The memory in a PLC system is mainly used to store system programs, user programs, and operating status data. PLC memory includes system memory and user memory.
(1) System memory
The system memory stores the system program written by the PLC manufacturer and is permanently stored in ROM, which cannot be changed by the user. It enables the PLC to have basic functions and perform the tasks specified by the PLC designer. The quality of the system program largely determines the performance of the PLC.
(2) User storage
User memory consists of two parts: user program memory (program area) and data memory (data area). User program memory stores various user programs written by the user using a PLC programming language for specific control tasks. Depending on the type of memory unit selected (RAM, EPROM, or EEPROM), the contents of user program memory can be modified or added to by the user. User data memory stores (remembers) the ON/OFF states and data of devices used in the user program. The size of user memory directly affects the capacity of the user program and is one of the important indicators reflecting PLC performance.
To facilitate reading, checking, and modification, user programs are typically stored in CMOS static RAM, with a lithium battery as a backup power source to ensure data integrity during power outages. To prevent interference from damaging the program in RAM, once the user program has run normally and requires no changes, it can be stored in a read-only memory (EPROM). Many PLCs now directly use EEPROM as their user memory.
Working data refers to data that frequently changes and is accessed during PLC operation. It is stored in RAM to accommodate random access requirements. Within the PLC's working data memory, there are storage areas for logic devices such as input/output relays, auxiliary relays, timers, and counters. The states of these devices are determined by the initial settings and operational status of the user program. If necessary, some data is maintained in its current state using a backup battery during power failure; this storage area that preserves data during power loss is called the data retention area.
Since the system program and operating data are not directly related to the user, the form and capacity of memory listed in PLC product catalogs or user manuals refer to the user program memory. When the user memory provided by the PLC is insufficient, many PLCs also offer memory expansion capabilities.
There are two main types of memory: one is random access memory (RAM) which can be read/written, and the other is read-only memory or erasable programmable read-only memory (ROM, PROM, EPROM, and EEPROM).
