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19 BUILT-IN I/O FUNCTION
19.1 High-speed Counter Function
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Precautions
■Count direction switch during measurement
The rotational speed measurement mode calculates rotational speed based on current value difference of high-speed
counters in the measuring unit time. You should therefore note that the input number of pulses may differ from the
measurement value when count direction of a high-speed counter is switched within the same measuring unit time.
■Operation when counting in the minus direction
Rotational speed can also be measured when pulses are input in the direction whereby current value of high-speed counter is
reduced.
■Operation at overflow of high-speed counter current value
Rotational speed measurement can continue even when current value of high-speed counter overflows during measurement.
■Relationship with the SPD instruction
Measurement time specified by operand of the SPD instruction is written in the special register for measuring unit time used
by the rotational speed measurement function. Measurement results of the SPD instruction are also stored in the special
register of measurement results.
If rotational speed measurement has already been started by the HIOEN instruction, the SPD instruction cannot be used for
the same channel.
Inversely, if pulse density is currently being measured by the SPD instruction, rotational speed measurement cannot be
started for the same channel.
■Other precautions
There are common precautions when using high-speed counters. For details, refer to Page 163 Precautions when using
high-speed counters.