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HIGH-SPEED I/O • FX5U / GX WORKS3

How do I calculate RPM from FX5U pulse counts?

RPM = pulses counted × 60,000 ÷ (encoder pulses per revolution × sample time in milliseconds). Use latched sample data, not a continuously changing accumulator.

Practical explanation

For 512 pulses measured in 1000 ms with a 1024 PPR encoder, speed is 30 RPM.

Use double-word arithmetic when the intermediate multiplication can exceed a 16-bit value.

Check in this order

  1. 1Confirm whether the encoder specification is pulses or counts per revolution.
  2. 2Latch and reset or subtract counts at an exact interval.
  3. 3Check rotation direction separately for quadrature input.

Ladder instruction example

DMUL D200 K60000 D210
DDIV D210 K1024000 D220

Example only—allocate devices and interlocks for your own project.

Devices and registers

D200: pulses sampled in 1000 ms
K1024000: 1024 PPR × 1000 ms
D220: whole RPM

Last reviewed: 2 October 2026

Official source: MELSEC iQ-F FX5 User's Manual (Application)

Related verified answers

How many built-in high-speed counter channels can an FX5U use?

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