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A cycle to read I/0 evaluate according to Program and take required output. and its continuous unless any interrupt occurs.
PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.
Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step.
Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.
Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.
After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.
Scan is a cycle in PLC which consits of four steps.
1) Reading the input status
2) Execution of program
3) Diagnostics
4) Updating the output status
After this cycle continuous itself.
PLC SCAN is a cycle of the following steps:
1. Read all system digital & analog inputs (take state/value snapshot) and save it in CPU memory.
2. Execute user program.
3. Process communication requests by HMIs & SCADA workstations.
4. Perform self diagnosis for CPU, Memory, integrity of modules ....
5. Update the state/value of all system digital/analog outputs according to program logic & communication requests.
The order of these steps could be different according to PLC brand.
Time required to read load execute an instruction in program
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