PLC scan time and response calculator
Worst-case response is the sum of every delay in the chain, and each one contributes its full period, not half.
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m/s
Worst-case response
96.00ms
- Typical response
- 81.00 ms
- Jitter
- 15.00 ms
- Machine travel in that time
- 48.0
- Highest event rate it can follow
- 5.21 Hz
- Share of the delay from the scan
- 20.8 %
- Indicative safety distance Travel plus a margin. Real safety distances follow ISO 13855 and must be calculated properly.
- 158
Show the working
- PLC scan time = 10
- Input filter time = 5
- Output module delay = 1
- Network update period = 20
- Actuator response = 50
- Machine speed = 0.5
- Worst-case response = 5 + 10 * 2 + 20 + 1 + 50 = 96
- Typical response = 5 + 10 * 1.5 + 20/2 + 1 + 50 = 81
- Jitter = 96 - 81 = 15
- travel = 0.5 * 96 / 1000 = 0.048
- Machine travel in that time = 0.048 * 1000 = 48
- Highest event rate it can follow = 96 > 0 ? 1000 / (2 * 96) : 0 = 5.208 33
- Share of the delay from the scan = 96 > 0 ? 10*2/96*100 : 0 = 20.8333
- Indicative safety distance = 48 * 1.2 + 100 = 157.6
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- worst case = input filter + two scans + network period + output delay + actuator delay
- two scans because an input arriving just after a read waits a full scan before it is seen
Safety distance calculations are governed by standards and must not be based on a rough figure like this one.
Reducing scan time helps only until another term dominates. The actuator is usually the slowest link.