User manual

Chapter 3 Software Overview
NI-DAQ User Manual for PC Compatibles 3-126 ni.com
The SCXI-1200 module acquisition rate is limited by the rate at which your
PC can service interrupts from the parallel port. This is
a machine-dependent rate.
The filter setting on the SCXI-1100 and the SCXI-1122 dramatically
affects settling time. See the Specifications appendix in your SCXI module
user manual for details.
Note
The SCXI-1122 uses relays to switch the input channels; the module requires 10 ms
to settle when the relays switch, so the sampling rate in a channel scanning operation
cannot exceed 100 Hz. If you want to take many readings from each channel and average
them to reduce noise, you should use the single-channel or software-scanning method
shown in Figure 3-45 instead of the channel-scanning method shown in Figure 3-46.
This means you select one channel on the module, acquire many samples on that channel
using the DAQ functions, select the next channel, and so on. This increases the lifetime of
your module relays. When you have selected a particular channel, you can use the fastest
sample rate your DAQ device supports with the
DAQ
functions.
Analog Output Applications
Using the SCXI-1124 analog output module with the NI-DAQ functions
is simple. Call the
SCXI_AO_Write
function to write the voltages you
want to the module DAC channels. You can use the
SCXI_Get_Status
function to determine when the DAC channels have settled to their final
analog output voltages.
To calculate new calibration constants for
SCXI_AO_Write
to use for the
voltage to binary conversion instead of the factory calibration constants that
are shipped in the module EEPROM, follow the procedure outlined in the
SCXI_Cal_Constants
function description.
Table 3-13. SCXI-1200 Module Settling Rates
Gain
Maximum Acquisition
Rate
Settling Time
1 83.3 kS/s 12 µs
2to50 55 kS/s 18 µs
100 25 kS/s 40 µs