Negative number help

[roscho]

I have a wet bulb & dry bulb transmitter that will help control
A water tower system the dry bulb range is (-20 to 120 deg. F)
I know that BDC will not show negative numbers, does any one know where I can find some documentation
On setting this up? Also one more thing if the input sig is 4-20ma and the range of the transmitter is -20 to 120 deg does (4 ma=-20 and 20 ma =120)?
I ask this because when I scaled the wet bulb k100=high limit and k0 = low limit it’s off by about 20 deg (to high) and the range for the wet bulb is 0 to 100 deg f
Thanks for the help in advance Russ

[MikeS]

the 4-20mA card is going to give you a  0 to 4095 value. the full-scale range you have is 140 degrees, so scale the input to a rnage of 0 to 140. then subtract 20 from the scaled value.

to deal with the negative numbers you'll need to be working with Binary values, not BCD values. when you enter the pointer method ladder logic, i believe you can set it up to give your either BCD values or Binary values. choose binary at that point and you're ready to scale them. use the ANSCLB (Analog Scale Binary to Binary) IBox to do the scaling.