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Old December 10th, 2017, 04:58 PM   #1
cjd1965
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1734-ie8c raw value

Hi I have a 1784-IE8C which is configured for 4-20mA input, high engineering is 16383 and low enginering is 3277

The raw value from this channel is being read as -8. The scaling blow is 0-272 RPM but is showing -65.6 due to the -8

I have not done any trouble shooting yet so was looking for advice. I was going to connect 4mA from a calibrator and see what it read

I would say the card should read 3277 at 4mA so is -8 likely to be broken wire/under range?

I am on site tomorrow so can check diagnostics
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Old December 10th, 2017, 06:15 PM   #2
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I think you are already on it....

The -8 in that range is as close to zero you are likely to get with the calibration, so it looks to me like an open-circuit input, or a transmitter that isn't transmitting.

Go ahead and hook up the calibration source, but try to connect is as far into the field wiring as possible, and depending on the result of that, you will know which way to look for the problem.
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Old December 10th, 2017, 11:14 PM   #3
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A value of -8 is weird in this case.

In general, 4-20 mA modules from A-B present a broken wire condition by putting the value at the highest possible integer value for the channel, and (if available) setting an Under-Range bit.

The "out of range" value isn't the full scale value on a module that supports scaling, but rather the highest value that the datatype will support. On this module it would be hex 0x7FFF = 32767 decimal.

That's from memory, though, and it doesn't explain why cjd is seeing a negative value. There just shouldn't be a negative value available with these settings.

Check the under-range and over-range and channel fault tags when you get a chance, and have a printout of the Installation Instructions handy:

http://literature.rockwellautomation...n033_-en-e.pdf
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Old December 11th, 2017, 06:33 AM   #4
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Looking at appendix C of the user manual for the 1794-IE8 modules, when 4-20mA input mode is selected, the module internal scales the output data to 0000 @ 4.0 mA, and 7FFF @ 20 mA.

Your reading of -8 indicates your input 4/20 mA current is only just slightly below your "zero" range. It actually works out at 3.99585 mA

Your "current input" is actually measuring the voltage across a precision resistor switched into the circuit internally, the output value is then scaled as above. Note that the input voltage can go below the "zero-scale" value if the field transmitter doesn't accurately produce 4.00000 mA.

Analog instrumentation is never perfect, you will always get some errors, which can be caused by noise, digitising errors, poor calibration, temperature, etc.
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Old December 11th, 2017, 10:38 AM   #5
cjd1965
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Hi thanks for the input

I didnt go today because of the weather

I have attached a spreadsheet of the scaling, where 3277 counts is 0RPM so i can confused as to how -8 counts is only just less than 0 rpm
Attached Files
File Type: pdf scaling.pdf (40.6 KB, 9 views)
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Old December 11th, 2017, 12:27 PM   #6
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Let's be sure we are talking about the same module and eliminate any part number confusion. What I'm reading is that you have a 1734-IE8C, and that the "1784" above is a typo.

The 1734-IE8C is the POINT I/O platform 8-channel Analog Input for Current signals.

The 1794-IE8 is a FLEX I/O platform 8-channel Analog module, with separate terminals for voltage or current inputs.

POINT modules have onboard integer scaling with adjustable Min and Max values, while the older FLEX platform has only two configuration bits to select the input type and range.

FLEX is also a 12-bit module that left-justifies the data, so the first four bits are always zero and a module with only one signal bit would look like "8" in a raw view.

So let's clarify what module we're looking at.
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Old December 11th, 2017, 12:51 PM   #7
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Hi Ken
The exact module from the hardware config is 1734-IE8C/C

All the channels are setup the same, to 4-20mA , high engineering is 16383 and low enginering is 3277 and the scaling is done in the PLC code

I see i mis-entered in my first post of this thread, apologies
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Old December 11th, 2017, 05:03 PM   #8
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Cross-reference that Input tag and make sure you aren't inadvertently writing over or forcing it in some way.

A negative value simply should not be possible on that module. 4 mA should give you 3277.
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Old December 11th, 2017, 06:17 PM   #9
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What processor, what network, and what data format?
Anyway, it looks like there is no clamping by default, although I can't find a user manual for the ie8c only the installation instructions.
It will go as low as you tell it, it doesn't clamp at 4 mA, so the -8 would be consistent with a shorted input, and a small induced current.
Good luck.
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Old December 17th, 2017, 05:43 AM   #10
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Update

Hi it was a field wiring issue , it is a new intallation , done by contractors etc

I didnt help that the cards alarm features were disabled so the over/under range wasnt alarming on the HMI

thanks for the advice
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