PID control using Siemens S7 315-2 PLC

Hi everyone,
I am trying to use a Siemens S7 315-2 PLC to do level control. I have been getting some help from this site and have some of it figured out. Someone suggested using a Variable table(VAT Table) as part of the program because I could not get my data to update, however I could not find much literature on the Vat Table and am not sure if I have the addressing in the Vat table correct. Also, I was wondering if you are using a vat table, do you have to put your addresses into your PID block or can the PID block get the addresses from the Vat Table.
Right now the program is running, but when I put a signal into the PID block I am not getting anything out at the LMN(Manipulated Varaiable). I am thinking this has something to do with the Vat table not being set up properly.
I have the program attached to look at.
Any comments or suggestions would be appreciated.
Thanks

I use to work for Siemens doing building controls. What device are you programming? What are you programming it to control? Maybe I can help with these answers.

Hi John,
I am programming an S7 315-2 PLC. I am trying to program the controller for basic level control, basically, I will have a 4-20mA signal from a DP level transmitter input as the process variable and set a set-point of 50% and have an output from the PID block of the PLC to the Analoge output card to an current to pressure converter which would the use that sigal to control a pneumatic control valve to contorl the level.
This is the first time I have tried to program this PLC using the PID block so I am trying to keep it simple for now.
If you need more info, please let me know, and thanks for your reply.
Tomcoll

Tomcoll,

You had some overlapping addresses- MW40 and MD40.

The Scale FC and unscale FC are not necessary. You can put the PIW values directly into the FB41 if you use PVPER_ON and the PQW can be sent directly to your control value through LMN_PER.
Heres your PID with my take on it.

Jay

Hi Jay,
thanks for the response
I'll try this out to see how it works
tomcoll

May I know if this PID code works? Im having problem with my PID too...

It works. What are your specific questions