Introduction to Industrial Automation

6.8.1   Memory  Organization

The executable program, the user’s program (the automation program), the states of the I/Os, and 

various data of non-permanent nature are stored in the memory of a PLC. The term “memory orga-

nization” is described as the fragmentation of the memory in various sectors where each one has a 

Read-write

memory

Internal

registers

Arithmetic

logic unit

Program counter

Memory with

“operation system”

Microprocessor

Input/output

modules

Figure 6.30  Typical parts of a microprocessor used in a PLC.

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  Introduction to Industrial Automation

specific aim and corresponding size. Even if every PLC manufacturer has a different organization 

of the memory, the memory of a PLC more or less follows the same general structure. In Figure 

6.31, a typical fragmentation of a PLC memory is displayed in five sectors. The size of every memory 

sector, and thus the overall size of the memory, varies based on the size and characteristics of the 

PLC. For example, the size of the memory is different for a small PLC controlling 128 I/Os from 

a PLC controlling 1024 I/Os. The capacity of the memory, independently of its type, is defined by 

the number of digital bits that it can store. A memory of 1 kB size is able to store 1024 B, which 

means that under the assumption of instructions of 1 B, the memory can store up to 1024 program 

instructions, while if this is an I/O memory, it has the ability to store the state of 8192 discrete I/Os. 

Therefore, the same memory size indicates different issues if it refers to a program storage or to other 

internal units. For this reason, the total memory of a PLC is not such a dominant characteristic, 

such as in the case of PCs, as this memory can be easily extended when needed.

The operating system, stored from the manufacturer in a ROM memory, is forcing the micro-

processor to execute the various operations, such as scanning the inputs, updating the outputs, 

the execution of the user’s program, etc. Subsequently, the process of memory access in the various 

sectors of the RAM will be examined, in accordance with the previous functionalities.

 

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The user is programming a PLC in a language consisting of graphical instructions 

 or 

alphanumeric instructions (e.g., AND Input 1). The instructions of the user’s program, inde-

pendently from their form, are stored in binary form in the memory sector named “automa-

tion program” or “user program”. The operating system (executable program) is forcing the 

microprocessor to execute these instructions one by one and to translate them into equivalent 

Operating system

Input/output status memory

Memory for storage of

temporary system data

Data memory for

timers, counters, auxiliary bits

Memory reserved

for user-program storage

0000

0001

0002

Memory register

ROM

RAM

Memory organization of PLCs

Address