SCXI 1140 readings fluctuate between correct value and zero

[Chris Morison]

I have a SCXI Chassis with two 1531 modules, one 1520 module and one 1140 module, controlled by custom written software written in Labwindows/CVI.

When tested in the UK lab it all worked fine. When taken into the field in Romania I got strange signals from the 1140 (voltage input) module - at about a 50Hz cycle the signal would rapidly switch between zero and the correct signal. The signals from the other 3 modules were fine.

I assumed it had been damaged in transit, but when I got back to the lab it all works fine again.

The only difference between the two situations was the power supply level. Both were AC mains, UK was 240V, Romania was 220V, and I remembered to change the voltage selection wheel on
the supply to the SCXI chassis.

The symptoms are identical to those mentioned here but I'm not using DAQmx, and I only get the problem with the 1140 module.

I'm reluctant to turn the voltage selection wheel back to 220V when it has a 240 V input, and can't think of any way to recreate the situation I had in Romania.

Does anyone have any ideas on what could be causing this problem, or how I could test it locally?

Thanks.

[Steven Bird]

Chris, I am unsure why this should happen. However there are various power options available to you that you can convert your chassis to that NI sell (power cords). The Swiss 220VAC may help the situation if this is a power source issue.

What frequency is the supply in Romania?

I seems very strange that when you bring the platform back to the UK everything seems fine again? Could this be a loose connetion or way is very periodic and conclusive?

[Chris Morison]

The Romanian supply is 50 Hz.

In Romania I pulled the 1140 module out and checked that all the dip switches were firmly in place, and nothing else seemed loose.

The fault was extremely periodic, and was at exactly 50 Hz. It's as if there was only an output signal when the supply voltage was positive, and when the supply voltage was negative the output signal was zero.

I would really like a way to check the module in the lab with a 220 V supply.