ELK M1 Alarm panel

[rocfusion]

Hi Admin,

Thank you,  the checksum function works.  I am trying to do the reverse and verify the message received has a valid checksum

Code:

function checksum(str)
 local sum = 0
 for i = 1, #str do
   sum = (sum + string.byte(str,i)) % 256
 end
 x = 256-sum
 return string.format('%02X', 256 - sum)
end

function reversechk(str, cksum)
 local sum = tonumber(cksum,16)
 log(sum)
 for i=1, #str do
   sum= (sum + string.byte(str,i))
 end
 return string.format('%02X', sum)
end

local message= '16XK403923611081711000'
local chksum = checksum(message)
log(chksum)
-- chksum is 67
-- so the total message is '16XK40392361108171100067'
local isok = reversechk(message,chksum)
log(isok)

The reversechk function does not return a 0,  what am doing wrong?  I have the reverse calculation in C code,  maybe you can understand what they doing?

Code:

Calculate checksum on received and transmitted ASCII string
Example C code program
//INT8U is an 8 bit unsigned integer.
INT8U itAscRecBuf[82]; //ASCII receive character buffer
INT8U AscHexToBin(INT8U, INT8U *); //ASCII hex to binary conversion
INT8U AsciiToHex( INT8U); //Ascii to Hex conversion

//Calculate checksum on a received ASCII string, return checksum value.
//It should equal 0 if good.
INT8U CalcCheckSum(void)
{
    INT8U i,length, cc;
    length = AscHexToBin(2, &itAscRecBuf[0]);
    //get length value, first two characters
    
    cc = AscHexToBin(2, &itAscRecBuf[length]); //get checksum value at end of string.
    for (i=0;i<length ;i++ )
        {
            cc += itAscRecBuf[i]; //get string value and add it to checksum
        }
        return(cc);
        //good checksum should equal 0 }
    //ascii hex to binary, width 1 or 2
    INT8U AscHexToBin(INT8U Width, INT8U * DataPtr) //
        {
            INT8U aVal; // accumulated value
            aVal = AsciiToHex(*DataPtr);
            DataPtr++;
            if (Width == 2)    //two digits wide, else 1 digit wide
                {
                    aVal = aVal << 4;
                    aVal += AsciiToHex(*DataPtr);
                }
            return(aVal);
         }
    //Ascii to Hex conversion
    INT8U AsciiToHex( INT8U Value )
        {
            switch ( Value )
                {
                    case 'A': return( 10 );
                    case 'B': return( 11 );
                    case 'C': return( 12 );
                    case 'D': return( 13 );
                    case 'E': return( 14 );
                    case 'F': return( 15 );
                    default: return( Value - 0x30 );
                }
        }

Thanks,

Roger