Chapter 2
Hardware Overview
2-12
ni.com
PFI Lines as Inputs
You can select PFI 1 or PFI 2 as inputs for a trigger or a reference clock.
Refer to the
Synchronization
section for more information about the use of
reference clocks in the NI 5911.
PFI Lines as Outputs
You can select PFI 1 or PFI 2 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 can also output its 10 MHz reference on the RTSI bus clock
line or a PFI line so that additional NI 5911 devices 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 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 RTSI bus cable. One of the RTSI bus triggers must be designated
as a synchronization line. This line is 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.
To synchronize the triggers of multiple NI 5911 devices, one digitizer must
receive a trigger as described in the