ControlLogix processor w/1794 Flex remote I/O

Christoph

Member
Join Date
Sep 2003
Location
Indiana
Posts
345
I have an application with a rack of flex i/o on RIO
My issue is
I am set up for 4-20 mA input. my measurement device when at
stand still fluctuates .021mA and does so when incrementing up
or down. This causes my data to change by 1000 counts. Is
there a way to filter that noise out? module is a 1794-IE4XOE2
 
Do you have shielded wiring grounded at one end?

Or apply a filter with a little math. See text below.
 
What is your scaling? (What are you scaling the 4-20mA to?)

That sounds like noise. .021mA is only a range of 4 steps for the module's A/D converter. 12 bit A/D, = 4096 steps. Maximum range is 21mA, so 1 step = 0.00512mA.

Strangely, the spec sheet shows a resolution of 5.12uA (Microamps), I seriously doubt that to be correct.
 
And what is the best way to scale the signal a scl block? that is the only option I see for this configuration. with no scaling I see data counts as high as 31500. If this is 12 bit I would not expect to see that higher than 4095 but I do.
 
I connected my Fluke meter which sources 4-20 mA and the
data in the plc was still noisy. Not sure how to proceed at this point.
 
I connected my Fluke meter which sources 4-20 mA and the
data in the plc was still noisy. Not sure how to proceed at this point.

Then filter it, see post #2.

Scaling to engineering units is a separate issue.

See page c-2 of the manual below for raw data values for a given input range. (note values are in hex. Go figure why Allen Bradley tech writers did that)

4ma = 0000 hex = 0 counts decimal

20ma = 7878 hex = 30840 counts decimal

https://www.google.com/url?sa=t&rct...dRh-qy4nvrwda8ZHpETSnXA&bvm=bv.45645796,d.cGE
 
Last edited:
Page C-2 of that manual is the most important one for understanding this module's data encoding.

FLEX Analog modules have an unusual characteristic: they left-justify the data bits into a 16-bit signed word.

This means that the least-significant bits 0, 1, 2, (and 3 if you're using bipolar mode) are always zero. When you are looking at the raw integer value, a single A/D increment looks like a value of 8.

There are quite a few ways to deal with the analog data in the FLEX module. You can just scale from minimum to maximum across the analog range, or you can bit-shift the lower 15 bits so that the value is a more 'conventional' 0-4095 like it would be with an SLC-500 analog module.

I don't have one of these modules to test with; maybe 4 counts is an ordinary A/D noise floor fluctuation, or maybe Christoph's installation has crosstalk from another channel or the power supply.
 
One last comment on 'why did A-B do it that way'; one of the design principles of FLEX was that it be totally universal, able to work with PLC-2, PLC-3, PLC-5, SLC, and any future controllers (this was about 5 years before ControlLogix was introduced).

So you find a lot of math mechanisms and scaling in the FLEX product line that uses raw or simplified data representation that requires more user math in the SLC or PLC or Logix platform, but can also be done in the PLC-2 platform.

And in some cases, they just fell back and said "we'll do it in hex so we don't confuse the PLC-2 guys who are used to BCD".
 
Found my root cause. I had to use a second DC power supply
could not use the same power as on the DC input modules
and I had to ground the neg. side of the supply and it smoothed right out. Thanks for the scaling info it came in handy. But when 4-20 mA is used with this card all 16 bits are used last one being a sign bit and 30840 counts. Which doesn't click with me if you have 15 bits I would expect 32767. Its
working I'm happy now. Thanks guys.
 

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