Hi 5Zkgkdr,

A function block (FB) stores its In, Out, and some of its InOut parameters in the instance data block. From a technical point of view, it makes no difference whether you supply data to an In parameter of the FB or write it to the corresponding variable in the instance data block.

However, you should not do this because it is not a good way to program and it makes the program harder to understand and analyze because the cross-references do not work as well when writing to the instance data block as they do when using the block interface.

Kind regards
Stefan

5Zkgkdr

Hi,every master.

I have a question as the title,according to the file "id81318674_Programming_guideline_DOC_v16_en",as follow picture:

is there anyone can give me some example?

Information that is exchanged between building blocks (OB / FC / FB) should go through the input / output area of the code block.

Per example assume we have FB1/DB1 and FC1 being called in OB1. All the data that is linked from FB1 to FC1 should be visible in OB1. FC1 should not secretly read the values of DB1. When looking inside OB1, there is no doubt which information flows between the code blocks.

This recommendation comes at a price. That means you have to have extra memory that carries the data between FB1 and FC1. But the loss of memory is not as big a problem as it was with the old S7-300 PLCs.

Do these notes help?

I routinely break this guideline, but only with "global" variables, like GLOBAL DB tags that are truly global. There are some instances where internal FB instance DBs have values not exposed via the FB, especially some Siemens blocks.  Sometimes those internal tags are useful outside the block and we are not allowed to modify these "instructions", so the only solution is to touch the internal instance DB tags - and in fact, Siemens does the same thing in their posted sample projects.

They only do it with a very small number of highly complicated blocks, and typically these are blocks essentially brought over from the S7-300/400 world and they want migration to s7-1500 to be easy, so they leave the block interface alone and we have to resort to messing around with internal instance tags in some situations.

We should not be doing any of this in our own FCs and FBs.

However, they are guidelines, not rules.  Sometimes not obeying a guideline makes more sense in the bigger picture, but we should avoid it unless and until it makes it harder to work on, troubleshoot, commission, or implement.

5Zkgkdr

Hi,every master.

I have a question as the title,according to the file "id81318674_Programming_guideline_DOC_v16_en",as follow picture:

is there anyone can give me some example?