NI 6528/6529 User Guide and Specifications 8 ni.com
Features
The NI 6528/6529 features digital filtering, Real-Time System Integration
(RTSI), and change detection. The NI 6528 also features programmable
power-up output states and a watchdog timer.
Digital Filtering
Use the digital filter option available on the NI 6528/6529 input lines to
eliminate glitches on input data. When used with change detection, filtering
can also reduce the number of changes to examine and process.
You can configure the digital input channels to pass through a digital filter,
and you can program the filter interval the filter uses. The filter blocks
pulses that are shorter than half of the specified filter interval and passes
pulses that are longer than the specified interval. Intermediate-length
pulses—pulses longer than half of the interval but less than the
interval—may or may not pass the filter.
The filter operates on the inputs from the optocouplers. The optocouplers
turn on faster than they turn off, passing rising edges faster than falling
edges. The optocouplers can therefore subtract up to 150 s from a low
pulse.
Table 1 lists the pulse widths guaranteed to be passed and blocked.
You can enable filtering on as many input lines as is necessary for your
application. All filtered lines share the same timing interval, which ranges
from 400 ns to 200 ms.
Internally, the filter uses two clocks: the sample clock and the filter clock.
The sample clock has a 100 ns period. The filter clock is generated by a
counter and has a period equal to one half of the specified timing interval.
The input signal is sampled on each rising edge of the sample clock, which
is every 100 ns. A change in the input signal is recognized only if it
maintains its new state for at least two consecutive rising edges of the filter
clock.
Table 1. NI 6528/6529 Digital Filtering
Filter Interval
Pulse Width Passed Pulse Width Blocked
Low Pulse High Pulse Low Pulse High Pulse
t
interval
t
interval
+ 150 s t
interval
t
interval
/2 (t
interval
/2) – 150 s
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