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Chapter 2 Hardware Overview
NI 5911 User Manual 2-12 ni.com
PFI Lines as Inputs
You can select PFI1 or PFI2 as inputs for a trigger or a reference clock.
Please see the Synchronization section below for more information about
the use of reference clocks in the NI 5911.
PFI Lines as Outputs
You can select PFI1 or PFI2 to output several digital signals.
Reference Clock is a 10 MHz clock that is synchronous to the 100 MHz
sample clock on the NI 5911. You can use the reference clock to
synchronize to another NI 5911 configured as a slave device or to other
equipment that can accept a 10 MHz reference.
Frequency Output is a 1 kHz digital pulse train signal with a 50% duty
cycle. The most common application of Frequency Output for the NI 5911
is to provide a signal for compensating a passive probe.
Synchronization
The NI 5911 uses a digital phase locked loop to synchronize the 100 MHz
sample clock to a 10 MHz reference. This reference frequency can be
supplied by an internal crystal oscillator or through an external frequency
input through the RTSI bus clock line or a PFI input.
The NI 5911 may also output its 10 MHz reference on the RTSI bus clock
line or a PFI line so that other NI 5911s or other equipment can be
synchronized to the same reference.
While the reference clock input is sufficient to synchronize the 100 MHz
sample clocks, it is also necessary to synchronize clock dividers on each
NI 5911 so that internal clock divisors are synchronized on each different
device. These lower frequencies are important because they are used to
determine trigger times and sample position.
To synchronize the NI 5911 clock dividers, you must connect the digitizers
with an NI RTSI bus cable. One of the RTSI bus triggers must be
designated as a synchronization line. This line will be an output from the
master device and an input on the slave device. To synchronize the
digitizers, a single pulse is sent from the master NI 5911 to the slaves. This
pulse supplies the slave devices with a reference time to clear their clock
dividers. Hardware arming cannot be used during an acquisition using
multiple devices. For more information about synchronization, refer to
your NI-SCOPE Software User Manual.