S7 analog inputs

CharlesM

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• Jul 1, 2006

• #1

I am trying to scale a +/-10VDC analog input in Step 7. I put the input PIW520 into a var table and it shows -27000. In the ladder program it shows at 36000. I have a simple scale block that I made for other things but it only works for 0-10vdc not +/-10. So this is the first time I have gone -. I have tried using the FC105 but I am having trouble with it as well. Anyone used FC105?

 

CharlesM

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• Jul 1, 2006

• #2

The problem I was having is related to words & ints. I changed everything to ints and it started working. I changed my scale block to do - numbers by checking the input to see if it was less than 0. If it was I mul by -1 to change to + and set a bit. After the scale was done if the bit was set then I mul the scaled value by -1 to make it -.

I realy like most things about Siemens. But some of things should not be this hard.

 

lendyck

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• Jul 1, 2006

• #3

Scaling

If you use the scaling formula, it will not matter what ranges you are working with.

Output=(ScaledMax-ScaledMin)/(InputMax-InputMin)*(Input-InputMin)+ScaledMin

Siemens PLCs do take a bit of getting used to, but from what I've seen, they are on top for flexibility and performance.

 

SimonGoldsworthy

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• Jul 1, 2006

• #4

If you set max=+10.0, min=-10.0 and bipolar=true the output from FC105 will be +/-10.0

 

Pandiani

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• Jul 2, 2006

• #5

As far as I know, Siemens analog module represent analog signal like integer number in your program. Integer number is in range (0-27648). If you want to convert it to a -/+10 V use FC105 as SimonGoldsworthy described. Alos to have as little headache as possible define PIW520 as type INT in your simbol table.

 

lendyck

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• Jul 2, 2006

• #6

The analog input should be from -32768 to 32767. If this is to be scaled to a small range, it may be beneficial to convert the number to a real (float) type.

 

Pandiani

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• Jul 2, 2006

• #7

lendyck said:

The analog input should be from -32768 to 32767. If this is to be scaled to a small range, it may be beneficial to convert the number to a real (float) type.

Click to expand...

No, normally values will be in range (0-27648), values out of that range means some kind of error. If you use FC105 and set up bipolar to 1, lo limit to -10 and high limit to +10, FC105 will convert that range (0-27648)INT into range of real numbers (-10, 10).
Try it and see