Tim Jones, Jason Kreienheder, Dan Biggs, Jaime Escobedo
We work for a large aerospace company, and utilize much of the NI offered hardware on multiple test stands. In addition, many of my colleagues have adopted similar design patterns on their stands. In short, we use a PXI chassis with an E-Series DAQ board, cabled to an SCXI-1001. Within the SCXI, the following boards are commonly utilized:
SCXI-1520�s and SCXI-1121�s for strain gauges
SCXI-1126 for Frequencies
SCXI-1102/1102B/1102C for temperatures and voltages
SCXI-1104�s
SCXI-1161 Relays
(and a few others less commonly.)
We have experienced, on MANY stands and with different independently working developers, seemingly random problems with incorrect readings. After much testing and trial and error, we have come to the following understanding of the situation:
1. If we start from a completely shutdown system, then turn on the SCXI and PXI chassis, and then the PC, we periodically get incorrect readings when scanning the SCXI channels.
2. If the readings are bad, they stay bad. If they are good, they generally stay good. Shutdown and restart may, or may not fix the bad readings.
3. It does not matter whether the readings are from a LabVIEW based DAQ program or through the test screen in Measurement and Automation Explorer.
4. It appears to continue through all versions of traditional DAQ, and LabVIEW from at least versions 4 through 6.1. The jury is still out on version LabVIEW 7.0 and 7.1 with traditional and DAQ MX.
5. SCXI chassis that had been powered on but unused for long periods of time (days or weeks) seems to go from reading correctly to reading incorrectly. We have not been able to identify any repeatable phenomenon, and the stands are significantly varied.
6. The reset button on the SCXI chassis, ONLY when pressed while NI-DAQ is completely unloaded from memory, seems to help remedy the bad readings. Using the LabVIEW based �Reset Device� VI with any configuration does not have the identical effect. Programmatically resetting the DAQ board, SCXI chassis, and SCXI individual modules, does not have the same effect as pressing the physical reset button. Some NI reps have stated that the Reset Device vi should have the same effect as the pushbutton, but that doesn�t appear to be the case in practice.
7. The BEST PRACTICE at this time is to power up the SCXI chassis, followed by the PXI chassis, and then the PC (note, dual processor Dell PC�s may need to be turned on within 8 seconds of the PXI/MXI-3 bridge for reliable boot, which shows as amber-green-amber-green on the Dell diagnostic LED�s.) Then log in and allow the MXI-3 optimizer software to finish (this should be part of a normal startup if you are using MXI-3.) Now go back to the SCXI chassis and press the reset button for about 1 second. This seems to clear any possible random bits, settings, or other problems that come up randomly during startup. Finally, run MAX or your LV software and take readings.
8. We primarily use Dell machines, which may be a common factor.
I welcome any and all comments regarding this problem and procedure, and the group of us are especially interested in hearing from NI as to how this could happen, and from other developers who have independently experienced this, whether resolved or not.
Tim Jones and colleagues.
