symmetric positive-negative scaling in RSLogix

Hi again fellows,

I’m a bit confused about a matter regarding analogic input scaling in a RSLogix 5000 ladder routine. Hope you can help…
The analogic input gets sampling readings 4-20 mA covering a variable range -2000 to 2000 mV (this is the raw span my REDOX device can measure). I want to obtain finally those readings scaled from 0 to 10000.
What is blurring me is the minus sign: The operator will commonly enter setpoint values included among -230 and 380 mV through an HMI device (raw values).
Generally, when I work with a positive span of values I use the next formula for the scaling:
Scaled value = ( 10000* raw value) / highest value of raw span
And so far, so good.
But how to do when I get a symmetric -/+ span? I wonder whether I have to perform a bit more complex calculation or is there any other easiest way.
What is the common programming approach for this particular case?

Thanks in advance,

If its a 1756 Analog Input Module it will do the scaling to engineering units for you in the module, no need for any code.

Just go into the configuration for the channel and set your four scaling points and the input range

mmm...:-( nope, it is not a 1756. It is a 1769 IF8 so I can't define scaling span. For the rest of the analogic inputs I've been using in this project I've already choosen "Percent Range" which is giving the scaling between 0 to 10000

daba said:

If its a 1756 Analog Input Module it will do the scaling to engineering units for you in the module, no need for any code.

Just go into the configuration for the channel and set your four scaling points and the input range

Click to expand...

Setting up the module will allow you to enable each channel as well as selecting "engineering units" for each channel.
As Daba said, no code is required.
It can be done in the configuration window of the module.

Sorry fellows, I explained myself wrong:

Yes, of course, I get a 4-20 mA signal through the analogic input so I'll get it scaled once I set up correctly the parameters under the module configuration tab.

But still the reason why I need to know the relationship between the raw span and the scaled span is because the operator will enter in the HMI a setpoint value (in units of the raw variable, for this particular case, mV, as we are measuring REDOX-ph in a waste water process ). So I have to say to the PLC how to turn that entered setpoint into a scaled which can be used as a reference value for making calculations with it (for example, comparing such setpoint value with the current value read by the analogic input.)

Hope this time, I've explained right what is exactly my problem.
Thanks again.

Let me see if I understand correctly.
You have a field device that sends a 4-20ma signal to the PLC, the engineering units for the field device are -2000 to 2000 mV.
Your PLC input module is converting the 4-20mA signal to raw counts, with a range of 0-10000. In the PLC logic, you want to scale the raw counts back into Engineering units.

Assuming my understanding is correct, here is the formula you need:

((Raw Counts / Max Raw Counts) * (Eng. unit span)) + (eng. unit offset from zero)= eng. unit

Example1:

Field device is outputing 12mA, PLC raw counts is 5000

(5000/10000) * 4000 + (-2000) = eng. unit

0= eng. unit

Example2:

Field device is outputing 20mA, PLC raw counts is 10000

(10000/10000) * 4000 + (-2000) = eng. unit

2000= eng. unit

Example3:

Field device is outputing 4mA, PLC raw counts is 0

(0/10000) * 4000 + (-2000) = eng. unit

-2000= eng. unit

buhnen said:

Sorry fellows, I explained myself wrong:

Yes, of course, I get a 4-20 mA signal through the analogic input so I'll get it scaled once I set up correctly the parameters under the module configuration tab.

But still the reason why I need to know the relationship between the raw span and the scaled span is because the operator will enter in the HMI a setpoint value (in units of the raw variable, for this particular case, mV, as we are measuring REDOX-ph in a waste water process ). So I have to say to the PLC how to turn that entered setpoint into a scaled which can be used as a reference value for making calculations with it (for example, comparing such setpoint value with the current value read by the analogic input.)

Hope this time, I've explained right what is exactly my problem.
Thanks again.

Click to expand...

If you use the formula I posted, then all parameters will be in engineering units. You can then compare the setpoint in mV with the field input also in mV.

Thank you very much Ken, I'll try it like that