Advantech PC-LabCard PCI-1784 - Manual (Page 53)

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2-pulse mode
In 2-pulse mode the PCI-1784 uses two input pulses as counting
sources: one for clockwise (CW) and one for counterclockwise (CCW)
counting. The counter will increment whenever a rising edge occurs on
channel A. It will decrement whenever a rising edge occurs on channel
B.
pulse/direction mode
In pulse/direction mode the PCI-1784 uses one input line (A) for pulse
input and one line (B) for direction. If channel B is high (1), the
counter will decrement whenever a rising edge occurs in channel A. If
channel B is low (0), the counter will increment whenever a rising
edge occurs on channel A.
Disabled mode
PCI-1784 will not accept input, but you can access all its registers.
You select the mode by programming the card's registers: BASE+00H
for CH0, BASE+04H for CH1, BASE+08H for CH2 and BASE+0CH
for CH3. See Appendix C for more information.
D.3 Digital noise filter
Noise immunity is the most important requirement for reliable encoder
interface operation. The PCI-1784 conditions the input signal with a
four stage digital filter. This filter reduces glitches (digital noise) or
spikes by sampling by sampling the input at 1, 2, 4 or 8MHz. The filter
output waveforms change only when an input has the same value for
four consecutive sampling edges. The filter thus rejects noise or pulses
shorter than four sampling clock periods. You can optimize noise
immunity by selecting the lowest sampling frequency that compatible
with the highest input rate you expect.
The PCI-1784 accepts up to 2 MHz quadrature frequency at 8 MHz
filter sampling speed. At 2 MHz sampling speed it can still accept up