Chapter 3 Theory of Operation
SCXI-1121 User Manual 3-18 www.natinst.com
entry. The list in Slot 0 will determine which module is being accessed and
for how many samples. It is important that you make sure that the lists on
the data acquisition board and Slot 0 are compatible so that the samples are
acquired as intended. See your SCXI chassis manual for more information.
Single-Module Multiplexed Scanning

Single-Module Multiplexed Scanning (Direct)

This is the simplest multiplexed scanning mode. Directly cable the
SCXI-1121 to the data acquisition board as shown in Figure 3-8. The
module sends SCANCLK onto TRIG0, and Slot 0 sends SCANCON back
to the module. SCANCON will be low at all times during the scan except
during changes from one Slot 0 scan list entry to the next, when
SCANCON pulses high to make the MUXCOUNTER reload its starting
channel. Notice that although you are using only a single module, you can
put many entries with different counts in the Slot 0 FIFO, so that some
channels are read more often than others. You cannot change the start
channel in the module Configuration Register during a scan.
Figure 3-8. Single-Module Multiplexed Scanning (Direct)

Single-Module Multiplexed Scanning (Indirect)

In this mode, the SCXI-1121 is not directly cabled to the data acquisition
board. Instead, you connect another module to the data acquisition board,
and the analog output of the SCXI-1121 is sent over Analog Bus 0, through
the intermediate module, and then to the data acquisition board. The
SCXI-1121 receives its MUXCOUNTER clock from TRIG0, which is sent
by the intermediate module, as illustrated in Figure 3-9. Slot 0 operation is
the same for direct connection scanning.
Cable
Assembly
Data Acquisition Board
Analog Input
Timing Output
TRIG0
SCANCON X
SCANCLK
MCH0
SLOT 0
Four Isolated
Floating
Single-Ended
Inputs
SCXI-1000 or SCXI-1001 Chassis
SLOT X
SCXI-1121