An essential tool for engineers and programmers, the PLC Scan Time Calculator allows users to predict how long it will take a Programmable Logic Controller (PLC) to do a full scan of the control logic. Users may identify performance bottlenecks, guarantee real-time response, and optimize PLC programs by knowing the scan time.
What is PLC Scan Time?
The PLC scan time is the total duration for the PLC to execute the following sequential operations:
- Reading Inputs: Capturing the status of field devices (sensors, switches, etc.).
- Processing Logic: Executing all instructions in the program.
- Updating Outputs: Sending control signals to actuators, motors, or other devices.
If the scan time exceeds the system’s required response time, delays in automation processes may occur, potentially leading to inefficiencies or safety concerns.
Formula for PLC Scan Time Calculator
The total scan time of PLC is calculated using the following formula:
This provides a baseline estimate for how long the PLC program execution phase will take.
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PLC Scan Time Calculator Parameters in Detail
Number of Instructions
- Represents the total number of logical operations in the PLC program.
- Includes ladder rungs, function blocks, or lines of structured text, depending on the programming language.
- Larger programs with complex logic result in longer scan times.
Instruction Execution Time
- The time required by the PLC’s CPU to process a single instruction.
- Varies based on the complexity of the instruction (e.g., mathematical operations may take longer than simple logic gates).
- Refer to the technical documentation of the PLC model for precise execution times (e.g., 0.01 ms for Siemens S7-1200 basic instructions).
Factors Affecting Scan Time
- Program complexity affects scan time as larger and more intricate programs result in higher scan times.
- Adding loops or advanced calculations can significantly increase the time. CPU performance plays a role, with high-speed CPUs executing instructions faster, thus reducing scan times.
- Selecting a PLC with adequate processing power for the application is crucial.
- The type of instructions also matters, as simple instructions like AND or OR execute faster than complex operations like floating-point arithmetic or PID control.
- I/O update time contributes to total scan time, with efficient communication with field devices helping to minimize delays.
- Communication overhead, including data exchange between PLCs, HMIs, and SCADA systems, can increase scan times, especially in communication-intensive programs.
Example Calculation on PLC Scan Time
You are programming a Siemens S7-1200 PLC with the following details:
- Average Instruction Execution Time: 0.01 ms per basic instruction.
- Number of Instructions: 10,000.
- I/O Scan Time: 10 ms (for reading inputs and updating outputs).
Step-by-Step Calculation:
Logic Execution Time:
Logic Execution Time=10,000×0.01 ms=100 ms
Total Scan Time:
Add I/O scan time to logic execution time:
Total Scan Time=100 ms+10 ms=110 ms
Result:
The total scan time for the program is 110 ms.
How to use this PLC Scan Time Calculator ?
- Count the instructions in your program by analyzing the total number of operations.
- Many PLC programming environments offer tools to display instruction counts.
- Obtain execution time from the PLC manufacturer’s manual for both basic and advanced instructions.
- Include I/O and communication overhead by factoring in delays from reading inputs, updating outputs, and data exchange.
- Apply the formula by multiplying the number of instructions by the execution time and adding any additional delays.
- Optimize as needed by simplifying program logic or selecting a higher-performance PLC if the calculated scan time exceeds requirements.
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When and Where to use this PLC Scan Time Calculator ?
- Use the calculator during program design to estimate scan times early, ensuring the program meets real-time constraints.
- It is valuable for performance optimization, helping evaluate existing programs to identify and eliminate unnecessary delays.
- During troubleshooting, it can diagnose and resolve issues related to slow PLC response times.
- For system integration, use it to verify that the PLC scan time aligns with the control cycle requirements during commissioning.
Additional Tips PLC Scan Time optimization
- Regularly monitor actual scan times using PLC diagnostic tools to ensure consistent performance.
- Consider advanced optimization techniques, such as replacing repeated code with subroutines or functions to streamline execution and minimizing complex calculations within the main program to reduce delays.
- Always include buffer times for future expansions to prevent performance degradation as system demands grow.
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