The recommended excitation voltage for my pressure transducers is + 10 VDC. I prefer to use external excitation as per the NI 9237 manual, total power for the module is limited to 150 mW, which in my case is "Four 350 ohm full bridges at 3.3 V". (I will be using (4) full bridge 350 ohm pressure transducers). I am using Scan Engine for EtherCat and have the NI 9237 in a NI 9144 Chassis connected to a cRIO-9081. For troubleshooting, I have two transducers on a pressure calibrator and another RJ50 connection to a NI 9949 breakout. If I select "External Excitation" and have my 10 V power supply connected to the lower EX+, EX- connector, I get an invalid value for my pressure transducer. This is after scaling the value in calibration. The Raw Value is also incorrect. If I leave it set to "External Excitation" but pull the EX+, EX- connector, the EX+, EX- voltage (pins 6 & 7) will go to 4.735 V and I will get a CORRECT pressure reading. If I select Internal Excitation of 3.3V, I will get a correct pressure reading, but I am a little concerned about resolution as the recommended excitation is + 10 VDC for the pressure transducers. My question is: why am I unable to use the external excitation? I have another application with a PXI and EtherCAT chassis and it works OK. I've also tried placing a NI 9237 module in the cRIO-9081 chassis and get the same invalid value that I describe above.
