Below is a typical PID logic for a SCADAPack.
Highlighted below are three block that facilitates reverse acting function. Techanically what is does is as follows.
(1) MULF (Multiply floating-point values)
Multiplies output values from the PID function block by -1
(2) ADDF (Add Floating-point Values)
Add the output from MULF from Step (1) by 100 (which give the reverse acting output from PID)
(3) FTOU (Floating-point to Unsigned Integer)
Assigned the output from ADDDF from Step (2) into an unsighed integer
If you want to use direct acting, just use the output from the PIDA function block (Modbus register 40590)
I hope it helps.