Chapter 1
Installing and Configuring the SCXI-1126
©
National Instruments Corporation
1-5
Multiplexed Mode (Recommended)
In multiplexed mode, an E Series DAQ device connects to only one module
in a chassis and reads from CH0 on the rear signal connector. If the
SCXI-1126 is the cabled module, all of the SCXI-1126 channels are
multiplexed onto CH0 along with the signals from other SCXI modules that
are multiplexed on the SCXIbus. If the SCXI-1126 is not the cabled
module, all of its channels are placed on the SCXIbus and are then sent to
the E Series DAQ device through CH0 on the cabled module. The output
signals for channels 1 through 7 on the SCXI-1126 are always sent to the
rear signal connector and drive pins 5 through 18. In multiplexed mode,
these output signals are ignored by the E Series DAQ device and are
short-circuit protected. Use multiplexed mode if your sampling rate is
<333 kS/s. Most applications sample rate is
≤
10 kHz.
You also can use multiplexed mode to perform scanning operations with the
SCXI-1126. The SCXI chassis is programmed with a module scan list that
dynamically controls which module sends its output to the SCXIbus during
a scan. You can specify this list to scan the modules in any order, with an
arbitrary number of channels for each module entry in the list. When
scanning the SCXI-1126, you can start scans with any channel, but you
must scan the channels in a consecutive, ascending order.
Note
The SCXI-1126 parallel outputs continuously drive the rear signal connector pins
even when the module is configured in multiplexed mode.
While in multiplexed mode, the minimum scan interval is 3
µ
s. This is the
smallest interval in which you can switch between analog channels on the
module and still measure accurate voltages; specifically, the accuracy at
3
µ
s is 0.012% of full scale. The 3
µ
s scan interval gives you a maximum
sampling rate of 333 kHz. Because this rate is much higher than the 1 kHz
maximum output bandwidth of a single SCXI-1126 channel, you can
sample multiple channels on multiple SCXI modules without
undersampling one of the SCXI-1126 channels. Refer to Appendix A,
, for more information.
