PLC the PLC operating system with basic intelligence, able to complete PLC Designer provides a variety of work. User programs designed by the user, it allows the PLC can complete features specific to user requirements. User program memory capacity in bytes (Byte, b for short) as the unit.
1. PLC used physical memory
(1) random access memory (RAM)
CPU can read the data in RAM, you can also write data to RAM, RAM is also called the read/write memory. It is a volatile memory, after a power outage, the information stored will be lost.
High working speed of the RAM, cheap, easy to overwrite. Turn off PLC after the external power supply, use lithium-ion battery to save user programs and data in RAM. Lithium battery needs to be replaced, by the PLC signal notifies the user. RAM with lithium-ion batteries can be used to store a user‘s programs and data.
(2) read–only memory (ROM)
ROM contents can be read out, and cannot be written to. It is non-volatile, power gone, can still save the stored content, ROM used to store the PLC operating system.
(3) Flash memory and EEPROM
Flash memory (Flash EPROM) referred to as the FEPROM, electrically erasable programmable read–only memory referred to as EEP-ROM. They are non-volatile, they can be with a programming device programming, both non-volatile ROM and RAM random access benefits, but the time required for writing information to them much longer than the RAM. They are used to store user programs and save the important data you need power.
2. Micro memory card
FEPROM based micro memory card MMC for short, when the electricity was used to save the user program and some of the data. MMC is used to load a memory (Load Memory) or portable media.
If item in the MMC was key, but forgot the password, use only the Siemens programmer card reader slot Siemens card reader with USB interface or use MMC to delete the original contents of the MMC can be used as an empty card is not encrypted. Only in the de-energized State, or when the CPU is in the STOP State, to remove the storage card.
3. CPU storage
Store by the load of CPU memory, system memory and working memory (see Figure 2-5). Working memory is similar to the computer‘s memory, loaded memory is similar to a computer‘s hard disk, or USB flash drive.
(1) loading memory
CPU load memory for saving does not contain symbolic addresses and comment block, block and system data (configuration, connections, and module parameters, and so on). When you download a program, user program (logical block and block) is downloaded to the loaded memory. When in the PLC, and copy the executable part of the CPU memory load to the working memory, symbol tables and save notes in a programming device. In power, the need to save data is automatically saved in loaded in memory.
S7-300 using MMC (micro memory card) mounted memories. Production S7-300 CPU must now insert the MMC to download and run the user program. CPU and MMC are ordered separately.
S7-400 load of the CPU with integrated memory (RAM with backup battery), the FEPROM memory card can also be used to extend the loading memory or RAM memory card (see Figure 2-10).
(2) the working memory
Working memory is integrated in the CPU–speed access RAM memory, CPU when running user programs and data are stored, such as blocks, function block, function and data blocks. In order to ensure the program execution speed and does not take up too much memory, only related to program execution and working memory by man. When using the mode selector switch reset CPU memory, programs in the RAM is cleared, programs not cleared in the FEPROM.
(3) system memory
System memory is the CPU provides RAM for user program area, used for storing user programs manipulate data, such as image input, process image output, memory, timers, and counters, blocks the stack (a stack), interrupt stack (I stack) and the Diagnostics buffer.
System memory also includes temporary memory (a local data stack, namely l stack), used to store program block called a temporary variable (TEMP). Blocks are carried out in the temporary variable to be valid, you may be overwritten.
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