Memory is the component that stores information, programs, and data in a PLC.
The process of putting new information into a memory location is called writing. The process of retrieving information from a memory location is called reading.
Types of Memory
The common types of memory used in PLCs are
- Read Only Memory (ROM) and
- Random Access Memory (RAM).
A ROM location can be read, but not written. ROM is used to store programs and data that should not be altered.
For example, the PLC’s operating programs are stored in ROM. A RAM location can be read or written. This means the information stored in a RAM location can be retrieved and/or altered.
Ladder logic programs are stored in RAM. When a new ladder logic program is loaded into a PLC’s memory, the old program that was stored in the same locations is over-written and essentially erased.
The memory capacities of PLCs vary. Memory capacities are often expressed in terms of kilobytes (K). One byte is a group of 8 bits.
One bit is a memory location that may store one binary number that has the value of either 1 or 0. (Binary numbers are addressed in Module 2). 1K memory means that there are 1024 bytes of RAM. 16K memory means there are 16 x 1024 =16384 bytes of RAM.