Analog Scaling in RSLogix 5000 for 1769-IF8

rta53

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rta53
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Feb 2003
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OK, I apologize up front for having to ask about this. I am still learning to deal with the transition from Logix 500 to 5000. There is no module scaling for the 1769-IF8. It has to be done in 5000. I have been using the SCP instruction in SLCs but of course 5000 does not have this. In PLC5s I used the formula (X|4095)*100 in a CPT instruction if I wanted to scale my input 0 to 100. This was easy because the raw data was 0-4095. But if I use raw data in the IF8 for a 4-20mA input it is -32767 to +32767 where 4mA is about -29800 and 20mA is around 28900. If I use Engineering Units it is of course 4000 to 20000. What math do I use to scale these ranges say to a range of 0 to 150?
 
rta53 said:
OK, I apologize up front for having to ask about this. I am still learning to deal with the transition from Logix 500 to 5000. There is no module scaling for the 1769-IF8. It has to be done in 5000. I have been using the SCP instruction in SLCs but of course 5000 does not have this. In PLC5s I used the formula (X|4095)*100 in a CPT instruction if I wanted to scale my input 0 to 100. This was easy because the raw data was 0-4095. But if I use raw data in the IF8 for a 4-20mA input it is -32767 to +32767 where 4mA is about -29800 and 20mA is around 28900. If I use Engineering Units it is of course 4000 to 20000. What math do I use to scale these ranges say to a range of 0 to 150?
Click to expand...

There is scaling that you can use right in the module itself. Go to the module properties (right click) and then go to the configuration tab. Select the type of input 4-20mA, and then enter in your min and max engineering units.
 
I already pointed out that there is no in module scaling for the 1769-IF8. This is a Compact Logix system.
 
does the SCL function exist in Compact Logix? In a control logix system, you can use it in function block. If that's the case and you don't want to use FB, create a new function in FB that has the SCL function inside it, and then save it under a new name, and use in ladder.
 
keithb said:
Hi

Everything you need is in the manual
Click to expand...

I already read the manual. It shows me how to configure the module. What I am looking for is a "math for dummies" answer. I don't know if FB is available in Compact Logix but I plan to just use ladder logic anyway.
 
all of the math is here ... but it's probably a little more detailed than you wanted ... if all else fails, dig in ...

edit: just a suggestion: PRETEND that there is an SCP in RSLogix5000 ... show us the entries that you would use to set it up for your project ... we'll work the math and post the CPT for you ... but personally I'm only online for a few more minutes ... I'm sure others would be capable and willing to help ...
 
Last edited:
Ron,

I had previously read some of your post on this matter of scaling and it is indeed very detailed. However I am running short of time to do a lot of reading right now. I was hoping for something much more brief. I'm not looking to become a math whiz at this, I'm just trying to get my project finished. I would be happy just to have an SCP instruction and be done with it. Maybe I need to ask my question in a different way. :oops: What is the simplest way to scale a 4-20mA analog input in RSLogix 5000 in order to get the engineering units I need? In other words how do I get from (-32767 to +32766) to (0 to 150) using a CPT instruction.

Randy
 
try this ... I'll double-check it on a working system in a few minutes ... but I think that this is right ...

SCP4RTA.JPG
 
Scaling for Dummies

You can use the formulas shown in the spreadsheet below in a compute function. This is a universal solution, and works for floating point as well - the factor is just zero in that case.
 
Last edited:
Hey Tom,

I wanted to look at you spreadsheet but our proxy server is blocking my open or download options. Could you change the file extension and repost?

Marc
 
where did you get your original numbers? ...

Randy,

you asked:
What is the simplest way to scale a 4-20mA analog input in RSLogix 5000 in order to get the engineering units I need? In other words how do I get from (-32767 to +32766) to (0 to 150) using a CPT instruction.
Click to expand...

I think that I answered that specific question in my earlier post ... but now I've had a few more minutes to look at your problem ... I'm wondering about those values that you gave ... did you intend those to be just "examples" or as valid data ranges? ...

as near as I can tell from the manual, when using “plain vanilla scaling”, 4 mA into your module gives a data reading of 6241 ... not the “-29800” value that you first quoted ... and 20 mA on your module gives a data reading of 31206 ... not the “around 28900” value that you first quoted ...



using the more-or-less “standard/book” settings, here’s the CPT that should work ... specifically, 4 mA will give 0 for a scaled value ... and 20 mA will give 150 for a scaled value ...



SCALED4RTA.JPG

hope this helps ...



 
Last edited:
Analog Scaling

OK - I cleaned it up a little, made it a zip, added a graph, and made it more general purpose. It should work correctly for any signal range, any data range, and any process variable range. If that isn't the case, I'd appreciate it if someone would advise me of the errors.Anaolg scalling technique seems to be a very common question - we see some variation of this about once a month on this site alone.
 

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