High Speed I/O
High Speed Counter (HSC) Functions
Four dedicated inputs are available that can be configured for one of four modes of operation. Those modes are Fre-
quency, Count (totalize), Pulse width or period (pulse) and Quadrature measurement. For some modes, more than
one HSC input may be consumed. The measurement value is provided to ladder in a %AI register.
NOTE: While the high-speed input circuitry has a resolution of 1μs, measured edge transitions must not occur faster
than 100μs for accurate measurements. Keep in mind that pulse width measurements utilize both the rising and fall-
ing edges of the waveform, thus the pulse width must exist longer than 100μs.
NOTE: The edge polarity selection in the mode parameter for the totalize and pulse width functions (Digital/HSC
Input Configuration) assume Positive Logic regardless of the associated I/O board’s jumper setting for the Digital
DC inputs polarity. If Negative logic is configured when using these functions, the opposite edge polarity must be
selected in the mode parameter.
Frequency
In frequency mode, the frequency of the input signal is written to the accumulator in terms of Hertz (cycles/second).
When using frequency mode, four update selections are provided which specify the width of the sample window.
NOTE: Selecting a shorter sample window provides a quicker measurement (faster response) but lowers the fre-
quency accuracy (resolution) and increases the minimum frequency measurement limit.
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