This section describes the RAM (Random Access Memory) size for different areas of the Modicon M251 Logic Controller.
The RAM size is 64 Mbytes.
The RAM is composed of 2 areas:
odedicated application memory
oOS memory
This table describes the dedicated application memory:
|
Area |
Element |
Size |
|---|---|---|
|
System area 192 Kbytes |
System Area Mappable Addresses %MW0...%MW59999 |
128 Kbytes |
|
System and diagnostic variables (%MW60000...%MW60199) This memory is accessible through Modbus requests only. These must be read-only requests. |
||
|
Dynamic Memory Area: Read Relocation Table (%MW60200...%MW61999) This memory is accessible through Modbus requests only. These must be read-only requests. |
||
|
System and diagnostic variables (%MW62000...%MW62199) This memory is accessible through Modbus requests only. These can be read or write requests. |
||
|
Dynamic Memory Area: Write Relocation Table (%MW62200...%MW63999) This memory is accessible through Modbus requests only. These can be read or write requests. |
||
|
%MW64000…%MW65535 Reserved |
||
|
64 Kbytes |
||
|
User area 8 Mbytes |
Symbols |
Dynamic allocation |
|
Variables |
||
|
Application |
||
|
Libraries |
System and Diagnostic Variables
|
Variables |
Description |
|---|---|
|
PLC_R |
Structure of controller read-only system variables. |
|
PLC_W |
Structure of controller read/write system variables. |
|
ETH_R |
Structure of Ethernet read-only system variables. |
|
ETH_W |
Structure of Ethernet read/write system variables. |
|
PROFIBUS_R |
Structure of PROFIBUS DP read-only system variables. |
|
SERIAL_R |
Structure of Serial Lines read-only system variables. |
|
SERIAL_W |
Structure of Serial Lines read/write system variables. |
|
TM3_MODULE_R |
Structure of TM3 modules read-only system variables. |
For more information on system and diagnostic variables, refer to M251 PLCSystem Library Guide.
This table describes the memory addressing for the address sizes Double Word (%MD), Word (%MW), Byte (%MB), and Bit (%MX):
|
Double Words |
Words |
Bytes |
Bits |
||
|---|---|---|---|---|---|
|
%MD0 |
%MW0 |
%MB0 |
%MX0.7 |
... |
%MX0.0 |
|
%MB1 |
%MX1.7 |
... |
%MX1.0 |
||
|
%MW1 |
%MB2 |
%MX2.7 |
... |
%MX2.0 |
|
|
%MB3 |
%MX3.7 |
... |
%MX3.0 |
||
|
%MD1 |
%MW2 |
%MB4 |
%MX4.7 |
... |
%MX4.0 |
|
%MB5 |
%MX5.7 |
... |
%MX5.0 |
||
|
%MW3 |
%MB6 |
%MX6.7 |
... |
%MX6.0 |
|
|
%MB7 |
%MX7.7 |
... |
%MX7.0 |
||
|
%MD2 |
%MW4 |
%MB8 |
%MX8.7 |
... |
%MX8.0 |
|
... |
... |
... |
... |
||
|
... |
... |
... |
... |
... |
|
|
... |
... |
... |
... |
||
Example of overlap of memory ranges:
%MD0 contains %MB0 (...) %MB3, %MW0 contains %MB0 and %MB, %MW1 contains %MB2 and %MB3.
NOTE: The Modbus communication is asynchronous with the application.
