Questions of Siemens PLC Control~ PLS HELP

Hi all, I have 3 Questions of Siemens PLC program.

1. I need to write a PI control in the program. As I know I set D_set as 0 then it is a PI Control. But what is the meanings of the others ?? I have no idea what value or type should I set in order to be a PI control function

2. What can I do if i want to find the max. and min. value of 9 temperature readings ? Does any functional blocks can do that ?? Or need to write a program to compare all the value by myself ?

3. Since in the program, I need to check the time schedule to do some actions, but I have no idea of time schedule . Does any functional blocks can do that ?? If yes, will it be configed by hours? days? weeks? months? years? or any format?

Can anyone help me ?? PLZ~~

First of all a few questions back for you:

1. what type of PLC are you using? I assume it is an S7-300/400.
2. if I'm correct about question 1, then what type of PID are you using? There are two available: FB41 CONT_C and FB42 CONT_S. More info can be found if you turn to help on standard and system functions and search for these function blocks, but it is easier to select the function block in the instruction browser and then press F1.[/list=1]
   
   The answer to your second question is: yes, you'll have to write that function yourself.
   
   The answer to the third question is: use an organisational block, more specific OB30 to OB31. These are the cyclic interrupt blocks. You can set the time interval in milliseconds, range 0 to 32767. Which one(s) is (are) available depends on the CPU used.
   
   Kind regards,

There are also Time-Delay Interrupt OBs and Time-of-Day Interrupt OBs.
Time-Delay Ints are fired after specified time after calling SFC32 (SRT_DINT).
Time-of-Day Ints are fired at specified time of day and can be repeated at some defined intervals.

question nr 2.

Hello aprilchu!

To answer(?) your question nr 2:

The function FC 25-max and FC 27-min in the standard library -> IEC func. is the closest "ready to use"-functions in your case. The only problem is that they only can handle 3 values at a time. So the easiest way to solve your problem must be a construction of your own.

Good luck!

Question 3:

I used FC18 (Less or Equal to DateTime) and FC12 (Greater or Equal to DateTime) to create a time schedule. This works fine as long as you are using the DateTime variables.

Regards
Borte