You have an 8 bit signed magnitude input value coming into the PLC via I0-7.
And you want it to scale so that:
11111111b = -127d = -10V
01111111b = +127d = +10V
If the above assumption is correct, you can use two linearization functions.
But first you will need to only use the function “I to MI” on inputs 1-7.
Before the linearization function, use input 0 (the sign bit) as a condition to enable the positive or negative linearization function.