Chapter 4
Theory of Operation
4-10
ni.com
Theory of Multiplexed Mode 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-1521/B.
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-1521/B 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.
If the scanned SCXI-1521/B module is not directly cabled to the 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 DAQ device through the AI 0 pin
on its rear signal connector.
If the DAQ device scans the cabled module, the module routes its input
signals through the AI 0 pin on its rear signal connector to the DAQ
device AI 0.
