SCXI Cascading: SCXI-1121 to SCXI-1140

Cascading SCXI modules gives users greater flexibility in conditioning analog signals. It allows you to combine the functionality of multiple SCXI modules to perform one data acquisition process. This example program demonstrates the use of two SCXI modules in a cascaded system. The first stage is an SCXI-1121, which provides high voltage isolation and signal amplification, as well as excitation and bridge completion for transducers that require these features. The output signals from the SCXI-1121 are passed to the SCXI-1140. The SCXI-1140 sample and hold module allows all 8 channels to be sampled simultaneously, when using a non-simultaneous sampling data acquisition device.

While cascading SCXI modules provides additional opportunities for creating complete signal conditioning solutions, users should excercise greater care when configuring and calibrating a cascaded SCXI system. Improper programming of cascaded modules can actually lead to decreased accuracy in your measurements. Cascading SCXI modules is thus recommended only for more advanced users.


The following rules should be followed when using cascaded SCXI examples:
1) Do not input virtual channels
2) No thermocouple applications
3) No current-input
4) No calgnd as a generic channel string, but calgnd may be used within an application
5) No channel strings using shunt, cjtemp, or mtemp
6) No random sampling
7) All Channels of a module must have the same gain
8) No RDA
9) DAQ devices and 1st stage modules have their gains adjusted; all others modules have gain of 1
10) No 4Hz filtering
11) Not guaranteed to work with SCXI-1200 or remote SCXI

Hardware Setup:
Use the SCXI-1352 cascading kit to connect the 1121 to the 1140. Make sure to use the cable with two 50-pin connectors, and make sure the red wire is connected to the pins closest to the sidewall of the 1140's case. If applicable, connect the 68-pin cable to the DAQ card and the back of the 1140 (which may require an adapter). In addition, verify all jumpers are set to the desired configuration.

NI MAX configuration:
Set the gains and filter settings according to the hardware configurations. It is recommended to perform a test verifying that each module is working. Make sure the 1121 is configured in parallel operating mode. Configure the terminal block gain, if applicable.

VI Settings:
Device: Device number for the DAQ device controlling the cascaded modules
Input Limit: Expected voltage range of input signals
Scan Rate: Samples per second
Buffer Size: Number of buffers to be stored
Scans to Read at a Time: Number of scans retrieved for each iteration of AI Read
Interchannel Delay: Time to wait before acquiring data form the next channel

How the VI Functions:
In a cascaded module, LabVIEW cannot directly account for the gains from the 1st stage module (SCXI-1121). Instead, LabVIEW communicates with the second stage module (SCXI-1140) to acquire data. This VI programmatically retrieves the gains for the 1120 module and multiplies these gains by the user supplied input limits, to provide the "apparent" input limits for the DAQ device. After reading the scans from the buffer (AI Read.vi), the data are divided by the 1121 gains to correctly scale the readings.

The VI will temporarily set the operating mode of the SCXI-1121 and the SCXI-1140 to multiplex mode, because this operating mode allows LabVIEW to retrieve the gains for the SCXI-1121. The SCXI-1121 operating mode is then set back to parallel, which allows the cascaded modules to begin data acquisition. The program will stop if the STOP button is pressed or a fatal error occurs.

Requirements

Filename: 1121_1140.llb

Software Requirements

Application Software: LabVIEW Full Development System 6.0
Language(s): LabVIEW

Hardware Requirements

Hardware Group: Multifunction DAQ (MIO)
Driver: Traditional NI-DAQ (Legacy) 6.9.1

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