PID Control Loop (S7-200) 

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you can search internet...... absolutely u will get many control loop programs to work on

if available select the same address while configuring hmi..... and if not then select any other address and move the data to variable memory

what make of VFD is....??

If u trying to give speed rpm refrence from vfd, then it should also be dine by taking vfd in local operation, and if u r trying from hmi side, put the operation in remote, and for all these u have to go for scaling block....... thanks