Filling an S7 ARRAY

Hello guys!

I'm new to STEP7, so could you, please, help me with the following problem I face:

I need to write values from a current sensor into a data block (DB3) once an OB1 cycle. The process (high-current pulse) is very fast (45 ms) so i need to quickly get as much values as possible and write them to an array of REAL.

What I have come up with so far is that I got the current value and it's updated every scan cycle and stored in DB2. So I need to pick up this value and store in DB3 as an array item.

The problem is that I need to increment the array index and write the number (from DB2) to the next array item. I have no idea of how to do this. And basically, don't know how to work with arrays with a variable array index.

Could you please help me?


--
Pavel Maslov
R&D Institute for Electro-Physical Apparatus
St. Petersburg, Russia

You may be interested in the library function FC87 LIFO, which you can find in the standard library under TI-S7 converting blocks.
FC85 FIFO may be an alternative.

Read/Write array example attached:

Thanks everyone!
I chose the FC_84 function, which adds data to an array.

JesperMP, you might have misunderstood - I need to form an array, not use its items. You did helped me though, cos had it not be you, i would not have even considered to use built in functions from the Standard Library

Great that I could be of help, despite that my suggestion was not exactly what you wanted.

I forgot to mention that these library blocks uses words, and not REALs (double-words). But I guess you have it figured out by now.

help!

Guys!

I thought that this would be easy. In fact the FC 84 block does not want to work whatsoever. I decided to run a simple test - write a value (W#16#44) using this function. Look what I've tried to do:

-------------------------------
1) I created DB3:

2) I called the FC 84 block. The inputs of the block are as follows:

3) As a result - the PLC goes SF. It doesn't update the DB3 data block. Strange.
-------------------------------

Anybody knows what's wrong?

Is FC84 loaded into the PLC?

Check your Diagnostic buffer for more information (CTRL + D)

Cycle time??

Hello again!
Thanks STL??? - I forgot to upload FC84, silly me =) Everything works fine.

I have another question. Are there any methods that can lessen the cycle time?

I made a DB, which consists of 5000 WORDs. The PLC cycle time is approximately 1ms (the maximum is 8ms) - so I'm writing values of a 5 second process with 1 to 8ms discretization, when in fact i need only ~46ms to catch.

Perhaps, the cycle buffer (ring buffer) would work faster than FC 84? Can I somehow decrease the cycle time, say, to 500 us or even less than that? Because as I've said earlier the process is very fast and I need as much values as possible.

I use a S7 313C controller and CP 342-5. Data is read from an ABB FOCS Fiber-Optic Current Sensor via Profibus at 10ms/78 samples speed. I measured (Ctrl-D) the cycle time of a simple communication via PROFIBUS (ABB->PLC), which equals ~1ms.

What do you guys think about that?

A 313C is not suited as a datalogger for anything less than 10 ms datalogging time.
The 10 ms is the shortest timed interrupt that can be configured.

I would find out what logging speed you need, and depending on that you find out which hardware you need to achieve that.