Reverse Scaling (408 to zero) in Logix 500

Hi, I have a vacuum transmitter, it's range is 30 to 0 inches of mercury. My spec calls for inches of water, no problem, use SCP to scale 0-16733 to 0-408 and done.

Now the tricky part, the transmitter outputs ~20ma at 'rest' ie. no vacuum.

SO I have a PID loop which works backwards, is there a way to reverse my numbers so that my operator can enter a meaningful setopint and read meaningful transmitter output while looped?

I can't set my scaling block in reverse
I also can't rewire or add a signal conditioner. This must be done in software (and tonight!)

Thanks all. Best PLC forum ever.

Tim

I think I figured this out, subtract pv from 408. Sound right?

You should be able to set the SCP block to to this. There is absolutely nothing that keeps you from using SCP to scale 0-16383 to 408-0. The SCP instruction does not require that input min be less than input max or that scaled min be less than scaled max.



This will give you 408 in N7:6 when I:8.0 is 0 and 0 in N7:6 when I:8.0 is 16383.

Alaric,

Thanks, I gave up when I tried reversing the input side. Should have tried the sacaled side like you pointed out. Works like it should, saves a conversion and potential eror.

Much Appreciated.

Tim