as a byte and can represent 256
states or the numbers from 0 to 255. This is how computers represent
numbers and values – by cascading bits. The PLC is no different.
Analog signals
involve detecting different levels of a signal. For example, how fast
you want to run a motor. With analog signals, the user can turn a
potentiometer that generates a varying voltage or current (analog
input) that tells the PLC to send an analog output to the motor
indicating the speed that the motor should run. Technically you would
need something like a motor inverter between the PLC analog output and
the motor to essentially amplify the PLC analog output to control the
motor voltage or frequency and vary the motor speed.
You can represent
the position of the potentiometer using 12 bits (0 to 4095). Zero
voltage (or current) would be represented by 0 in the PLC and 4095
would represent the maximum voltage (or current). Theoretically
(assuming no noise, perfect linearity, and a few other things), the
PLC can run the motor at one of 4096 different speeds. Analog signals
can be used to bring many types of continuous signals into the PLC –
temperature, pressure, level, position, etc. Once in the PLC memory,
these signals can be manipulated to control motors, valves, and other
field devices.
Let me reemphasize
that the function of a PLC is to read these discrete and analog
inputs, run some logic based on those inputs, and then write discrete
and analog outputs to control the available environment.
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