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Memory Types

Chapter-5

• Modern PLC'S use modules that have solid- state memory. The memory size for each type of memory is specified in bytes with one byte equal to eight consecutive binary bits.

• Because modem memory circuits can store a large number of bytes, the following metric unit prefixes are used to indicate memory size:

1. kilo (k),

2. Mega (M), and

3. Giga (G).

• There are a variety of solid-state memory types, but these are the most common types.

Read-only memory

• (ROM) stores information developed by the PLC manufacturer that cannot be changed.
  

• This type of memory is non-volatile, meaning that the information contained will not be lost in the event of a power failure.

Erasable, programmable read-only memory

• (EPROM) is protected from unauthorized changes. EPROM can be read from, but not easily changed. EPROM storage is sometimes referred to as firmware.
  

• Module firmware is often designed to be updated when required. This is also a non-volatile memory type.

Random access memory

• (RAM) is also called read/write memory and is used for temporary storage of programs and data.
  

• RAM is volatile. meaning that information will be lost in the event of a power failure unless power to the circuits is backed up by a battery or charged capacitor.

Flash memory

SIMATIC Memory Cards

• Flash memory is a type of electrically-erasable programmable read-only memory (EEPROM).

• Flash memory is a non-volatile memory type that, among other things, is used for SIMATIC Memory Cards.

• These cards are available in a range of memory sizes and can be used to store or transfer a PLC user program and associated data

CPU Memory Organization

• PLC CPU'S have a memory organization that defines the available storage for various types of information.
  

• PLC'S of a specific model typically include a range of CPU'S that vary in the amount of user memory available.

• Descriptions of user memory types vary from PLC to PLC, but the following user memory types are typical for SIMATIC S7 PLC'S.
  

• Load memory is non-volatile storage for the user program and related data.

• When a user program is downloaded to the CPU, it is first stored in the load memory area.
  

• This area is located either on a memory card or in the CPU and is maintained through a power loss.

• Work memory is volatile storage for some elements of the user program. The CPU copies these elements from load memory into work memory during program execution.

• This is volatile memory, so the information is lost when CPU power is lost, but is restored by the CPU when power resumes.

•  Retentive memory is non-volatile storage for a limited amount of work memory information.

• Much of the data used by the user program is stored in data bocks in the program.
  

• However, CPU'S have global memory areas that are accessible by the user program.

• These include input (I) and output (Q) process image tables and bit memory (M).

Chapter 6 we will discuss about Power Supply Module,Signal Module,Communication Module,Special Purpose Module

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