Step 7 help with Strings and UDTs

I've done a search on this forums (and Siemens forums) trying to get my head around UDT's and strings.

I found an example by Daniel Chartier from a long while ago detailing a way of explaining UDT's by imagining a record collection:


I started to create this as an example to help myself understand UDTs and also strings but I can't find any way of 'MOVE'ing a value into a string.

The accumulators etc only hold 4 characters but even....


... doesn't work.

I know that you can MOVE a string from one place to another with BLKMOVE but not sure how you initially put a value into it (or maybe you can't???)

I've read how other people prefer to use CHARs etc...

This isn't for a real program, just me trying to understand more about the darker depths of S7.

I've posted similar questions before and found some of my old questions as I searched. I found an example posted by LD utilising an ARRAY of UDTs etc but it was a bit above my head.

I basically would like to see how you could do a very very (very) simple example of a UDT utilising a couple of data types including a STRING. Just to see how it would be used in a very simple program.

Anyone got anything like that? Or any reading material they could link me to??

Berger only seems to give a brief overview and other forms of reference are either too complicated or only reference RS Logix.

Many thanks for any help

;-)

Manipulating STRINGs in STL is really cumbersome.
'ABBA' is 4 CHARs, not a STRING[4]. A STRING in addition to the chars uses the 1st byte to hold the reserved string length, and the 2nd byte to hold the actual used string length.
Copying STRINGs can be done with SFC20 BLKMOV. Concatenating and joining STRINGs can be done by using library functions.
But generating the contents of STRINGs dynamically in code in STL, LAD or FBS is code intensive and error prone.

In SCL you could write
"CDs".artist := 'ABBA' ;

edit: damn, forgot a ":" in the above SCL code snippet.

Hi Jesper,

Thank you for the info. I've never done any SCL. I'm not sure my software can do it (I'm just using basic Step 7).

I did read something about the first 2 bytes of the string but even:

L 'a'
T CDs.Artist

doesn't work....

Artist is showing as String[4] in the variable declaration table.

uptown47 said:

Code:

L  'ABBA'
T  CDs.Artist   // This is a [B]field in my UDT[/B]

... doesn't work.

Click to expand...

This makes me ponder, if you did declare a data in datablock (or stat of FB) of type "yourUDT" ?

What I suggest you to do is, first learn to use UDT with basic, bool, int, dint etc. datatypes. Then learn ot use STRING datatype. And just after that lear how those are used together.

Turpo

I actually just referenced the UDT in the STAT var.

When I declared the 3 UDT's in the DB it was the same problem.

It wouldn't let me write to the 'string' part of the DB.

??

How did you monitor it? Can you use string and succesfully monitor it with STRING datatype without UDT?

You cannot monitor STRINGs in a VAT in STEP7.
But you can monitor the individual bytes of the STRING.
Point the VAT to the individual bytes, displayed as decimals for the 1st and 2nd byte, and as Chars for all the subsequent bytes.

I'm not sure I understand what you mean. I haven't managed to monitor it because it won't let me write anything to it.

I've zipped the software up as a library in case you get a chance to look at it.

I'm off to the dentist in a mo and not back until Monday. Would love any suggestions on how to get this work or any clarification on what I'm missing.

Many thanks

;-)

uptown47 said:

L 'a'
T CDs.Artist
doesn't work....

Click to expand...

Thats what I am trying to tell you.

In the above code, 'a' is a single CHAR, whereas CDs.Artist is a complete STRING[4]
The biggest datatype you can address with "T" (transfer) is a doubleword (4 bytes).
In STL, LAD or FBD, a STRING can only be manipulated by pointers.
So the code editor will refuse to accept T CDs.Artist

Just for completeness sake...
You CAN very tediously set individual characters of a STRING in STL, LAD and FBD.

Like:

L 'A'
T "CDs".artist[1]
L 'B'
T "CDs".artist[2]
L 'B'
T "CDs".artist[3]
L 'A'
T "CDs".artist[4]

As already stated, strings can only be accessed by area pointers. Here's some example processing that will load a hardcoded string into the artist field. It also shows a VAT table being used to monitor the results.

Code as follows:

Jesper, LD,

A big thanks for your posts. I've just read through them and downloaded the example that LD posted.

I'm going to go through that this morning and will post back with any questions.

Reading the posts though it seems straight forward enough and Jespers comment about passing a Char into a String makes perfect sense so I can see why that wasn't working.

Many thanks

;-))