存储器主要用于存放系统程序、用户程序及工作数据。存放系统软件的存储器称为系统程序存储器;存放应用软件的存储器称为用户程序存储器；存放工作数据的存储器称为数据存储器。常用的存储器有RAM、EPROM和EEPROM。RAM是一种可进行读写操作的随机存储器存放用户程序，生成用户数据区，存放在RAM中的用户程序可方便地修改。RAM存储器是一种高密度、低功耗、价格便宜的半导体存储器，可用锂电池做备用电源。掉电时，可有效地保持存储的信息。EPROM、EEPROM都是只读存储器。用这些类型存储器固化系统管理程序和应用程序。根据PLC梯形图程序扫描原则，按先左后右先上后下的步序，逐句扫描，执行程序。遇到程序跳转指令，根据跳转条件是否满足来决定程序的跳转地址。从用户程序涉及到输入输出状态时，PLC从输入映象寄存器中读出上一阶段采入的对应输入端子状态，从输出映象寄存器读出对应映象寄存器，根据用户程序进行逻辑运算，存入有关器件寄存器中。对每个器件来说，器件映象寄存器中所寄存的内容，会随着程序执行过程而变化。
The memory is mainly used to store system programs, user programs and working data. The memory storing system software is called system program memory; The memory storing application software is called user program memory; The memory storing working data is called data memory. Ram, EPROM and EEPROM are commonly used. Ram is a kind of random access memory that can be read and written to store user programs and generate user data areas. The user programs stored in RAM can be easily modified. RAM memory is a kind of semiconductor memory with high density, low power consumption and low price. Lithium battery can be used as backup power supply. In case of power failure, the stored information can be effectively maintained. EPROM and EEPROM are read-only memories. Use these types of memory to solidify system management programs and applications.
According to the scanning principle of PLC ladder diagram program, scan sentence by sentence and execute the program according to the step sequence from left to right, from top to bottom. When a program jump instruction is encountered, the jump address of the program is determined according to whether the jump conditions are met. When the user program involves the input and output status, PLC reads out the corresponding input terminal status acquired in the previous stage from the input image register, reads out the corresponding image register from the output image register, performs logical operation according to the user program, and stores it in the relevant device registers. For each device, the contents registered in the device image register will change with the execution of the program.
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