Checksum in ST

Hello,

I am an intern at a company and I am trying to get an AB CompactLogix PLC to control a piece of machinery. I need to send checksum values to the controller of the machine that I am trying to control to get it to accept commands. I have the checksum code in C, but it needs to be in Structured Text. I've been working on this for a few days now and I can't seem to get anything that works. Is there anyone that can help?

The checksum function in C is:

int checksum(char*data){
int cs=0;
while(*data!=null){
cs+=*data;
data++;
}
return 255-(cs%256);
}

There is no way to directly translate this to A-B structured text because it uses pointers. To be creative about it you could probably copy your string data into an array of bytes and sum all of the characters in order to eliminate that loop.

If you want to stay true to the code you will have to extract each character from the string and add it to a variable of type BYTE. Then you'd return 255-(cs mod 256). Hope that helps.

Good luck,

Yosi

Why not simply:

For i := 0 to (MyString.Len - 1) By 1 Do
cs := cs + MyString.Data
End_For;
Result := 255 - (cs MOD 256);