Chapter 4 Theory of Operation
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National Instruments Corporation
4-3
Multiplexed Hardware Operation Theory
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 module being scanned 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 CH0 signal on the rear signal
connector. If the E/M Series DAQ device scans the cabled module, the
module routes its input signals through the CH0 signal on the rear signal
connector. The power of SCXI multiplexed scanning is its ability to route
many input channels to a single channel on the E/M Series DAQ device.
Note
The SCXI-1125 parallel outputs continuously drive the rear signal connector output
pins even when you configure the module in multiplexed mode. If the module is cabled to
an E/M Series DAQ device in multiplexed mode, the differential inputs 1 through 7 on the
E/M Series DAQ device cannot be used for general-purpose analog input. Refer to
Appendix D,
, for more information on available pins on the rear signal
connector.
Multiplexed mode is typically used for performing scanning operations
with the SCXI-1125. 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. You
can specify this list to scan the modules in the chassis in any order, with an
arbitrary number of channels for each module entry in the list. You can
randomly scan the channels on the SCXI-1125, meaning channels can be in
any order and occur multiple times in a single scan. When performing
multiple scans, the list pointer of the module automatically wraps around
and starts scanning with the first channel in the scan list.
Operating in Parallel Mode
You can configure the SCXI-1125 to operate in parallel mode as described
in Chapter 1,
. In parallel mode, all eight analog
output channels on the SCXI-1125 are connected to eight analog input
channels on the E/M Series DAQ device. The CJC channel is not
accessible. Every SCXI-1125 configured for parallel mode must have a
E/M Series DAQ device directly cabled to it.
