Programming instructions

Chapter 21 Common SCXI Applications
© National Instruments Corporation 21-21 LabVIEW Data Acquisition Basics Manual
If you are not using the DAQ Channel Wizard, there are special
considerations for addressing the channels. When you daisy-chain multiple
chasses to a single DAQ device (non-Remote SCXI), each chassis
multiplexes all of its analog input channels into a separate onboard analog
input channel. The first chassis in the chain uses onboard channel 0, the
second chassis in the chain uses onboard channel 1, and so on. To access
channels in the second chassis, you must select the correct onboard channel
as well as the correct chassis ID. The string
ob1!sc2!md1!0
means
channel 0 on the module in slot 1 of SCXI chassis 2, multiplexed into
onboard channel 1.
Remember to use the correct chassis ID number from
the configuration utility and to put the jumpers from the power supply
module in the correct position for each chassis.
When an MIO/AI Series device is cabled by a ribbon cable or shielded
cable to multiple chasses, the number of reserved analog input channels
depends on the number of chasses. On MIO Series devices, lines 0, 1,
and 2 are unavailable. On MIO-E Series devices, lines 0, 1, 2, and 4 are
unavailable. For more channel information refer to the LabVIEW
Online
Reference
, by selecting
Help»Online Reference…
.
When you access digital SCXI modules, you do not use onboard channels.
Therefore, if you have multiple chassis, you only have to choose the correct
SCXI chassis ID and module slot.
When you use Remote SCXI to address analog input channels, specify the
device number of the SCXI-1200 that is located in the same chassis
containing the analog input module from which you take samples.
You can perform DAQ operations on channels in multiple SCXI chassis at
the same time. For example, the first element of your
channels
array could
be
ob0!sc1!md1!0:31
, and the second element of the
channels
array
could be
ob1!sc2!md1!0:31
. Then, LabVIEW would scan 32 channels
on module 1 of SCXI chassis 1, using onboard channel 0, then the
32 channels on module 1 in SCXI chassis 2, using onboard channel 1.
Remember that the
scan rate
you specify is how many scans per second
LabVIEW performs. For each scan, LabVIEW reads every channel in the
channels
array. One restriction is that the channel list for each module must
be consecutive.
You can practice reading channels from different chassis by using the
channel strings explained above in the Easy VIs.