Step 7: Indexing to Specific Word within a DB

I'm writing a Rate-of-Change routine that uses the WSR function within Step 7 to progressively fill a data block. I then use the two extrema to calculate the rate of change.

My problem is that early in the acquisition cycle, the upper words in the DB are 0, so I don't want to use them in my calculation. I'm using a counter to keep track of where in the DB is the first non-zero data point but I don't know how to grab the value.

Is there a means by which I can use an index (the value from my counter) to grab the value of a specific word within a data block (for example, word #4)?

Once the data block is full, it's not a problem for I just use the first and last words. For the first part of each cycle, though, I need to use only the first word and whichever is the highest populated word.

Hello!
There is a thing called indirect accessing in the siemens help files. There you will find an example of what you are looking for.

You must write a pointer to acces the value, was it 20 bit long or longer Hmmm?

Is this for an S7-200 CPU ?

I don´t think so?
S7 200 doesn´t have to much commands...

Thanks, I'll look into that. Does it essentially function as do pointers and indirect addressing in C++?

The data are 16 bit words.

This is running in a 315 CPU

Beggar

Don't get too hung up on using pointers. These are for true indirect addressing. If all you want is indexed addressing then you can certainly have an address of the format DB20.DBW[Index]

"Index" should be declared as a double-word variable, then you just load the appropriate value in to it. So let's say you use MD60 as "Index", just stick a value in to MD60 and hey presto, you can either have DBW[MD60] or DBW[Index]. Or of course if you want to reference bytes or reals or other datasizes in the datablock, just use DBB[Index] or DBD[Index] etc.

regards

Ken

Thank you, Ken.

For "Index", is there a way to extract the value of a counter into the DWORD variable?

Unless I've misunderstood what you want, all you need is:

Cheers

Roy