National Instruments SCXI-1125 Operating in Parallel Mode

Chapter 4 Theory of Operation

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, Common Questions, 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, About the SCXI-1125. 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.