Micrologix 100

MODULE

ARCHITECTURE

ONE

To understand the Allen-Bradley MicroLogix 1000, you need to start
with the basics. This first module explains the basic operation of
programmable controllers and details the specific features of the
MicroLogix 1000 PLC.

AND

This first module is broken up into four sections:
1. MicroLogix 1000 basic principles of operation

1
OPERATION

2.MicroLogix specifications
3. I/O structure and memory system
4. Addressing notation

®

Ke y Po i n t s

After finishing this module, you will:
s understand the basic operating principles of the MicroLogix—
how it works, what the components are, and what the
components do
s know the three basic specifications for the MicroLogix 1000—
how the program is written, how data is represented in thesystem, and what configurations are available
s grasp the MicroLogix’s intricate memory system—how the
I/O is set up, what makes up the memory system, and how the
memory system is organized
s understand the unique MicroLogix addressing notation

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1-1 MicroLogix 1000 Basic Principles of Operation
The MicroLogix 1000 programmable logiccontroller may appear to be like any other PLC, but it has special features, specifications, and capabilities that make it a unique tool for implementing process or machine control. The MicroLogix 1000 follows many of the same basic principles of operation that all
PLCs follow. At the end of this section, you will know:
• what PLCs do
Measures

• why PLCs are invaluable to industrial facilities
•what makes up a PLC
• how a PLC operates

PLC Fundamentals
A MicroLogix 1000 is a programmable logic controller—an
industrial computer that controls a machine or process. A PLC
interfaces with the field input and output devices that are part of
a control application. Then, through the control program stored
in its memory, the PLC uses the data supplied by the input
devices to manipulate orcontrol the output devices. The overall
PLC process, which is shown in Figure 1-1, is very simple. A
PLC measures or senses signals coming from a machine or process. Then, through its internal program, the PLC provides control back to the machine or process.

Inputs
MicroLogix
1000
Outputs

Process
or
Machine

Control
Figure 1-1. PLC operation.

Programmable logic controllersprovide many benefits over traditional electromechanical control systems. One of the best benefits is that PLCs make it easier and less costly to change a
control system. They eliminate the need to rewire the input and
output devices if the control requirements change. If the control
requirements for a PLC application change, all you need to do is
2

Module 1

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change the control program. Another benefit of PLCs is that
they are more powerful and more accurate than electromechanical systems.

PLC
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CPU

PLC Components
A PLC is made up of two basic components (see Figure 1-2):
• the input/output (I/O) system
• the central processing unit (CPU)

I/O System
Figure 1-2. A PLC and itscomponents: the central processing unit (CPU) and the
input/output (I/O) system.

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(a)

To PLC

From PLC

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Input/Output System. The input/output system is made up of
two components, the input interface and the output interface
(see Figure 1-3).
An input interface is a bank of terminals that physically connects input devices, like push buttons andlimit switches, to a
PLC. These input devices provide data to the PLC. The role of
an input interface is to translate data from the inputs into a form
that the PLC’s central processing unit can understand.

M

(b)

Figure 1-3. (a) A PLC’s input interface interprets the data from the input devices
and then sends it to the CPU. (b) A PLC’s output interface interprets
the data from the CPU...