PLC user program execution process - Database & Sql Blog Articles

Brand AVX TPSE226M035R0125 Low impedance tantalum capacitor AVX 22

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

Photocoupler

For plc users, when writing user programs or selecting devices, you must be aware of the three phases described below, which are the principles of the user program execution process.
The PLC adopts a centralized processing method, that is, a centralized batch processing mode for inputting a scan signal, executing a user program, and outputting a refresh.
(1) Input scan In this stage, the PLC reads the input signals on all the input terminals in a scanning manner, and stores the input signals in the input image area, and the input image memory is refreshed. During the program execution phase and the output refresh phase, the input image memory is isolated from the outside world, and its contents remain unchanged until the input scan phase of the next scan cycle is refreshed. It can be seen that when the PLC executes the program and processes the data, it does not directly use the input signal at the scene, but uses the data input into the image area during the sampling. If the input device can make the PLC input form a closed loop, the internal input relay corresponding to the input end number is stored as "1", which is equivalent to the relay coil being turned on. During the execution of the program, the contact corresponding to the number moves; if the input device enables the input to be open, the internal input relay corresponding to the input number is stored as “0”, which means that the relay coil is not turned on, and the program is executed. During the process, the contact corresponding to the number does not operate. If the state of the input device outside the PLC changes during the non-input scan phase of the PLC, the internal input relay will not change, and it will not be refreshed according to the input state at this time until the next input scan phase. Therefore, for input signals less than ten milliseconds, it is often not collected.
(2) Execution program During the execution of the user program, the PLC scans one by one from top to bottom and from left to right in the order of the ladder program, that is, in the order of the mnemonic instruction list. However, when a program jump instruction is encountered, the program jump address is determined according to whether the jump condition is satisfied. During the execution of the program, the PLC takes the current state of the input variable from the input image area, and then performs a logic operation or other operation determined by the program, and stores the operation result in the corresponding internal relay according to the program instruction, including the output relay and the internal auxiliary. Relays, timers, counters, etc. The signal of the output relay is stored in the output image area, that is, the output relay corresponds to the same numbered output point outside the PLC.
During the execution of the program, in the same cycle, the previous logic result affects the subsequent contacts, that is, the program executed later may use the latest intermediate operation result; but in the same cycle, the subsequent operation results do not affect the previous logical relationship. . The final state (on or off) of all internal relays except the input relay during this scan cycle will affect the opening and closing of each contact in the next scan cycle.
(3) The result of the operation of the output refresh program is stored in the output image area and not sent to the output port. During the output refresh phase, the PLC sends the output variable in the output image area to the output latch, which then generates the control output for this cycle through the output module. If the status of the internal output relay is "1", the output relay contacts close. The state of all output devices is maintained for one scan cycle.