CPU

The central processor unit (CPU) is a microprocessor system
that contains the system memory and is the PLC decisionmaking
unit. The CPU monitors the inputs and makes decisions
based on instructions held in the program memory. The
CPU performs relay, counting, timing, data comparison, and
sequential operations.

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Analog Outputs

An analog output is a continuous, variable signal. The output
may be as simple as a 0-10 VDC level that drives an analog
meter. Examples of analog meter outputs are speed, weight,
and temperature. The output signal may also be used on more
complex applications such as a current-to-pneumatic transducer
that controls an air-operated flow-control valve.

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Analog Inputs

An analog input is a continuous, variable signal. Typical analog
inputs may vary from 0 to 20 milli amps, 4 to 20 milli amps,
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.

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.

Actuators

Actuators convert an electrical signal from the PLC into a
physical condition. Actuators are connected to the PLC output.
A motor starter is one example of an actuator that is connected
to the PLC output. Depending on the output PLC signal the
motor starter will either start or stop the motor.

Sensor

A sensor is a device that converts a physical condition into an
electrical signal for use by the PLC. Sensors are connected to
the input of a PLC. A pushbutton is one example of a sensor
that is connected to the PLC input. An electrical signal is sent
from the pushbutton to the PLC indicating the condition (open/
closed) of the pushbutton contacts.

Terminology

The language of PLCs consists of a commonly used set
of terms; many of which are unique to PLCs. In order to
understand the ideas and concepts of PLCs, an understanding
of these terms is necessary.

Sensor
Actuators
Discrete Input
Analog Inputs
Discrete Outputs
Analog Outputs
CPU
Programming
Ladder Logic
Ladder Logic Diagram
Statement list
Function Block Diagrams
PLC Scan
Software
Hardware
Memory Size
RAM
ROM
EPROM
Firmware

Hexadecimal

Hexadecimal is another system used in PLCs. The hexadecimal
system has the following characteristics:

16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

Base 16

Weights Powers of base 16 (1, 16, 256, 4096 ...)

The ten digits of the decimal system are used for the first ten
digits of the hexadecimal system. The first six letters of the
alphabet are used for the remaining six digits.

A = 10 D = 13
B = 11 E = 14
C = 12 F = 15

The hexadecimal system is used in PLCs because it allows the
status of a large number of binary bits to be represented in a
small space such as on a computer screen or programming
device display. Each hexadecimal digit represents the exact
status of four binary bits. To convert a decimal number to a
hexadecimal number the decimal number is divided by the base
of 16.
To convert decimal 28, for example, to hexadecimal:

1r 12
16)-----
28

Decimal 28 divided by 16 is 1 with a remainder of 12. Twelve is
equivalent to C in hexadecimal. The hexadecimal equivalent of
decimal 28 is 1C.

The decimal value of a hexadecimal number is obtained by
multiplying the individual hexadecimal digits by the base 16
weight and then adding the results. In the following example
the hexadecimal number 2B is converted to its decimal
equivalent of 43.

160 = 1
161 = 16
B = 11

BCD

Binary-Coded Decimal (BCD) are decimal numbers where
each digit is represented by a four-bit binary number. BCD is
commonly used with input and output devices. A thumbwheel
switch is one example of an input device that uses BCD. The
binary numbers are broken into groups of four bits, each group
representing a decimal equivalent. A four-digit thumbwheel
switch, like the one shown here, would control 16 (4 x 4) PLC
inputs.

Logic 0, Logic 1

Programmable controllers can only understand a signal that
is On or Off (present or not present). The binary system is a
system in which there are only two numbers, 1 and 0. Binary 1
indicates that a signal is present, or the switch is On. Binary 0
indicates that the signal is not present, or the switch is Off.