Chapter 4
Theory of Operation
SCXI-1502 User Manual
4-4
ni.com
The CJ SENSOR input channel is used to read the sensor temperature from
the terminal blocks that have one, such as the SCXI-1300 and SCXI-1303.
The temperature sensor is for cold-junction compensation of thermocouple
measurements. The CJ SENSOR channel also passes through a 5 Hz
lowpass filter to reject unwanted noise on the SCXI-1502. Along with the
other 32 input channels, the CJ SENSOR is multiplexed to the output
buffer, where it can be read by the DAQ device.
Theory of Multiplexed Operation
In multiplexed mode, all input channels of an SCXI module are
multiplexed into a single analog input channel of the DAQ device.
Multiplexed mode operation is ideal for high channel count systems.
Multiplexed mode is typically used for performing scanning operations
with the SCXI-1502. The power of SCXI multiplexed mode scanning is its
ability to route many input channels to a single channel of the DAQ device.
The multiplexing operation of the analog input signals is performed
entirely by multiplexers in the SCXI modules, not inside the DAQ device
or SCXI chassis. In multiplexed mode the SCXI-1502 scanned channels are
kept by the NI-DAQ driver in a scan list. Immediately prior to a multiplexed
scanning operation, the SCXI chassis is programmed with a module scan
list that controls which module sends its output to the SCXIbus during a
scan through the cabled SCXI module.
The list can contain channels in any physical order and the multiplexer can
sequence the channel selection from the scan list in any order. The ordering
of scanned channels need not be sequential. Channels can occur multiple
times in a single scan list. The scan list can contain an arbitrary number of
channels for each module entry in the scan list, limited to a total of
512 channels per DAQ device. This is referred to as flexible scanning
(random scanning). Not all SCXI modules provide flexible scanning.
The module includes first-in first-out (FIFO) memory for storing the
channel scan list defined in your application code. NI-DAQ drivers load the
FIFO based on the channel assignments you make in your application. You
need not explicitly program the module FIFO as this is done automatically
for you by the NI-DAQ driver.
When you configure a module for multiplexed mode operation, the routing
of multiplexed signals to the DAQ device depends on which module in the
SCXI system is cabled to the DAQ device. There are several possible
scenarios for routing signals from the multiplexed modules to the DAQ
device.