Modicon PLC2 Instruction Reverse/Direct Acting


Thread Starter


The PID2 instruction does not seem to follow the Modicon Book definition of Direct Acting/Reverse Acting PID.Is the book definition of the PID2 instruction Reverse/Direct acting configuration wrong? Plus wrong in all other documentation that I can find?

Both the manual and the online software help show that you must short the third input on the PID2 block for Reverse acting and leave the third input open for Direct acting. However, observed results show the opposite is true. Could someone just clear this up for me? Is this just a known documentation error?

Thanks for the help.
Perhaps the problem is a difference in definitions. Direct acting means that as the
error increases, the control variable (output) increases. The error is defined as SP - PV, as illustrated in the PID2 documentation. I could see where you might be confused if you defined the error to be PV - SP.

I've used the PID2 instruction numerous times without surprises.

An easy way to remember the sense (for me) of direct/reverse acting: increase in setpoint produces increase in output, while measurement remains constant.

Aside: the PID2 instruction's reset feedback feature allows easy implementation of constraint control; many other plcs don't offer reset feedback with their PIDs...