SCXI Quick Start Guide 30 ni.com
Can I programmatically detect whether or not my chassis is
powered on?
You can determine whether or not you can communicate with configured
modules. In NI-DAQmx, use a LabVIEW VI that you can obtain from
ni.com/express by typing rdtscp, or in Traditional NI-DAQ (Legacy),
use
SCXI_ModuleID_Read.
All my channels float to a positive rail when I try to take a
measurement. How do I correct the problem?
Make sure that the signal reference settings for the DAQ device match the
SCXI module. For example, if the device is configured for NRSE, make
sure that the cabled SCXI module shares the same configuration. Matching
configurations can require a change to the jumper setting of the module.
I am acquiring measurements with an SCXI-1100 or SCXI-1122 that
are incorrect or have crosstalk between channels. What could be
causing incorrect measurements?
If you have enabled the 4 Hz filter on the SCXI-1100, are using high gains,
or have the interchannel delay set to a fast rate, the amplifier may not have
enough time to settle for each channel. NI-DAQmx automatically sets the
correct interchannel delay. However, in Traditional NI-DAQ (Legacy), the
default interchannel delay is automatically set to the shortest interval
possible for the DAQ device, but no faster than 3 ms if you are using
multiplexed mode. This is called the scan interval. You can specify a longer
interchannel delay to allow the filter and amplifier to settle to desired
accuracy. Remember that interchannel delay is set separately from scan
rate. The following equation is suggested for determining a suitable delay
value.
Check the device user manual for accuracy and settling time specifications.
I am using one of the following modules—SCXI-1100, SCXI-1102/B/C,
SCXI-1112, or SCXI-1125—with one of the following terminal
blocks—SCXI-1300, SCXI-1303, or SCXI-1328—to measure
temperature with a thermocouple. How do I stop the thermocouple
reading from fluctuating?
Average the temperature readings to minimize fluctuations. Also, ensure
proper field wiring techniques. Most thermocouples are floating signal
sources with low common-mode voltage; they require a path for bias
currents from the SCXI module amplifier to ground. Make sure that you
have grounded the negative lead of each floating thermocouple through a
resistor. Refer to the terminal block documentation for impedance values.
For grounded thermocouples, ensure that there is not a high common-mode
voltage present on the thermocouple ground reference.
Interchannel Delay
1
Scan Rate Number of Channels×
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