Difference between FB and FC's

Hello Jeffrey;
Basically, memory.
An FB is a function that "remembers", or "keeps score" of its last operations.
Both FCs and FBs can hold parameters (IN, OUT, IN-OUT and TEMP), it allows the re-use of the blocks with different calling environments. But FBs have an extra type of parameter: STATIC not available in FCs.
When you call an FB, you are required to generate an instance DB (IDB) that accompanies this particular call of an FB; this DB contains all the STATIC parameters of the FB, and these are available at any time by any other block in your program. You could do this with an FC, a general DB and MOVE instructions that would write results of the FC program in that DB. With an FB it is automatic.
If you want an example, look at the contents of the System Function Blocks library in Step 7, and look at the grouping of the SFCs and the SFBs. SFCs include SET RTC; this transfers a DT value to the real-time clock of the CPU. Once executed, there is no reason for the function to look back at what it had done. Same thing for SFC20, BLKMOV; once the data has been transfered, no need to remember. But look at the first SFBs: CTU, CTD, TON, TP... Once you have started counting or timing, you might need to look at the value of your accumulated value, or modify the preset. The IDB makes it always available, memorized and accessible, even if the block is not being processed at this exact moment by the CPU.
As an exercise call SFB4, TON, in a Lad network; you will be promped for a DB number; put any DBx not presently used to create the IDB. Once the calling block saved, open the blocks folder and see that are the elements of this DB; try modifying them.. The structure od the IDB is linked exclusively to the structure of the FB and is not accessible to you.
Now go back to the program, and use any instruction to use the IDB varables in your program. You should be able to use them as any other data in the CPU.
Hope this helps,
Daniel Chartier

• Divide and organize the user program. Divided in small parts the units can be easily understood. This helps specially maintenances and alterations of the program.
• Rationalize and optimize the programming. FC and FB can be created and used as users’ library. So, an algorithms is programming just one time, and can be used many times in one our more projects.
• Reduce and rationalize resources of the CPU (e.g. memory occupation).

Mojo

Here's another reason to use FC's for machine control vs. FB's:
Let's say you have a machine with four "widgets". All four "widgets" are the same. You could create the logic for a widget in an FB and then call the FB four times. Each call would have its own instance DB. Now you go online to the machine because there is a problem with Widget #1. You open the FB that controls all four widgets. Which widget are you looking at???? You will have to stop calling the FB for Widget's 1, 2, and 4 so you can monitor the logic for Widget #3.

You can monitor a specific instance without stopping other instances. Check out this link:
http://support.automation.siemens.com/WW/view/en/21671563

So you can see if you have something to do like scale analog variables or determine an average and you want to call this logic multiple times the FB is the way to go. But for general programming of machine controls I much prefer the FC.
IMHO, I would disagree on the Scale function. FB's with static variables are good for retaining data from call to call. You don't need to retain intermediate calculations from call to call with a scale or unscale function. FC105 and FC106 from the TI-S7 Converting Blocks library are quite good at scale and unscale. But that is my opinion.

Bill

follow up question published by adammukhrizis split to a seperate thread with the subject FB or FC program which use 4 to 20ma to control pump with FC and FB method?.

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New question published by luis schwenke is split to a separate thread with the subject FC in Tia Portal to set a solenoid.

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