Using 'MID' Instruction With Custom String Type

tbreww96

Member
T
Join Date
Aug 2018
Location
Ohio
Posts
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So, I am a beginner-slightly intermediate level programmer, and I am looking for a way to access specific data, and pull it out of a larger string. Not hard right, just use the MID command to pull it out!

Wellll, the string that I am attempting to pull data out of is a custom string data type, with a possible length of up to 4096.

The MID command will not work with any string larger than 82 characters. :angr:
I have to find a way to pull this data out; whether it be by splitting the custom data type up into regular 82-character strings, or using an instruction that will work with strings larger than 82 characters. Help! & thanks in advance!
 
To start, three questions:

  1. What PLC part number?
  2. What PLC Firmware version?
  3. What programming software version?
The more exact your answers, the more exact our answers can be.


Also, what do you mean by a "custom string data type"? Are you using an array of characters instead of a string?
 
mk42 said:
To start, three questions:

  1. What PLC part number?
  2. What PLC Firmware version?
  3. What programming software version?
The more exact your answers, the more exact our answers can be.


Also, what do you mean by a "custom string data type"? Are you using an array of characters instead of a string?
Click to expand...
@mk42

1) PLC is AB 1769-L36ERMS. I am using this PLC for bench testing.
The application PLC will be a 1769-L30ER

2) Vs 30.012

3) RSLogix 5000 Vs 30.00.00

Technically in this version of Logix, all strings are an array of characters or SINT's (in the ASCII style) with the length in an attribute called 'LEN' of DINT type. The only difference is, in a regular string, there can only be 82 characters, and the array of SINT's is always a SINT[82].

In this string type, the array is a SINT[4096].

I've attached an image that maybe helps show this:
Capture.png
 
That's one heck of a string! So, in practice, is there any real chance that the content of the string will really be that long? If not, perhaps you can look at its LEN value and if that is less than or equal to 82, copy the customer STRING to your own internal STRING with an SINT[82] size and work with that.

Just a thought
 
Steve Etter said:
That's one heck of a string! So, in practice, is there any real chance that the content of the string will really be that long? If not, perhaps you can look at its LEN value and if that is less than or equal to 82, copy the customer STRING to your own internal STRING with an SINT[82] size and work with that.

Just a thought
Click to expand...
I did think about that. Unfortunately, the strings I am dealing with are consistently 300-500 characters.
 
You could roll your own fairly easily.

COP INPUT.DATA[START_INDEX] OUTPUT.DATA[0] NUMBER_OF_CHARS
MOV NUMBER_OF_CHARS OUTPUT.LEN

I would have to read the MID instruction's manual to find out what you want to do when you mess up the array index. Eg.
if start_index >= input.len
if start_index >= LEN(input.data)
 
AustralIan said:
You could roll your own fairly easily.

COP INPUT.DATA[START_INDEX] OUTPUT.DATA[0] NUMBER_OF_CHARS
MOV NUMBER_OF_CHARS OUTPUT.LEN

I would have to read the MID instruction's manual to find out what you want to do when you mess up the array index. Eg.
if start_index >= input.len
if start_index >= LEN(input.data)
Click to expand...
I thought the same! I tried it, and it still seems to be unhappy! I'm stumped!
Capture1.png
 
tbreww96 said:
The MID command will not work with any string larger than 82 characters.
Click to expand...

In Logix Rev 18, at least, the MID instruction is not limited to the default 80-character string type. So long as your string can fit into a user-defined string type of the desired length, MID will work over the whole length. I just tested with a 200-character string, finding a 3-character substring at position 151 to 153.

Also, I created a 4096-character string to make sure that was not a limit.
 
Mispeld said:
In Logix Rev 18, at least, the MID instruction is not limited to the default 80-character string type. So long as your string can fit into a user-defined string type of the desired length, MID will work over the whole length. I just tested with a 200-character string, finding a 3-character substring at position 151 to 153.

Also, I created a 4096-character string to make sure that was not a limit.
Click to expand...
That's extremely interesting. I will keep trying; maybe there is something else going on here.
 
tbreww96 said:
@mk42

1) PLC is AB 1769-L36ERMS. I am using this PLC for bench testing.
The application PLC will be a 1769-L30ER

2) Vs 30.012

3) RSLogix 5000 Vs 30.00.00

Technically in this version of Logix, all strings are an array of characters or SINT's (in the ASCII style) with the length in an attribute called 'LEN' of DINT type. The only difference is, in a regular string, there can only be 82 characters, and the array of SINT's is always a SINT[82].

In this string type, the array is a SINT[4096].

I've attached an image that maybe helps show this:
Capture.png
Click to expand...

Thanks for circling back with those answers! The logix experts are already on the case, so I'm going to let them do their thing.

That said, your response is way more detailed than most new posters on this forum, so props to you, best of luck, and welcome aboard!
 

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