1746-ni8??

keving9

Member
Join Date
Mar 2004
Posts
33
I have a system that has been in service for over 7 years. Last week one of the level indicators was showing over range. Transmitter was working fine, putting out the correct ma for level but the engineering units in the SLC5/03 were incorrect. What it boiled down to was as long as I was using a 2 wire transmitter or simulator the eus were incorrect. If I use a 4 wire transmitter, everything is fine. Power supply is fine and as I said the correct ma are being put out but not read correctly when in 2 wire mode. A spare channel on this card reacted the same way. There is another ni-8 in this rack using the same power supply and some 2 wire inputs and it works fine. Almost has to be the ni-8 card, just wondered if anyone else had seen this. It's not convenient to shut the system down now to replace it. I was able to come up with a 4 wire xmitter temporarily. Comments?
 
When you say you are using a 2 wire simulator, and still having problems, do you mean feeding simulator directly into input? Or inplace of transmitter?
George
 
The 2 wire simulator is in place of the transmitter, using the same power supply as the transmitter. When I feed the current in without using the power supply, everything works.
 
Not knowing what simulator your using, I'll ask this question. Does it have an actual Transmitter simulator vs a 4-20mA out mode? Can you hook it up directly to plc input in 4-20mA out mode?

George
 
Yes it does have a transmitter simulator mode. I hooked it up right at the ni-8 card in that mode and got the inaccurate reading on the plc. However when I hook it up in the 4-20ma output(source)mode the plc reads it correctly.
 
The transmitter simulator mode assumes the power supply is in the loop. Going directly into the transmitter(SORRY should have said PLC), you do need to use 4-20mA out mode of calibrator. Unless the Analog Input module is different than the ones I have used in the past.

George
 
Last edited:
In the transmitter simulator mode with the power supply in the loop, the simulator puts out the correct ma but the plc doesn't read the correct ma. When I use the 4-20 ma output mode, using the internal power supply of the simulator it reads correctly.
 
OK, I'm now on the same page as you.:nodi: Is the Level transmitter set for Failsafe high? Can you turn that off for testing? If so, what does it now read?

George
 
I've removed that transmitter and replaced it with a 4 wire transmitter for the time being so I can't try that. But even with my transmitter simulator I wasn't getting correct readings.
 
The common element creating a problem seems to be the power supply that supplies the 2 wire transmitter. Not that it is bad, but that when it's used, it creates a problem.

If I read it right, when you don't use the 2 wire loop power supply, things work (like 4 wire transmitter).

When you do use the 2 wire loop power supply things don't work, like when you use the 2 wire transmitter or the simulator connected so that it uses the loop power supply.

All this sounds to me like a ground loop, common mode problem, where the introduction of the power supply for the 2 wire loop powered unit is producing a ground loop or common mode problem.

It is very typical for AI cards to accept inputs, up until the common mode rejection level is reached and then additional inputs cause problems.

Or you could have a flakey card.

A 4 wire transmitter could likely isolate its output from the power supply. Not guaranteed, but it could, hence it could work because it doesn't produce a ground loop.

A simulator could easily run off batteries. Does yours? Does the card read correctly when the simulator is connected to the input using the simulator's batteries?

The big question is why, if this is ground loop problem, did it work OK for 7 years, and then cause a problem.

I know "nothing's changed", but are you the only guy working on the system? Has any loop power power supply in the system been replaced?

Could someone have grounded a shield wire that wasn't grounded before?

Could a grounded shield wire be scrapping against a low side connection?

Could someone have grounded the negative 24Vdc return side?

As an aside, I use the term "engineering units" to mean degrees or psi or inches of level. When you say the "eng units aren't right", I assume you mean the level reading itself isn't correct, compared to the mA value and a known level in the tank. Right?

Dan
 
My simulator does run off batteries. It's actually a Fluke 787. When I use it as a transmitter simulator,ie putting it in the circuit in place of the level transmitter, it puts out the correct ma but the plc reads incorrectly. When I use it as a source ie using the battery of the Fluke to supply the current the plc reads correctly. I am the only person who works on this system although the possibility of somebody else doing something exists. I agree the power supply appears to be the common denominator, but there are actually 2 power supplies supplying 4 NI-8 cards and this is the only card with problem. I tried using the other power supply with the same results. A spare channel on this same card acts the same way making me lean towards a flaky card. I was curious if anyone else had seen this. We're to be down in about 6 weeks when it should be more convenient to shut the system down and replace the card. I guess I'll get by till then.
When I referred to engineering units I was actually referring to the channel data word which reads 3500 to 20500 corresponding to 3.5 to 20.5 ma.
 
Appendix A in the 1746NI8 user's manual gives the spec:
Common Mode Voltage Range: ±10.5V (21V maximum between any two terminals)

Note A-B's other statement under maxiumm cable impedance:

Current Source (transmitter properly wired to its power supply):
250 ohms maximum loop impedance, to meet common-mode voltage requirements.

This card has fairly poor isolation characteristics - it can tolerate only 10V of common mode voltage. That's not a lot in industrial environments.

When you connect the channel that is loop powered, the card's ±10.5V common mode is being exceeded, the card's amplifiers are being pulled one way or the other and you're getting wrong readings.

Everything you've done confirms this:

The AI card channel is functional and it can produce a proper reading with a floating, battery powered signal from the Fluke, because it's a floating input.

When any device, transmitter or simulator, which is loop powered provides the signal, the card reacts to the excessive common mode in the loop and gives a bad reading.

I'll go out on a limb here and say it isn't a bad AI card. I think it's working within its design limits. If you replace the card, I think you'll get the same results. It's a common mode grounding problem that's developed in the wiring/power supply.

If it were my plant, I'd put in a 4-20mA isolator (4-20mA in /4-20mA out, each side isolated from the other)

High odds that isolating the loop powered signal will give you a good reading on your existing card.

The alternative is to establish what changed and fix that. But trying to find the reason what's changed and why can be challenging, to say the least. Which is why people still use isolators.

Dan
 
Makes sense to me Dan. I'm certain the isolator would work. Hadn't thought of using one. I'll give it a try.
Thank you!
 

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