PLC Scan Time
t_total = t_io + t_logic + t_comms
Quick answer
This calculator solves t_total = t_io + t_logic + t_comms for Total Scan Time. With I/O Scan Time 44 ms, Logic Execution Time 94 ms, Communications Time 9.4 ms, the result is 147.4 ms. It solves in 4 directions, so you can enter the values you have and get the one you need.
Calculator
Formula
Description
A PLC (Programmable Logic Controller) executes its program in a cyclic scan consisting of three main phases: reading all inputs, executing the user logic program, and updating outputs and handling communications. The total scan time determines the worst-case response latency of the control system. An input change occurring just after the input scan phase will not be processed until the next cycle, resulting in a maximum response time of nearly two full scan cycles. Fast processes like high-speed counting or motion control may require dedicated hardware interrupts rather than relying on the normal scan cycle.
Variables
- t_total — Total scan cycle time (s)
- t_io — Time to read inputs and write outputs (s)
- t_logic — Time to execute the user program (s)
- t_comms — Time for network communication and housekeeping (s)
Practical Notes
Typical PLC scan times range from 1-50 ms depending on program size and I/O count. Modern PLCs offer periodic tasks with fixed execution rates for time-critical control loops, event-driven tasks triggered by interrupts, and background tasks for non-critical housekeeping. Watchdog timers detect scan time overruns that indicate program errors or excessive load.
Symbols and units
| Symbol | Meaning | Default unit | Accepted units |
|---|---|---|---|
| t_total | Total Scan Time | ms | ps, ns, µs, ms, s, min, h |
| t_io | I/O Scan Time | ms | ps, ns, µs, ms, s, min, h |
| t_logic | Logic Execution Time | ms | ps, ns, µs, ms, s, min, h |
| t_comms | Communications Time | ms | ps, ns, µs, ms, s, min, h |
Step by step
- 1. What you know: I/O Scan Time 44 ms, Logic Execution Time 94 ms, Communications Time 9.4 ms
- 2. The relation: t_total = t_io + t_logic + t_comms
- 3. Substitute: t_io = 44 ms, t_logic = 94 ms, t_comms = 9.4 ms
- 4. Result: Total Scan Time = 147.4 ms
Worked examples
Minimum range
Given: I/O Scan Time 0.22 ms, Logic Execution Time 0.47 ms, Communications Time 47 µs
Result: Total Scan Time = 0.737 ms
Low range
Given: I/O Scan Time 2.2 ms, Logic Execution Time 4.7 ms, Communications Time 0.47 ms
Result: Total Scan Time = 7.37 ms
Typical
Given: I/O Scan Time 44 ms, Logic Execution Time 94 ms, Communications Time 9.4 ms
Result: Total Scan Time = 147.4 ms
High range
Given: I/O Scan Time 1.1 s, Logic Execution Time 2.35 s, Communications Time 0.235 s
Result: Total Scan Time = 3.685 s
Maximum range
Given: I/O Scan Time 11 s, Logic Execution Time 23.5 s, Communications Time 2.35 s
Result: Total Scan Time = 36.85 s
Solving for each variable
Solve for Total Scan Time
I/O Scan Time 0.22 ms, Logic Execution Time 0.47 ms, Communications Time 47 µs → 0.737 ms
Solve for I/O Scan Time
Total Scan Time 100 µs, Logic Execution Time 0.47 ms, Communications Time 47 µs → -0.417 ms
Solve for Logic Execution Time
Total Scan Time 100 µs, I/O Scan Time 0.22 ms, Communications Time 47 µs → -0.167 ms
Solve for Communications Time
Total Scan Time 100 µs, I/O Scan Time 0.22 ms, Logic Execution Time 0.47 ms → -0.59 ms
How Total Scan Time changes with I/O Scan Time
| I/O Scan Time (input) | Total Scan Time (result) |
|---|---|
| 9.68 ms | 0.2374 s |
| 24.2 ms | 0.2519 s |
| 48.4 ms | 0.2761 s |
| 72.6 ms | 0.3003 s |
| 96.8 ms | 0.3245 s |
| 145.2 ms | 0.3729 s |
| 0.1936 s | 0.4213 s |
| 0.3872 s | 0.6149 s |
| 0.6776 s | 0.9053 s |
| 0.968 s | 1.196 s |
The same result in other units
| Unit | Value |
|---|---|
| µs | 147400 |
| ms | 147.4 |
| s | 0.1474 |
| min | 0.002457 |
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<p><a href="https://vbatpower.com/formulas/plc-scan-time/">PLC Scan Time</a> Calculator by vbatpower.com</p>Frequently asked questions
How do you calculate PLC Scan Time?
Use t_total = t_io + t_logic + t_comms. With I/O Scan Time 44 ms, Logic Execution Time 94 ms, Communications Time 9.4 ms, Total Scan Time works out to 147.4 ms.
What is the formula for PLC Scan Time?
The formula is t_total = t_io + t_logic + t_comms. A PLC (Programmable Logic Controller) executes its program in a cyclic scan consisting of three main phases: reading all inputs, executing the user logic program, and updating outputs and handling communications.
How do you find I/O Scan Time when you know the other values?
Rearrange for I/O Scan Time: with Total Scan Time 100 µs, Logic Execution Time 0.47 ms, Communications Time 47 µs, I/O Scan Time is -0.417 ms.
How do you find Logic Execution Time when you know the other values?
Rearrange for Logic Execution Time: with Total Scan Time 100 µs, I/O Scan Time 0.22 ms, Communications Time 47 µs, Logic Execution Time is -0.167 ms.
What units does PLC Scan Time use?
Total Scan Time is shown in µs, ms, s, min. Values are held in SI base units internally, so you can mix prefixes without converting by hand.
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