Analog Help!

Hi Guys,

This may be a stupid question so I am sorry. I have a SLC 5/03 with a temp input to my analog card. From what I can tell the card is scaled 3279 to 16384. I am assuming this is the decimal equivalent of the binary word. My OIT is reading 52 deg F and the raw decimal number is 9979. My question is how do I calculate my analog scaling. ##deg F to ##deg F. Is there a calculation I can use for this. I am getting my butt kicked here. Thanks alot.

Look for the SCP (Scale with Parameters) instruction. As long as the 5/03 you are using has OS301 operating system. The SCP will allow you to take tin INPUT MIN (3277) and INPUT MAX (16384) and scale it engineering units SCALED MIN/MAX (ex.: 0 to 100 degrees). Then the scaled result is moved to an integer or floating-point file of your choice.

Thanks for the reply, I just checked all of the logic and there is no SCP instruction. How else are analogs scaled? Thanks

You could scale it yourself by calculating how much a degree F is to so many units. Do you really need this scaled can you not just use the value you are getting.

The SCP instruction wouldn't be in the logic, unless you had already place it there. You will have to create a new rung with the instruction. As previously stated, if your 5/03 has OS301 or higher operating system (which can be verified by looking at the PROC tab under Processor Status), you can use this instruction. It is found under the Advanced Math tab of the instruction palate. The rung you create should look similar to this:

Greetings ryand,

the SCP rung offered by mgvol should do the trick - IF (as he said) your processor supports SCP instructions ... the good news is that most of them do ...

but ... since you asked:

How else are analogs scaled? Thanks

Click to expand...

there is a lot more information (probably more than you really wanted to know) in this thread ... and if you want it in an easy to print .PDF format, here are the links for part 1 and part 2 ...

hope this helps ... and welcome to the forum ...

3279 to 16384 looks like the card is set for PID scaling. Is there a PID instruction in your program that is using this analog input? If there is then the 14th word of whatever control block the PID is using will have the scaled process variable in it.

Thanks to all who replied I appreciate the time that you took to help me out. I am trying to go through this logic that someone else wrote and decipher what they were thinking. I am fairly new to PLC's so I still have some questions. What I am trying to do is find out what to scale my DCS too. I have temp out on the PLC and have no idea what the scaling is. There is no PID blocks or SCP blocks, so I am a little confused. Thanks again.


Ryan

I can't find where the scaling is but I think this CPT block has something to do with it, hopefully someone can tell me what the purpose of this is for.

((N7:70)*15)+I:7.0)/16 Destination is N7:70 and the input is I:7.0
The raw value for I:7.0 at this moment is 10206. N7:70 gets moved to F8:100. The value of N7:70 coming into the MOV is 10213 and then F8:100 is 178.3 (why did this value change I thought the MOV just moved the numbers?) After that F8:100 with a value of 10213 gets moved again to it's final output of 0:8.2 of 6258. (Againg why the change in value?) My question is how do I know what to scale my DCS for?

They are taking this input into the PLC for it's use but I am trying to bring this point out to my DCS so I can have indication on my screen. Thanks again so much for the help!!!!!

Ryan

My OIT is reading 52 deg F and the raw decimal number is 9979

Click to expand...

Option 1
Look at the OIT, find out which address it is reading and what the scaling parameters are, use the same for your DCS.

Option 2
Add the logic for the SCP instruction to the PLC, then bring the scaled results of that instruction, straight into the DCS without any scaling.

Ryand, When you say the value is only moved yet it changes:

Say this is your program scan

Move N7:0 into N7:1 All ok, no change in value
Logic
Logic
Add 1 to N7:1 and place in N7:1 Now N7:1 is a different value
Heaps and heaps of logic
End of scan Process cycles back and repeats

Now N7:1 spends most of the scan time as 1 more than N7:0. Remember the PC only reads the values from the PLC at a very slow rate compared to what is really happening so you nearly always see N7:1 as the higher value. If you looked long enough you may see it flicker to the lower value every so often.
Now a another way to do it is

Move N7:0 into N7:3 All ok, no change in value
Logic
Logic
Add 1 to N7:3 and place in N7:1 Now N7:1 is a different value
Heaps and heaps of logic
End of scan Process cycles back and repeats

When done this way N7:1 still ends up the same value but you can see the intermediate maths.

Hope this makes sense.
Regards Alan Case

Thanks for all of the replies it is starting to make sense. I appreciate the scan time response for some reason I didn't even think about that. Thanks