S7, Function calls, and DB's...

I know there is a way to do this...but I'm sufferring from brain-lock at the moment, and can't remember how

I need to be able to pass a word value to a function call, and open up a datablock referenced by the word value for manipulation.

Essentially, I have DB100 through DB140, all very large, and identical. I want to pass say 100 to a function call to work with DB100 in it.

Passing BLOCK_DB doesn't work, as FC's can't take that as IN/OUT.

Can anyone point me in the right direction?

Thanks in advance

I just did something exactly like that.
I had to pass the DB number via an intermediary variable (MW100)

In declaration section:

CIM211_INBOX_DB  INT



In code sction:

L     #CIM211_INBOX_DB            // Open the CIM 2.11 INBOX DB

T     MW   100

OPN   DB [MW 100]

Woot! Jesper, you are a lifesaver. That will work just fine for what I need.

And thanks for the ultra-fast response

I have an inner feeling that there must be another way to do it. Maybe a bit more elegant.
But what the hell - if it gets the job done, why bother.

Agree, there should be a more elegant way... but, uhhh...

You are only half a lifesaver I'm afraid. Now I find that I can indeed open the DB properly, and transfer values out of it, but nothing I write to it stays. /sigh

This is for the FM355-2 Temp/PID control module by the way, and interfacing many of them to an HMI. It seems now that while I can directly write the HMI value to the DB:

L DB30.Z1.Temp_SP
T DB100.OP.SP_RE

That works. So does the direct form of:

L DB30.DBD44
T DB100.DBD12

However, OPN'ing the DB in my function call and using it similarly:

L Temp_SP
T DBD12

doesn't work. I know I have the right DB open, as moving the actual PV out is fine.

Think this could be a problem that DBD12 (OP.SP_RE) is defined as static by the DB?

-- Edit...
In monitoring the actual values in the FC, the proper setpoint is being moved to the DB, but on exiting the FC, it reverts. I can also change it using a variable table. This is wierd.

I am using a shared DB, and I think you are using an instance DB.
If so you should use OPN DI in stead of OPN DB.
Dont know if that will make the difference though.

Also, if it would mean that you have to write with T DID in stead of T DBD, I dont know.

Hello Rdrast;

However, OPN'ing the DB in my function call and using it similarly:

L Temp_SP
T DBD12

Click to expand...

If you OPN'ed DB30, this will write to DB30.DBD12. Is this what you nwant to do? You might have to complete the address to
T DB100.DBD12

Hope this helps,
Daniel Chartier

Okay, Yes, indeed, after exprimenting, I found I had to open the stupid data block as an INSTANCE (DI) block to allow writes to it.

Very strangely, moving values into it worked fine all through the function call (SP was constant inside the FC), but it would revert to the original values on exiting the FC.

This seems to imply to me that opening it as a DB was creating a local copy of it, whereas opening it as a DI acted more as a pointer to the actual Datablock.

/shrug... My head hurts, I'm going to run with it now.

Thanks for the help all

I think Daniel nailed it.

By the way, do you need to open ONE or TWO DBs "dynamically".
If you need to open TWO, then you could make use of OPN DB (shared) and OPN DI (instance). Together with L DBD, T DBD, L DID, TDID etc.. it allows you to manipulate two DBs.