Problem with RSLogix 4 to 20 scaling

[hewilson425]

Hey Guys,
I am trying to scale 0 to 36000 feet in RSLogix 500 in 4 to 20 mA. It worked great when the max length was only 25000 feet but our max has increased. I guess since the increased need for resolution, it scaled values in the middle of the scale have gotten innaccurate. Has anyone else run into this problem? Can anyone help me solve it? I'm using an SCP instruction......input min-max, 0-36000, and scaled min max 6242-31208. Thanks

 

[rdrast]

Are you using integers for your registers?

 

[Mickey]

input min-max, 0-36000, and scaled min max 6242-31208

Got that backwards? Or is this scaling for an output card?

Its just a straight line formula, check your 4-20ma.

What PLC are you using? Analog card? Input or output?

Can you post its program? Zip it first.

 

[James Mcquade]

hewilson425,

If this is scaling for an input card the input min / max should be 3277 to 16384 for the current signal and 0 / 36000 for the scaled feet distance and the result would go into a floating point register since an integer cannot be larger than 32767.


james

 

[Paullys50]

Step back....

What exactly is the problem - Accuracy? Fluctuation?

Is this an analog input or analog output?

Are you using floating point registers?

Did you bust out y=mx+b on paper to verify what you're expected values are?

0 - 36,000 is an extremely large range to handle over the range of 16mA. Depending on the resolution of your analog card, expect the value to be constantly floating back and fourth, I'm sure you saw that at 0 - 25,000, it's just amplified now with the extended range.

At the very minimum, post some screen shots.

 

[ndzied1]

It would probably be helpful to know exactly which analog card you are using and if it is configurable, how you have it configured.

This is almost definitely an issue with trying to use an integer for a value > 32767.

 

[Bit_Bucket_07]

Your analog output module only has 14,996 counts of resolution. Therefore a change of at least 2.4 feet in the operator input (or output value calculated via program logic) will be required in order to change the analog output's current value.

 

[Mickey]

How do you guys know what the scaling/resolution of the OP's analog card is without the OP telling us which card the OP has?

 

[Mickey]

Here is a handy tool for scaling. All items in yellow can be changed.

 

[Bit_Bucket_07]

How do you guys know what the scaling/resolution of the OP's analog card is without the OP telling us which card the OP has?

All analog output modules for the SLC-500 use 6242-31208 scaling when using the 4-20 mA option.

 

[Mickey]

All analog output modules for the SLC-500 use 6242-31208 scaling when using the 4-20 mA option.

That is not true at all. It depends on the card and how the OP has it configured. See manual below for just one example Page 32)

We still don't know if its a input or output card. That was my main point. We also don't know the PLC the OP is using.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjABahUKEwjxq8bQ1IvIAhWFo4gKHY9PABQ&url=http%3A%2F%2Fliterature.rockwellautomation.com%2Fidc%2Fgroups%2Fliterature%2Fdocuments%2Fum%2F1746-um026_-en-p.pdf&usg=AFQjCNF0Bz0zx9_JRGZxMNcN98Nof7aiuw

Edit:
Im just being hard to get along with today. Sense I see the OP who is still around but not responding to all this banter.

 

[Bit_Bucket_07]

That is not true at all. It depends on the card and how the OP has it configured. See manual below for just one example Page 32)

We still don't know if its a input or output card. That was my main point. We also don't know the PLC the OP is using.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjABahUKEwjxq8bQ1IvIAhWFo4gKHY9PABQ&url=http%3A%2F%2Fliterature.rockwellautomation.com%2Fidc%2Fgroups%2Fliterature%2Fdocuments%2Fum%2F1746-um026_-en-p.pdf&usg=AFQjCNF0Bz0zx9_JRGZxMNcN98Nof7aiuw

Edit:
Im just being hard to get along with today. Sense I see the OP who is still around but not responding to all this banter.

If it was an input module, it wouldn't use that data range. That's how I knew it was an analog output.

 

[Mickey]

What if its flex I/O?

You see we just do not know what card the OP has and how its configured.

it scaled values in the middle of the scale have gotten innaccurate. Has anyone else run into this problem? Can anyone help

I guess none of this answers the OP's question above. My answer is no, I have never seen this with the SCP instruction.

 

[Bit_Bucket_07]

What if its flex I/O?

You see we just do not know what card the OP has and how its configured.

I guess none of this answers the OP's question above. My answer is no, I have never seen this with the SCP instruction.

No, I've never seen it either. It's possible that there is a calibration error, but that could be easily checked with a meter. I wouldn't expect a math error.

 

[hewilson425]

It's a micrologix 1100 with a 1762-OF4 analog output card. Mickey, your scaling chart helped me figure it out. I went 0-10V. I had to raise the max input value to get 10.0v at 36000 ft. most values are dead nuts, but a few are 4-6 feet off, which we can live with. thanks for the help guys!!!!!!