Plc 5/80e connected to 1746 ni8 card issue

[James Mcquade]

Hi all,

i have a plc 5/80e connected to a host of racks.
one of those racks is a slc 500 series
it is Rack 015 Group 6
the Ni8 card is Module 0

my problem is that we have several monitors connected to the input card and the zero point has drifted.

we are using a btw to the card with a configuration word of 3852.
the btr places the data in the next sequence of registers.

the program is setup for 4-20 ma.
the program math is set up for a result of 0.0 to 25 in the f8 file.

is it possible to get the math to read -1 to 25 and how would i do that in the configuration.

i'm not well versed in this area so any help would be much appreciated.

thanks in advance,
james

 

[Mickey]

is it possible to get the math to read -1 to 25 and how would i do that in the configuration.

Its possible but not in the configuration , in the math that scales the data to engineering units.

The configuration word value you posted would indicate the card is configured for 0-20ma with a scaled for PID data format.
That would give you 3277 counts at 4ma and 16383 counts at 20ma.

To scale that to -1 to 25 use compute instruction

((Raw Data-3277)*.00198))-1=Scaled Data -1 to 25

Post your program and point us to the math that scales the data now.

 

[James Mcquade]

Thanks Mickey.

regards,
james

 

[Mickey]

my problem is that we have several monitors connected to the input card and the zero point has drifted.

I may have misread your problem after reading it again, is this zero point the analog cards raw value at 4ma?

If so you may have to apply a known 4 ma and take note of the raw value counts, also do the same at 20 ma. Then use these new values to compute your engineering units 0-25.
If you can provide these values someone can help with the math.

 

[James Mcquade]

mickey,

thanks for the heads up.

we have several 4-20 ma monitors in the plant.
each is calibrated for 0 to 25.

what we have discovered is that on two of the units, the zero point has drifted into the negative area rather than staying at zero.

what i am trying to do is to see if anyone has been able to change the calibration / setup so the plc will be able to compute a -2 to a max of 23 or 25.

the problem is that this is the zero point of the meter.

if this was a slc 500 i wouldn't have a problem, i would use the scp command and a calibration meter to get my unit programmed.

regards,
james

 

[Mickey]

With the compute instruction or some simple math you can do the same as the "SCP" instruction. See link

http://www.plctalk.net/qanda/showpost.php?p=475356&postcount=5