MAN0878-09-EN_XLE_XLT_UserManual
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12.2 High Speed Counter (HSC) Functions
On units that support the HSC, four dedicated inputs are available than can be configured for
one of four modes of operation. Those modes are Frequency, 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 (see mapping
below).
NOTE: While the high-speed input circuitry has a resolution of 1
μ
s, measured edge transitions
must not occur faster than 100
μ
s for accurate measurements. Keep in mind that pulse width
measurements utilize both the rising and falling edges of the waveform, thus the pulse width
must exist longer than 100
μ
s.
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.
12.2.1 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 frequency accuracy (resolution) and increases the minimum frequency
measurement limit
. In this mode, the Disable and Latch special functions are allowed.
Please see
Section 12.2 for a description of these functions.