PLC Operation | Response time

 Operation of PLC 

Following are the major stages of normal operation of PLC  

1. Read inputs - The input memory is updated with the status of the physical inputs 
2. Execute program - In this process the program is written by the user and it is executed 
3. Update Outputs - In this, the status of the physical output is updated from the output memory 
   

In this type of operation, the steps of the program defined by ladder logic are executed one after the other by the PLC. The PLC processor requires a certain amount of time to execute any type of change in the output and to execute it programmatically. 

In this type of function, the PLC first performs internal processing operations and then the input samples are given to the PLC by the processor. In this way, the content is stored in memory. After this, the control program is executed. In memory, the store input values ​​are used in the output as control logic calculations. Finally, the output is aggregated and updated with the values ​​calculated. This process involves reading the input, executing the control program and customizing the output. The process of modifying the output is called scan and the time taken to perform the scan is called scan time. This time depends on the complexity and number of control functions for each cycle to perform. Thus, this scan time depends on the number of ladder diagrams and the complexity of the operations that occur on each cycle. Scan time can usually vary between 1 to 100ms. The scan time for the PLC is fixed.

Response Time of PLC Operation-

The time taken to complete a cycle is called response time, which depends on the length of the program and the execution rate of the PLC instruction. Response time is an important factor in selecting a PLC. This is equal to the sum of the input response time, program execution time, and output response time. Hence Total Response   Time = Input Response Time + Program Execution Time + Output Response Time 

The response time of a PLC depends on the processing time of the code. When the instruction and output execution are executed, the computer system does not read any new input signals. This is generally not a major problem, as most of the signals in industrial automation are quiet slow or last for long periods of time