Advantech PC-LabCard PCI-1784 - Manual (Page 54)

50
to 500 KHz quadrature input frequency.
A 3600 rpm motor with 2000 ppr encoder will nave a maximum
quadrature frequency of 3600Ч2000ч60＝120 KHz. In the above
example the 2 MHz sampling clock will have the noise immunity and
will meet the required input frequency.
The following table shows the maximum noise pulse width that the
filter will reject for each system clock frequency:
Clock frequency Maximum width
8 MHz 0.5 usec.
4 MHz 1 usec.
2 MHz 2 usec.
1 MHz 4 usec.
D.4 Latch mode
When you read a counter, you are actually reading a value latched into
a buffer. The PCI-1784 provides seven different latching modes, only
one of which is active at any given time. Make sure that you know
which latching mode is current whenever you read the counter.
Otherwise, you may read an old value or one that was latched at a
different time than you expect. You select the latching mode for each
channel individually.
The PCI-1784's latching modes are as follows:
S/W latch
Whenever you read a channel's data registers, the counter values will
be latched in buffer. Please refer to page 41 (BASE+28H) for detail
information.
Index latch
A rising edge on the channel's index input line will latch the channel's
counter value.