Memory section of the processor consists of hundreds or thousands of locations where information is stored. It is divided into two parts, User and Storage. The User memory is for storage of the program, which contains the ladder logic, or instructions that control the driven equipment. The Storage memory is used to store information such as Input/output status, timer or counter preset/ accumulated values and internal control relays etc. that are necessary for the processor to control the equipment.
The Memory chips used in processor can be separated into two distinct groups; Volatile (One that loses its stored information when power is removed) and Non Volatile (It has ability to retain stored information when power is removed, accidentally or intentionally). Back up Batteries included in the processor power supply protect volatile memory. The battery may be the sized dry cells rechargeable Nickel Cadmium or non-rechargeable alkaline, lithium types.
To change the battery we should back up our files and switch off mains before removing from PLC else it causes loss of program.
Memory types: The most common type of volatile memory is
RANDOM ACCESS MEMORY (RAM): Information can be written into or read from RAM chip and is often referred to as read/ write memory. Information stored in memory can be retrieved or read. While write indicates that the user can program or write information into memory. Random access refers to the ability of any location (address) in the memory to be accessed or used. RAM is used for both the user memory and storage memory in many PLCs. Since RAM is volatile, it must have battery backup to retain or protect the stored program. Various forms of RAM include MOS and CMOS - RAM (Complementary Metal Oxide Semiconductor – RAM). CMOS – RAM is very popular because it has very low drain current when not being accessed (15 micro amps) and information stored in memory can be retained by as little as 2 V DC. A typical fully charged lithium battery is rated 2.95 V at 1.75 amps / Hour and normally holds or protects a program for 60 days or longer.
The most common type non volatile memories are
READ ONLY MEMORY (ROM) : A common non volatile memory do not require battery backup and retains information or program when power is lost. Read only indicates that the information stored in memory can be read only and cannot be changed. Information in ROM is place by the manufacturer for the internal use and operation of the PLC and The manufacturer does not want the information changed or altered.
PROM (PROGRAMMABLE READ ONLY MEMORY) : It allows initial and/ or additional information to be written into the chip. PROM may be written into only once after being received from the PLC manufacturer. This type of memory is used to prevent un authorized program changes. Another method of restricting access to program is to use PASSWORD (Software Lock). They allow access to the program to those personal who know the correct password.
UVPROM – EPROM (ULTRAVIOLET PROGRAMMABLE READ ONLY MEMORY – ERASABLE PROGRAMMABLE READ ONLY MEMORY) : UVPROM is ideally suited when program storage is to be semi – permanent or additional security is needed to prevent unauthorized program change. UVPROM chip also referred to as EPROM. EPROM has a quartz window over a silicon material that contains the electronic IC’s. This window is normally covered by an opaque material, but when the opaque material is removed and the circuitry exposed to ultraviolet light, The memory content can be erased. Once erased, The EPROM chip can be reprogrammed using special programmer. After programming, the chip window must once again be covered with an opaque material such as electricians tape to avoid undesirable alteration of memory.
EAROM (ELECTRICALLY ALTERABLE READ ONLY MEMORY) : These chips can have stored program erased electrically. This is accomplished by applying different positive (+) and Negative ( - ) voltage values to specific circuit points (Supplied in the manufacturers literature).Once erased this chip also can be reprogrammed.
EEPROM (ELECTRICALLY ERASABLE PROGRAMMABLE READ ONLY MEMORY) : This is also referred to as Double EPROM or E2PROM. It is the chip that can be programmed using a standard programming device and can be erased by the proper signal being applied to the erase pin. This is primarily used as non – volatile backup for the user program RAM. If the user program in RAM is lost or erased, a copy of the program stored on an EEPROM chip can be downloaded in RAM. It is common on some PLCs for the processor to load the program from E2PROM chip into RAM memory, Each time the processor is powered up or after a power failure.
We study more on memory in next blog.
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