National Instruments NI 6115/6120, Руководство пользователя

Страница: 75 / 127

Chapter 4 Connecting Signals
NI 6115/6120 User Manual 4-38 ni.com
The GATE and OUT signal transitions shown in Figure 4-36 are referenced
to the rising edge of the SOURCE signal. This timing diagram assumes that
the counters are programmed to count rising edges. The same timing
diagram, but with the source signal inverted and referenced to the falling
edge of the source signal, would apply when the counter is programmed to
count falling edges.
The GATE input timing parameters are referenced to the signal at the
SOURCE input or to one of the internally generated signals on the
NI 6115/6120. Figure 4-36 shows the GATE signal referenced to the rising
edge of a source signal. The gate must be valid (either high or low) for at
least 10 ns before the rising or falling edge of a source signal for the gate to
take effect at that source edge, as shown by t
gsu
and t
gh
in Figure 4-36. The
gate signal is not required to be held after the active edge of the source
signal.
If you use an internal timebase clock, the gate signal cannot be
synchronized with the clock. In this case, gates applied close to a source
edge take effect either on that source edge or on the next one. This
arrangement results in an uncertainty of one source clock period with
respect to unsynchronized gating sources.
The OUT output timing parameters are referenced to the signal at the
SOURCE input or to one of the internally generated clock signals on the
NI 6115/6120. Figure 4-36 shows the OUT signal referenced to the rising
edge of a source signal. Any OUT signal state changes occur within 80 ns
after the rising or falling edge of the source signal.
FREQ_OUT Signal
This signal is available only as an output on the FREQ_OUT pin. The
frequency generator for the NI 6115/6120 outputs the FREQ_OUT pin.
The frequency generator is a 4-bit counter that can divide its input clock by
the numbers 1 through 16. The input clock of the frequency generator is
software selectable from the internal 10 MHz and 100 kHz timebases. The
output polarity is software selectable. This output is set to high-impedance
at startup.