Analog input scaling question - ML 1200

I have a 4-20mA level input that needs to be controlled (PID). But the level transmitter is configured such way that 4mA correspond to 100%(full) and 20mA correspond to 0%. Same thing with the output, the I/P for the filling valve is configured such that 20mA keeps it fully open(100% from PID). It is designed such a way for fail safe condition - If the transmitter or the I/P wire gets cut --> it closes the filling valve.

Now, how do I scale it for PID? I need to enter setpoint and show display PV. I am confused. If I use 'scaled for PID' then it relates to 4-20mA as 0-16383. But I want it the other way.. Please help!

Use 'Scale with Parameters' (SCP) and reverse your min and max values....


Hope this helps.

It says input max shouldn't be less than input min..

Make your input min=0 max=16383 and make your Output min=16383 max=0 it should reverse your 4-20mA transmitter output....

Thanks for your help!
Now I cannot use the Scaled to PID on the input directly.. right? I need to reverse it and connect to the PID?

If the output of your SCP instruction is 0 -16383, that is scaled for P.I.D. Simply use the output address you assigned in the SCP (i.e. N7:0) as the PID PV input.

Thanks..

Just to confirm.. On the Analog I/O configuration for 1762IF2OF2 module, I choose raw/proportional input type. Then scale it to 16383-0 and use that for the PLC - Process variable.

Another option might be to make E=PV-SP instead of the default E=SP-PV. This should have the effect of making your loop output reverse acting.