Chapter2 HardwareOverview
©NationalInstruments Corporation 2-15 NI5911 User Manual
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. Youcan use the reference clock to
synchronize to another NI 5911 configuredas a slave device or to other
equipmentthat can accept a 10 MHz reference.
FrequencyOutput is a 1 kHz dig ital pulse train signal with a 50% duty
cycle.The most common application of Frequency Output for the NI 5911
is to providea sig nal for compensating a passive probe.
Synchronization
TheNI 5911 uses a digital phase locked loo pto sy nchronize the 100 MHz
sampleclock to a 10 MHz reference. This reference frequency can be
supplied by a crystal oscillator on the board or through an external
frequency input through the RTSI bus clock line or a PFI input.
TheNI 5911may also o utput its 10 MHz reference on the RTSIbus clock
line or a PFI line so that other NI 5911 boards or other equipment can be
synchronized to the same reference.
While the reference clock input is sufficientto synchronize the 100 MHz
sampleclocks, it is also necessary to synchronize clock dividers on each
NI5 911 so that internal clock divisorsare also synchronized on th e
differentboards. These lower frequencies are important because they are
used to determine triggertimes and sample position.
Tosynchronize the NI 5911 clock dividers, you must connect the boards
with a National Instruments RTSIbus cable. One of the RTSI bus triggers
must be designated as a synchronization line. This line will be an output
fromthe master board and an input o n the slaveboards. To synchronize the
boards,a singlepulse is sent from the master to the slaves,which gives
them a reference time to clear the clock dividerson the boards. Hardware
arming cannot be used during a multiple board acquisition.