ABB 216DB61 HESG324063R100/J

程序执行阶段。在执行用户程序过程中，PLC按照梯形图程序扫描原则，一般来说，PLC按从左至右、从上到下的步骤逐个执行程序。但遇到程序跳转指令，则根据跳转条件是否满足来决定程序跳转地址。程序执行过程中，当指令中涉及输入、输出状态时，PLC就从输入映像寄存器中“读入”对应输入端子状态，从输出映像寄存器“读入”对应元件（“软继电器”）的当前状态。然后进行相应的运算，运算结果再存入输出映像寄存器中。对输出映像寄存器来说，每一个元件（“软继电器”）的状态会随着程序执行过程而变化。

输出刷新阶段。程序执行阶段的运算结果被存入输出映像区，而不送到输出端口上。在输出刷新阶段，PLC将输出映像区中的输出变量送入输出锁存器，然后由锁存器通过输出模块产生本周期的控制输出。如果内部输出继电器的状态为“1”，则输出继电器触点闭合，经过输出端子驱动外部负载。全部输出设备的状态要保持一个扫描周期。

Program execution phase. During the execution of user programs, the PLC follows the principle of ladder program scanning. Generally speaking, the PLC executes programs one by one from left to right and from top to bottom. But when encountering program jump instructions, the program jump address is determined based on whether the jump conditions are met. During program execution, when the instruction involves input and output states, the PLC "reads" the corresponding input terminal state from the input image register and the current state of the corresponding component ("soft relay") from the output image register. Then perform corresponding operations, and store the results in the output image register. For the output image register, the state of each component ("soft relay") will change with the program execution process.

Output refresh stage. The calculation results during the program execution phase are stored in the output image area instead of being sent to the output port. In the output refresh stage, the PLC sends the output variables in the output image area to the output latch, which then generates the control output of this cycle through the output module. If the status of the internal output relay is "1", the output relay contacts close and drive the external load through the output terminal. The state of all output devices should be maintained for one scanning cycle.