I have an application where I need to monitor a number of bridge-out load cells.
The cells use 350 ohm bridges and I’ll need to use an external power source. The cabling is about 30 feet between the NI 9237 and the load cells; so I plan to wire the remote sense lines. I’m using the D-sub version of the NI 9237 in a cRIO chassis using the FPGA interface mode.
As I understand the shunt calibration feature for the NI 9237 module, I will be able to simulate a change in the load value by activating the SC function. After reading through the Operating Instructions and Specifications document for the NI 9237, it remains unclear how to wire the shunt calibration resistor. As I understand it, the resistor needs to be wired between the SC+ terminal of the 9237 and the –Sig lead of the bridge. Please refer to the attached and confirm if I have the wiring scheme correct. From what I gather, the SC Switch is activated when the shunt cal function is activated within the 9237 module. Activation is programmatically done through an I/O node.
I realize that the lead titled SC- on the load cell is not necessary. However, it’s shown wired this way because the load cells have the contact available on the connector and the wiring is available in the cabling.
Another question concerns the activation of the shunt cal function. As I understand it, the 9237 module needs to first turn off through an I/O node before the shunt cal state can be changed. The SC state is then changed whereupon the 9237 is turned back on again through an I/O node. Do I have this right or is there more or less to it?
Dave
