Iterating through structure elements

Hi,

I'm currently working on RFID tag processing. I have the idea of creating a universal function to split the RFID string and write it in the structure of strings of different length. The problem is that the structure depends on the application.

The simplest solution would be to iterate through the structure and determine the length of each string. This would allow the calculation of offsets in the RFID string for reading and appropriate location of data in the structure.

Is it possible to iterate the structure by its elements? Or maybe it is possible to determine where in a structure the particular string starts and ends?

Hi kamlenk,

You might be in luck...

In a string, the first two bytes have the format USINT:

1.USINT: maximum length

2.USINT: current length

3. ..n. CHAR

So, if the whole structure consists of strings, you could look at Byte 0, which will potentially hint at the offset of the second string and so on...

This is merely speculation though as I don't have a PLC in reach for testing, but you might be able to handle it this way.

1st Offset: 0

2nd Offset: 0 + 2 + max_length of 1st string

3rd Offset: 0 + 2x2 + max_length of 1st + max_length of 2nd

...

There's maby an easier solution to this which I don't know, but someone smarter will point that one out :-)

kamlenk

Hi,

I'm currently working on RFID tag processing. I have the idea of creating a universal function to split the RFID string and write it in the structure of strings of different length. The problem is that the structure depends on the application.

The simplest solution would be to iterate through the structure and determine the length of each string. This would allow the calculation of offsets in the RFID string for reading and appropriate location of data in the structure.

Is it possible to iterate the structure by its elements? Or maybe it is possible to determine where in a structure the particular string starts and ends?

OK, maybe I have not expressed myself clearly enough.

I receive information from the Investor that I will get the data from the RFID tag. They tell me that the first 5 characters are the product ID, the next 7 are the batch number, etc. This is part of their system and I can not change anything in it.

Case 1:

1       2       3       4       5
0123456701234567012345670123456701234567...
NG01bUV891f43acXYZ123098472Wab1910ffcd10...
-----=======

- product ID
= batch No

The problem is that in another machine, the product ID takes 10 characters and the batch number has 4 characters, etc. (case 2)

I am trying to develop a universal function of retrieving data from an RFID string and placing data in a string structure. My idea is that I want to define the target string structure for the first case:

TYPE "User_data_type_1"
VERSION : 0.1
STRUCT
ProductID  : String[5];
BatchNo    : String[7];
etc...
END_STRUCT;
END_TYPE

and I expect that the function will check what elements this structure consists of and adequately fill them with fragments from the RFID string (for case 1 get first 5 chars from RFID string and put it in ProductID, get chars from x1.5 to x2.3 and put it into BatchNo, etc.).

For case 2 I'll prepare:

TYPE "User_data_type_1"
VERSION : 0.1
STRUCT
ProductID  : String[10];
BatchNo    : String[4];
etc...
END_STRUCT;
END_TYPE

and the same function will populate it properly (hence the universality of the function).

The more I think about it, the more I am convinced that it is impossible here

 

kamlenk

The more I think about it, the more I am convinced that it is impossible here

 

This is possible  to do. Did you mention yet which PLC you are using?

The S7-1x00 can use the VARIANT type that can handle any structure. You have the ability to detect from the variant what type it is and then adapt your code accordingly.

The other way is to have strings that are bigger than what you need. The structure is a fit all. Then you have a configuration definition that tells the code block how it looks. In turn the code block fill the spacing that it needs to.

Even though I will not offer my help to code the solution, my point is that it is possible.

My suggestion is for you to get starting with a code block suited for one type only. Get that done first. Thereafter look into how to make a selection or generalization.

Good luck.

kamlenk

OK, maybe I have not expressed myself clearly enough.

I receive information from the Investor that I will get the data from the RFID tag. They tell me that the first 5 characters are the product ID, the next 7 are the batch number, etc. This is part of their system and I can not change anything in it.

Case 1:

1       2       3       4       5
0123456701234567012345670123456701234567...
NG01bUV891f43acXYZ123098472Wab1910ffcd10...
-----=======

- product ID
= batch No

The problem is that in another machine, the product ID takes 10 characters and the batch number has 4 characters, etc. (case 2)

I am trying to develop a universal function of retrieving data from an RFID string and placing data in a string structure. My idea is that I want to define the target string structure for the first case:

TYPE "User_data_type_1"
VERSION : 0.1
STRUCT
ProductID  : String[5];
BatchNo    : String[7];
etc...
END_STRUCT;
END_TYPE

and I expect that the function will check what elements this structure consists of and adequately fill them with fragments from the RFID string (for case 1 get first 5 chars from RFID string and put it in ProductID, get chars from x1.5 to x2.3 and put it into BatchNo, etc.).

For case 2 I'll prepare:

TYPE "User_data_type_1"
VERSION : 0.1
STRUCT
ProductID  : String[10];
BatchNo    : String[4];
etc...
END_STRUCT;
END_TYPE

and the same function will populate it properly (hence the universality of the function).

The more I think about it, the more I am convinced that it is impossible here

 

 As #dp pointed out you haven't to this point indicated which CPU nor which TIA version.

It very doable since you will know the exact structure before hand. You can use TypOf () function to distinguish between the different PLC Data Types.

Have a look at this for more detail.

Happily I've achieved my goal.

I prepared the Function Block with input string and input/output variant. To the function's input I connected RFID string as my source of data. To the function's input/output I connected my structure of strings (each in different length) as my destination — I used UDT for this.

I serialize my variant to an array of bytes for easy access to individual bytes of my structured strings. The Serialize function returns size of my structure so I can check if the input string will fit into the target structure. Then I determine the length of the strings in my structure according to the idea of NiM_0021 (Thx!). Next, I copy, byte by byte, chars from the source string to the temporary array.
Finally I deserialize the temporary array to my structure. Done. Not much more than a dozen lines of SCL code.

This solution allows me to avoid explicitly providing the amount and lengths of elements (strings) of my structure. It's enough to define the structure elements correctly, and the function itself will copy to it the appropriate data from the string.

Now, I will prepare the second FB for transferring data from a structure to a string.

My only worry now is the Balluff function to read / write data from / to RFID tags. The one I have is outdated (with S5 timers and direct memory access) and is not suitable for closing in a function block. I am afraid, however, that this is not the right place to solve this issue.

Thank you all very much for your advice and inspiration.