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Chapter 3
Hardware Overview
The remainder of this chapter describes how these signals are used, acquired, and generated by
the NI PXI-665
x
hardware, and explains how you can route the signals between various
locations to synchronize multiple measurement devices and PXI chassis.
Clock Generation
The NI PXI-6653 and NI PXI-6652 can generate two types of clock signals. The first clock is
generated using the onboard DDS chip, and the second is generated with a precise 10 MHz
oscillator. The following sections describe the two types of clock generation and explain the
considerations for choosing either type.
Direct Digital Synthesis (DDS)
DDS is a method of generating a clock with programmable frequency. DDS consists of a
frequency tuning word, an accumulator, a sine-lookup table, a D/A converter (DAC), and a
comparator.
The frequency tuning word is a number that specifies the desired frequency. Each master clock
cycle, the frequency tuning word is added to the accumulator, which rolls over when it gets to
its maximum value. The accumulator value is used to get a point in the sine-lookup table, which
is converted to an analog voltage by the DAC. For example, if the sine table is 128 points long,
PXI_STAR <0..12>
In/Out
The PXI star trigger bus connects Slot 2 to
Slot <3..15> in a star configuration. The electrical
paths of each star line are closely matched to
minimize intermodule skew. An NI PXI-665
x
in
Slot 2 can route signals to Slots <3..15> using the
star trigger bus.
PFI <0..5>
In/Out
The Programmable Function Interface pins on the
NI PXI-665
x
route timing and triggering signals
between multiple PXI chassis. A wide variety of
input and output signals can be routed to or from the
PFI lines.
PXI_TRIG <0..7>
In/Out
The PXI trigger bus consists of eight digital lines
shared among all slots in the PXI chassis. The
NI PXI-665
x
can route a wide variety of signals to
and from these lines.
Note
: PXI_TRIG <0..5> are also known as
RTSI <0..5> in some hardware devices and APIs.
However, PXI_TRIG <6..7> are
not
identical to
RTSI <6..7>.
Table 3-3.
Signal Descriptions (Continued)
Signal Name
Direction
Description
