The working process of the large and medium-sized PLC

The working process of the large and medium-sized PLC newmaker welcome to e hall exhibition hall 3 PLC PLC programmable controller, PLC programming cable. 。 。 With the development of PLC technology, the form and function of PLC has very big development, from the comparison of a single independent units of the development in this modular structure, PLC network, distributed control, such as more representative AB Controllogix, MODICON Quantum, is a typical form of large PLC. These large PLC can be applied to the more important, big industrial control field, the control points can have thousands of points, the nature, this kind of circumstance, if still continue to use the PLC works cannot very good control tasks to complete this case, the fact is, large and medium-sized PLC way of working and small PLC has had the very big difference. A large and medium-sized PLC scan cycle is divided into six stages: stage 1, since the monitoring scanning in order to guarantee the reliability of the work, the PLC internal have since the monitoring or self-diagnosis function, since the monitoring function is by monitoring timer ( WDT、看门狗定时器) Completed, WDT is a hardware clock, main is to check and reset WDT since the monitoring process, if time to scan more than WDT value before the reset, the CPU will stop running, IO reset, give alarm signal, this failure is called WDT fault. WDT malfunction can be caused by the CPU hardware, may also be a user program execution time is too long, the scan time over the WDT time and cause, with programmer can eliminate the fault. The WDT Settings usually is 150 - 200 ms, time is less than 50 - general system 60ms。 In large and medium-sized PLC WDT can generally be modified. 2, exchange information with programmer user use the programmer ( Calculating concentration of programming software) User program of PLC controller to upload, download or use the PC in the SCADA system to monitor PLC control, PLC control of the CPU to hand over, in a passive state, the work is done or achieved confidence exchange after the prescribed time, the CPU to get the bus power, regain the initiative. In this phase, the user can through the programmer to modify memory program, start-stop CPU, IO control. 3, exchange information with digital processor DPU stage when equipped with DPU, just can have this stage. 4, the phase current of large and medium of communication and network PLC are large amounts of data exchange with fieldbus protocols, such as, use controlnet controllogix, quantum using MODBUS PLUS, in the period of the decade, PLC and network for data exchange. 5, user program scan stage when the PLC controller is in operation, a scan cycle phase scanning is contains the user program. In this stage, according to the instructions in the user program, PLC from the input state staging area and other soft components of the staging area will read about status, from the first instruction executed sequentially, every step of the implementation results in the output state the staging area. 6, IO service CPU Settings in the memory scanning stage two staging area, one is input the staging area ( The input image register) Output, one is the staging area ( The output image register) , the implementation of the user program, use the input values from the input of registers, output results in the staging area. In the input service ( Input by Yang and refresh) CPU will be the actual state of input into the input staging area; In the output services ( The output refresh and latches) Value at the same time, the CPU will output the staging area to the state of the output latch. Input data staging area depends on the actual state of the input point input service stage, in the user program execution phase, the input of the staging area changes over at the input data is away, the phase, the output of the staging area varies according to the results, but the output latch content remains the same.
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