S7 question - FC's vs FB's

[PLCKeef]

Just wondering what the adantages are or reasons for using a Function Block and it's associated Instance DB over a Function and a Global DB?

 

[Mike_RH]

A FB allows you to have multiple calls to it, each with its own data set - the instance DB.
A FC does not.

 

[PeterW]

You can have multiple calls to FC's and FB's, no difference there.

The main advantage I can see is the fact that the STAT's gives you the ability to remember states without needing to pass data in and out.

A multi-called FC generally would be for logic where you pass data in and perform some sort of operation which is complete that scan, you do have the IN_OUT which can remember states but this would normally be quite small, you wouldn't want too much although I have seen FC's and FB's with rediculous amounts of data passed in and out.

With a FB the STAT area gives you unique memory for that call, an area which can be quite large if required.

 

[PLCKeef]

I'm still not sure.
An FC can be programmed using formal operands as does an FB.
It's the Instance DB that I don't understand.

I'm thinking each time I call an FB , I have to set up it's DI with the values that the FB is going to use ?

I will have to experiment a bit.

 

[PeterW]

You program the FB and then you call the FB.

When you program in the call you are prompted to enter a DB number for the IDB, the system then creates the IDB, it will contain any IO parameters plus the STAT parameters.

I've seen some inexperienced programmers create a FB and don't have any IO or STAT's, they still have to create the IDB which is empty. A classic case of 'SHOULD HAVE BEEN A FC'.

The IDB at the end of the day is JUST A DB, you can access and change data anywhere in the program within the IDB in just the same manner as a global DB. The main difference is the fact that you cannot modify the DB make-up, i.e. you cannot open the IDB and add a few words.

 

[L D[AR2P#0.0]]

Whilst Siemens allow you to use functions with IN_OUT and OUT parameters, one method of making a distinction is to use FCs where a set of inputs to the FC produce a return value (RET_VAL) from the FC (i.e. do not use FCs with IN_OUT and/or OUT parameters).

For everything else, use an FB

 

[KalleOlsen]

Hi PLCKeef!

You find tutorials here. Check section B5.

Kalle