How a PLC controller works
1. Scanning technology
When the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution and output refresh. The completion of these three phases is called a scan cycle. Throughout the entire run, the PLC's CPU repeats the above three phases at a certain scanning speed.
1. Input sampling stage In the input sampling stage
The PLC reads all input statuses and data sequentially in a scanning manner and stores them in the corresponding cells in the I/O image area. After the input sampling is complete, it moves to the user program execution and output refresh phase. In both phases, even if the input state and data change, the state and data of the corresponding cells in the I/O image area do not change. Therefore, if the input is a pulsed signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.
2. User program execution stage
During the user program execution phase, the PLC always scans the user program in sequence from top to bottom (ladder diagram). When scanning each ladder diagram, the control circuit composed of each contact on the left side of the ladder diagram is always scanned first, and the control circuit composed of the contacts is logically operated in the order of first left and then right, first up and then down, and then according to the result of the logical operation, the state of the corresponding bit of the logic coil in the system RAM storage area is refreshed; Or refresh the state of the corresponding bit of the output coil in the I/O image area; or to determine whether to execute the special function instructions specified in the ladder.
That is, in the process of user program execution, only the state and data of the input point in the I/O image area will not change, while the state and data of other output points and software devices in the I/O image area or the system RAM storage area may change, and the ladder diagram ranked above, the program execution result will play a role in the ladder diagram of all coils or data used in the below; Conversely, the ladder below, the state or data of the refreshed logic coils can only be used for the program above it until the next scan cycle.
If the immediate I/O command is used during program execution, the I/O points can be accessed directly. Even if the I/O command is used, the value of the input process image register will not be updated, and the program will take the value directly from the I/O module, and the output process image register will be updated immediately, which is somewhat different from immediate input.
3. Output refresh stage
When the scanning user program is finished, the PLC enters the output refresh phase. During this period, the CPU flushes all output latch circuits according to the corresponding state and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of the PLC.
How a PLC controller works
Although many names such as relays, timers and counters are often used in the ladder diagram programs used in PLCs, PLCs do not have these hardware physically, but use memory and program programming to do logic control and editing, and connect external mechanical devices through output components for physical control. Therefore, the hardware space required by the controller can be greatly reduced. In fact, the PLC executes the ladder diagram program line by line, first reading the ladder diagram code into the CPU in a scanning way, and finally performing the control operation. The whole scanning process consists of three steps, "Input Status Check", "Program Execution", and "Output Status Update" are described as follows:
Step 1 "Input Status Checking":P LC first checks the status of each point switch or sensor connected to the input component (1 or 0 means on or off), and writes its status to the corresponding position Xn in memory.
Step 2 "Program Execution": Extract the ladder diagram program into the CPU line by line, if the program execution needs to input the contact status, the CPU will directly query and take it out from the memory. The calculation result of the output coil is stored in the corresponding position in the memory, and is not reflected to the output Yn for the time being.
Step 3 "Output Status Update": Update the output status in Step 2 to the PLC output contact, and return to Step 1. These three steps are called the PLC scanning cycle, and the time required to complete is called the PLC reaction time, if the PLC input signal time is less than this reaction time, there is a possibility of misreading. After each program execution and before the next program execution, the output and input states will be updated once, so this mode of operation is called "program end regeneration" at the output and input terminals.
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