Chapter 4
Theory of Operation
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National Instruments Corporation
4-3
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 thermocouple
measurements. The CJ SENSOR channel also passes through a 2 Hz
lowpass filter to reject unwanted noise on the SCXI-1102/B/C. 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
The SCXI-1102/B/C operates in multiplexed mode. which means that all
input channels of the SCXI module are multiplexed into a single analog
input channel of the E/M Series DAQ device. Multiplexed mode operation
is ideal for high channel count systems. The power of SCXI multiplexed
mode scanning is its ability to route many input channels to a single channel
of the E/M Series DAQ device.
The multiplexing operation of the analog input signals is performed
entirely by multiplexers in the SCXI modules, not inside the E/M Series
DAQ device or SCXI chassis. The SCXI-1102/B/C 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, but the SCXI-1102/B/C
multiplexer can only sequence the channels in the order 0, 1..31. The
SCXI-1102/B/C cannot skip channels or scan channels in random order.
The ordering of scanned channels must be sequential. The scan list is
limited to a total of 512 channels per E/M Series DAQ device.
When you configure a module for multiplexed mode operation, the routing
of multiplexed signals to the E/M Series DAQ device depends on which
module in the SCXI system is cabled to the E/M Series DAQ device. There
are several possible scenarios for routing signals from the multiplexed
modules to the E/M Series DAQ device.
If the scanned SCXI-1102/B/C module is not directly cabled to the E/M
Series DAQ device, the module sends its signals through the SCXIbus to
the cabled module. The cabled module, whose routing is controlled by the
SCXI chassis, routes the SCXIbus signals to the E/M Series DAQ device
through the CH 0 pin on its rear signal connector.
