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SCXI-1321 Bad Chips and Function Calls

8-4-04



This message is intended for the PSE of the SCXI-1321 and that person�s manager. Please route it accordingly. I would appreciate a phone response to 8**-2**-2*** so that I might share some specific experiences.

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To the managers of the SCXI product line,


I have used the SCXI-1121 with the SCXI 1321 for some time now, approximately 6 years. Our company generally uses them for full bridge pressure transducers and similar applications. I have some suggestions that would make them more saleable. I also believe that you will never know how many customers you have already lost on this product line, for the reasons given below.


1. For many years the shunt resistor on the 1321 has been vastly unusable. What SHOULD be possible is to command the shunt resistor through a Labview command, and then read the virtual channel to see the shunted value. That IS the purpose of the feature. What happens is that once you set the shunt command, then read the virtual channel, the very act of reading the channel UN-shunts the resistor. What you read is some value part way between, because the chip relay from Clare actually ramps open and closed, unlike a mechanical relay. If your timing loop to read the value is consistent, you will consistently read a BAD value. Several VI�s tried to command the shunt, then quickly read the virtual channel, then loop and do it again, and thought they had solved the problem. Further investigation showed that all that was really happening is that the readings appeared to be stable because of the fast loop time, but were actually mid-release of the shunt for all readings.
2. NI eventually starting saying �don�t use virtual channels with the shunt�. In fact, there is a channel string that starts with the word �shunt� that was recommended. The only problem with this is that the data that comes back is not scaled! Keep in mind that NI has been pushing �virtual channels� on users for many years as the way to do business, and so entire test stands are developed around them and MAX. Because we depend on MAX to do the scaling, how are we supposed to be able to read the data scaled? Does NI expect customers to duplicate the efforts of MAX for every virtual channel and scale it separately? Do I have to keep duplicate tracking of scaling information so that my virtual channels will work in the main code, but again elsewhere so that my calibration routine can read the data? This is messy, and for most customers, untenable.
3. I was told, literally, year after year and release after release, that the problem would be fixed in future versions of NI-DAQ. Each release proved that the problem was not resolved. I am sure that many customers were not repeat customers as they found the problems associated with using a simple shunt on the board. I myself needed this hardware due to the 1121/1321 parallel operation capability.
4. After years of trying to fight with the issues above, and what seemed like random problems with the shunting of channels, a second hardware related issue has finally been identified. See SRQ#600753 with Michelle Yagle. The Clare solid state relays installed on the 14 different 1321�s I bought ALL had the same defect. Imagine what this means for troubleshooting! You swap boards to try to troubleshoot, but they are all defective and so you falsely rule out the board, the problem didn�t resolve with a different board. The lot# 0027T12627 of chips covers a wide range of 1321 serial numbers (see my SRQ for details). That means this impacts a lot of potential customers. The part is rated by the manufacturer to 80C, 176F. This particular lot stops functioning as low as 96F! A great deal of testing with heat guns, shop air, and a thermocouple showed that all the chips in this lot fail somewhere between 96 and 110F, well below manufacturers specs or NI�s specs on the board. I also tested lot 98, from a board two years older, as well as new chips from lot 03, which both worked perfectly all the way up to 212F (100C).

Conclusions:

I believe that the people that have tried to use your 1321 have generally had a miserable time of it, unless they already do their own scaling (like some of the Alliance members do.) This is because the shunt feature is not compatible with virtual channels, because NI did not offer a reasonable fix for this problem over a four+ year period, and does NOT advertise this limitation when people are making hardware selections. Further, I believe there are many people that have boards with the same bad lot of chips on them that don�t know it. They may have elected to hardwire their own resistors after fighting with random problems (temperature related). They might have elected to hardwire their own resistors to get around the NI-DAQ limitation. They might not even use the shunt resistor. But one thing is certain: NI needs to clean up this board�s operation, make sure that customers of the serial numbers in my SRQ named above are notified of the bad chips, and fully test that the use of the relays is transparent to LV virtual channels in the future. People will not trust NI products with this type of problems, and will not buy the product again if all the features don�t work!

I have helped NI resolve similar issues with the SCXI-1126 years ago, which had a number of scaling issues in NI-DAQ (the board read great when you used 1k, 2k, 4k, 8k, etc. even ranges, but if you set a virtual channel to 0-2200, the readings were off.) I plotted the problem, documented it with my sales rep, and worked at length with tech support. The boards also had a lot of bad buffer amplifier chips that allowed cross talk between channels, causing a lot of other spurious problems. Those experiences were horribly frustrating, but I now buy the product with confidence. It took a lot of push on my part to convince NI there was a real underlying problem. This 1321 board is similar.

I want NI products to be the best they can be, as I have dedicated my career around them. Please take this feedback to heart, and contact me at 8**-2**-2*** to talk about these issues. Additionally, I am speaking at NI week this year and will be available for discussion. Contact me at the same number.

Sincerely,

Tim Jones
Test Equipment Design Engineer
Space Shuttle Program
Space, Land and Sea Enterprise, Hamilton Sundstrand
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Tim

Thank you for your feedback for the SCXI-1321 terminal block.

I wanted to see if it would be possible for you to move your application to NI DAQmx, the new driver for DAQ and SCXI as of NI-DAQ 7.0. I have tested the shunt calibration using a SCXI-1121 and SCXI-1321 and it works as it should. There are two ways to do the shunt calibration with DAQmx.

The first is to use the Calibration feature in the DAQ Assistant. When creating a DAQmx Task in either MAX or LabVIEW, you can select the �Device� tab and click on the Calibration button. It will then ask you if you either want to do a null or shunt calibration, or both. It will then do the calibrations selected and save the values to the task.

The other option to measure the shunted value is to use the AI.B
ridge.ShuntCal.Enable property node in LabVIEW. By setting this property to True, the driver will enable the shunt for the current measurements. If you are taking a strain measurement, the value will be converted to strain.

We are currently looking into the issues you are seeing with the Clare relays to see if other users could be affected.

Brian Lewis
Signal Conditioning PSE
National Instruments
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Brian,

Unfortunately the stand is government certified in LV 6.1, and cannot be migrated to the new DAQmx.

We have had some success in using the shunt with virtual channels by using the lower level DAQ vi's and placing the shunt command AFTER the AI Configure but before the AI Start. Still, without knowing what is under the hood, we can't be sure that the readings are going to be reliable. As you can see, the relay problems simply complicated matters.

I appreciated hearing back from the PSE. Thanks for your response.

Tim Jones
Space, Land and Sea Enterprise
Hamilton Sundstrand - United Technologies Corporation
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