What is PLC Memory?

✔ PLC program development for particular application require a program memory.
✔ Program memory read cyclically by the central processing unit (CPU).

Requirements of program memory :

1. To create new program and store the program in programming device and PC.
2. Program can not be lost during power failure or voltage fluctuation.
3. Program memory should be cost effective.
4. Program memory should be sufficiently fast operation of central processing.

There are three different types of memory are use :

1. RAM
2. EPROM
3. EEPROM

1. RAM 

    ✔ RAM (random access memory) is a fast and highly cost effective memory. 
    ✔ Main memory of PLC consist of RAM. 
    ✔ RAM Produced in High quantities. 
    ✔ RAM available at low cost.
    ✔ RAM are read/write memory and can be easily programmed and modified.

Disadvantages of RAM :

     ✔ RAM is volatile memory
     ✔ Program stored in RAM is lost the event of power failure
     ✔ RAM are backup by battery or accumulator.

2. EPROM 

     ✔ EPROM (erasable programmable read-only memory) is a fast and low cost memory which comparison with RAM.
    ✔ Advantages of EPROM is being non-volatile memory.
    ✔ Memory store the program When the event of power failure.
    ✔ The purpose of programming first erase memory and after a cooling period completely reprogrammed.
    ✔ This relatively process of erasing -cooling-reprogramming EPROM are very frequently use in PLC.
    ✔ EPROM is reliable and cost effective memories.

Notes :

• RAM is used during programming and commissioning of machine.
  
• After completing commissioning of machine, Program transfer to EEPROM memory.

3. EEPROM

   ✔ EEPROM (electrically erasable  programmable  ROM),
   ✔ EEROM (electrically erasable ROM),
   ✔ EAROM (electrically alterable ROM) or Auto Flash-EPROM available some times.
   ✔ EEPROM is used widely as an application memory in PLC.
   ✔ EEPROM is an electrically erasable memory which can be used for written to program.