PLC of the two basic working pattern and the scanning process

PLC, there are two basic types of working mode, namely, running ( 运行) Mode and stop ( 停止) Mode. In run mode, the PLC through repeated execution reflects the control requirements of the user program to implement the control function. In order to make the PLC output response in a timely manner could change at any time of the input signal, the user program is not executed only once, but constantly repeated, until the PLC downtime or switch to STOP work mode. In addition to execute user programs, in the process of each loop, PLC and complete internal processing, communication processing, such as work, a cycle can be divided into five phases, As shown in figure 1 5) 。 PLC cycles of the circulation way of working is called scanning works. Due to the speed of the computer executes instructions is extremely high, from the external input Output relation, process seems to be done at the same time. Internal hardware module is normal, will monitor the timer is reset, and some other internal work. At service stage, PLC communicate with other intelligent device with microprocessor, programmer type in the command response and update programmer display content. When the PLC in stopped ( 停止) Mode, the only perform the above operations. PLC controller is running ( 运行) Mode, and complete the other three phase of the operation. In the PLC memory, set up an area used to store the input signal and output signal of the state, they are referred to as the input images and output images registers, respectively. Other programming of PLC ladder diagram elements also have corresponding image storage area, they are collectively known as component image register. In the input processing stage, PLC connected the all the external input circuit, disconnect state reading input image register. External input circuit connected to the corresponding input image register for l state, ladder diagram of the corresponding input relays on the normally open contact, normally closed off contact. External input contact circuit disconnected, corresponding to the input image register to zero state, ladder diagram of the corresponding input relay normally open contacts disconnect, normally closed contact through. A programming element corresponding image register for l state, according to the programming element to ON, the image register for zero state, called the programming components OFF. During the program execution phase, even outside the state of the input signal is changed, the state of the input image register also will not change, then the input signal changes the state of only the next scan cycle is read into input processing stage. The PLC controller user program is composed of several instructions, instructions step by step in the memory number order. When there is no jump instruction, the CPU from the first instruction, detailed order to execute user programs, until the end of the user program. When executing instructions, or other elements from the input image register image register will be about the programming elements in 0/1 of the state to read, and according to the requirement of the instruction execution corresponding logic operation, operation results written to the corresponding component image register, as a result, the programming element image register ( Except for the input image register) The content of the changes with the execution of the program. In the output processing phase, CP / 7 to deliver 0/1 of the output image register state to the output latch. Ladder diagram in a certain output relay coil 'electricity', the corresponding output image register for 1 state. Signal after output isolation and power amplifier module, relay output module type of the corresponding hardware relay coil electricity, the normally open contact closure, the external load electricity work. If the ladder diagram output relay coil in the 'power', the corresponding output image register to zero state, after the output processing phase, type relay output module of the corresponding hardware relay coil power, the normally open contacts disconnect, external load without electricity, stop working.
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