Allen Bradley 1746-NI8 analog issue

Wondering if anyone has seen this and possibly knows how to fix.

We have an SLC 5/05 with an NI8 and an NI4 in slots 1 and 2 in the rack. Both analog cards are fed from weight transmitters. The transmitters are calibrated to output 0-10v or 4-20ma representing 0-33lbs on the scale. So 0 - 33 lbs is represented by 0 - 32768 counts on the cards. ( roughly 0.001 lbs = 1 count )

The problem is that the NI8 and the NI4 react differently from the same signal. The NI8 will only go up in increments of 3, sometimes 4 counts, whereas the NI4 will go up by 1. The NI8 is acting as though it only is using 14 bits ( 0 - 16384 ) or maybe less?

I have tried configuring the NI8 to use 0-10v, +/-10v, 4-20ma and 0-20ma. All seem to do the same. I have tried NI4 data format and proportional counts...same thing.

Anyone have any ideas?

the figure below is from page A-2 of the 1746-NI8 User Manual ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um020_-en-p.pdf

looks like you have 1 mV of resolution ... that means that the module's electronics can only "resolve" down to 0.001 volt per step ... in simplest terms, that's as "fine" as the module can "slice" the input signal ... in other words, each volt can be represented by 1000 steps ...

you say that your input signal ranges from 0 to 10 volts ...

so 10 volts multiplied by 1000 steps gives 10,000 steps of resolution to cover the entire input range ...

then 32,767 divided by 10,000 steps equals about 3.28 – which for an integer location is going to work out to about 3 "counts" per step ...

The NI8 will only go up in increments of 3, sometimes 4 counts,

Click to expand...

that's probably about right for the scaling values that you're using ...

just a guess, but if you're trying to get the two modules to read "about the same" then you might try setting the NI8 up to use NI4 operation ... that didn't work for you earlier? ...

.

Thanks for the quick reply.

According to the manual, Using NI4 data format, I should have .305 mv or am I reading this wrong? It looks like you are right though, it is only able to sense 1 mv. Configuring the card to use the NI4 format, didn't change anything.