RAM Memory Organization

Introduction

This section describes the RAM (Random Access Memory) size for different areas of the Modicon M218 Logic Controller.

Memory Mapping

The RAM size is 2 Mbytes composed of 2 areas:

o1024 kbytes System Area for Operating System memory

o1024 kbytes Customer Area for dedicated application and configuration data.

Memory containing Persistent and Retain variables is preserved and protected. The Persistent and Retain variables will be retained during power outages or when the controller is powered off.

This table shows the different types of memory areas with their sizes in the Modicon M218 Logic Controller memory:

Area	Element	Size (Bytes)
System Area 1024 kbytes	System Area Mappable Addresses %MW0...%MW59999	120000
	System and Diagnostic variables (%MW60000...%MW60199) This memory is accessible through Modbus requests only. These must be read-only requests.	400
	Dynamic Memory Area: Read Relocation Table (%MW60200...%MW61999) This memory is accessible through Modbus requests only. These must be read-only requests.	7600
	Reserved Memory Area (%MW62000...%MW62199) This memory is accessible through Modbus requests only. These must be read-only requests.
	Dynamic Memory Area: Write Relocation Table (%MW62200...%MW63999) This memory is accessible through Modbus requests only. These can be read or write requests.
	Reserved	920576
Customer Area 1024 kbytes	Variables (including Retain and Persistent variables, see table below)	838860⁽¹⁾
	Application
	Libraries
	Symbols	209716⁽¹⁾
⁽¹⁾ The memory size for variables, applications, libraries, and symbols are customized depending on the usage. It is recommended that variables, applications, and libraries take up to 80% of customer area, and symbols take 20% of the customer area.

1888 bytes Retention RAM
744 bytes	Retain Variables ⁽²⁾
144 bytes	Persistent Variables
1000 bytes	%MW0...%MW499
⁽²⁾ Not all the 744 bytes are available for the customer application because some libraries may use Retain Variables.

This table describes the retain data memory mapping for TM218LDD40DUPHNB:

8000 bytes Retention RAM
3000 bytes	Retain Variables ⁽²⁾
1000 bytes	Persistent Variables
4000 bytes	%MW0...%MW1999
⁽²⁾ Not all the 3000 bytes are available for the customer application because some libraries may use Retain Variables.

System Variables

For more information on System Variables, refer to the M218 PLCSystem Library Guide.

Library Sizes

Library Name	Average Size	Comment
M218 HSC	10 kbytes	Depends on the functions used.
M218 PLCSystem	25 kbytes	Always embedded in the application. The use of the functions does not consume additional memory.
M218 PTOPWM	10 kbytes	Depends on the functions used.
PLC Communication	20 kbytes	Depends on the functions used.
M218 Packaging	10 kbytes	Depends on the functions used.