What is a PLC Memory? Siemens PLC Memory Structure.

What is PLC Memory in Siemens?

Memory in general is just like a human brain. That is used to store the data or information. There are different types of memories in general, such as


⇒ It is also called Random Access Memory. It can be read or write memory, or we can say that it is the main or primary memory.

⇒ The programs or data that the CPU requires during execution will store in this memory.

⇒ It is a volatile memory, So all the data that is stored in this memory will be lost, when the power is turned off.


⇒ It is called Read-only memory. The memory which we just only read but can’t write on it.

⇒ It is a non-volatile memory, the information is permanently stored in it.


⇒ It is called Erasable and Programmable Read-only memory.

⇒ It is a type of read-only memory. But we can edit and reprogram it using ultra-violate light.


⇒ It is called Electrically Erasable and Programmable Read-only memory.

⇒ It can be erased and programmed electrically. But editing of memory is limited by several times.


⇒ It looks like an EEPROM, but it can be editable for unlimited numbers of times.

⇒ It works as a standalone memory storage device like a USB pen drive.

These are the types of memories in general.

PLC Memory Structure:

In general, the Siemens PLC system has four types of memories,

plc memory

Load Memory

Load memory is a non-volatile storage memory for the user program, data, and configuration. Therefore, when a program is downloaded to the CPU, it will first stored in the load memory area.

plc memory

Load memory can be expanded using memory cards, it is generally called MMC(Micro Memory Card).


Work Memory

Work memory is volatile storage for some elements of the user program while executing the program. Work memory is like a RAM of a computer.

The CPU copies some elements of the user project from load memory into work memory. This storage area is volatile and lost on power failure and is restored when CPU power is restored.

plc memory


System Memory

System memory can store all the addressing parts of the user program like addressing inputs, outputs, timers, counters, and bit memory.


Retentive Memory

PLC system provides some part of the memory to store data permanently. This part of the memory is called retentive memory.

This part of the memory will not be lost on power failure.

plc memory

User can review memory structure before modifying a program in the PLC, this is the best practice while editing any logic in the user program.

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