Calculating checksum - communication with new ANT BMS

stepus · May 7, 2022, 10:10am

Hi, can someone please help me with calc checksum ? Iam new with communicating over uart with protocols.
I have working sending request packet, and i get an answer from BMS. I can decode all data, but iam stuck on checksum, i tried internet calculators etc. And i didnt get same result as is received...
I dont know eather how is checksum calculated, because chineese documentation is low :D...
I sent reqest : 5A5A00000000.
Response from BMS is :
AA55AAFF025D0EC50EC50EC40EC50EC50EC50EC50EC50EC60EC50EC50EC50EC50EC50EC40EC40000000000000000000000000000000000000000000000000000000000000000000000003F003D09000026504100000389002CE55A0019001C00180017FFD8FFD8010100000000000000000000090EC6030EC40EC4100000007F007B02AD0000000000001911
First 4 bytes are header :AA55AAFF, then is value of first cell 025D - high byte first, and continous to 2 cell value etc as in PDF.
But last bytes two bytes 1911 is checksum. And i dont know how to calculate it.
In datasheet is something like first 4 bytes (header) are not present in checksum ?
thanks for help

140byteprotocol.pdf (346.5 KB)

gcjr · May 7, 2022, 11:24am

seems there may be useful information re: checksum on the right side of the pdf that has been cut off

and where is "Remark 4"?

gcjr · May 7, 2022, 11:30am

looks like it's the sum of bytes, skipping the first 4 bytes and not including the 2 byte checksum

horace · May 7, 2022, 11:51am

try this

unsigned char data[]="AA55AAFF025D0EC50EC50EC40EC50EC50EC50EC50EC50EC60EC50"\
"EC50EC50EC50EC50EC40EC4000000000000000000000000000000000000000000000000000000000"\
"0000000000000003F003D09000026504100000389002CE55A0019001C00180017FFD8FFD8010100000000000000000000090EC6030EC40EC4100000007F007B02AD0000000000001911";

int main(void) {
printf("%s\n", data);
unsigned short checksum=0;
for(int i=8; i<strlen(data)-4;i+=2) {
    char d[5]={0};
    memcpy(d, &data[i],2);
    printf("string %s\n", d);
    char text[20];
    unsigned short int hexvalue;
    sscanf(d, "%2x",&hexvalue);
   // hexvalue &= 0xffff;
    checksum+=hexvalue;
    printf("hex value %0x  %0x\n", hexvalue, checksum);
}
printf("checksum %x\n", checksum);
//checksun=
printf("checksum %0x\n", checksum);
}

a run gives

AA55AAFF025D0EC50EC50EC40EC50EC50EC50EC50EC50EC60EC50EC50EC50EC50EC50EC40EC40000000000000000000000000000000000000000000000000000000000000000000000003F003D09000026504100000389002CE55A0019001C00180017FFD8FFD8010100000000000000000000090EC6030EC40EC4100000007F007B02AD0000000000001911
string 02
hex value 2  2
string 5D
hex value 5d  5f
string 0E
hex value e  6d
string C5
hex value c5  132
string 0E
hex value e  140
string C5
hex value c5  205
string 0E
hex value e  213
string C4
hex value c4  2d7
string 0E
hex value e  2e5
string C5
hex value c5  3aa
string 0E
hex value e  3b8
string C5
hex value c5  47d
string 0E
hex value e  48b
string C5
hex value c5  550
string 0E
hex value e  55e
string C5
hex value c5  623
string 0E
hex value e  631
string C5
hex value c5  6f6
string 0E
hex value e  704
string C6
hex value c6  7ca
string 0E
hex value e  7d8
string C5
hex value c5  89d
string 0E
hex value e  8ab
string C5
hex value c5  970
string 0E
hex value e  97e
string C5
hex value c5  a43
string 0E
hex value e  a51
string C5
hex value c5  b16
string 0E
hex value e  b24
string C5
hex value c5  be9
string 0E
hex value e  bf7
string C4
hex value c4  cbb
string 0E
hex value e  cc9
string C4
hex value c4  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 00
hex value 0  d8d
string 3F
hex value 3f  dcc
string 00
hex value 0  dcc
string 3D
hex value 3d  e09
string 09
hex value 9  e12
string 00
hex value 0  e12
string 00
hex value 0  e12
string 26
hex value 26  e38
string 50
hex value 50  e88
string 41
hex value 41  ec9
string 00
hex value 0  ec9
string 00
hex value 0  ec9
string 03
hex value 3  ecc
string 89
hex value 89  f55
string 00
hex value 0  f55
string 2C
hex value 2c  f81
string E5
hex value e5  1066
string 5A
hex value 5a  10c0
string 00
hex value 0  10c0
string 19
hex value 19  10d9
string 00
hex value 0  10d9
string 1C
hex value 1c  10f5
string 00
hex value 0  10f5
string 18
hex value 18  110d
string 00
hex value 0  110d
string 17
hex value 17  1124
string FF
hex value ff  1223
string D8
hex value d8  12fb
string FF
hex value ff  13fa
string D8
hex value d8  14d2
string 01
hex value 1  14d3
string 01
hex value 1  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 00
hex value 0  14d4
string 09
hex value 9  14dd
string 0E
hex value e  14eb
string C6
hex value c6  15b1
string 03
hex value 3  15b4
string 0E
hex value e  15c2
string C4
hex value c4  1686
string 0E
hex value e  1694
string C4
hex value c4  1758
string 10
hex value 10  1768
string 00
hex value 0  1768
string 00
hex value 0  1768
string 00
hex value 0  1768
string 7F
hex value 7f  17e7
string 00
hex value 0  17e7
string 7B
hex value 7b  1862
string 02
hex value 2  1864
string AD
hex value ad  1911
string 00
hex value 0  1911
string 00
hex value 0  1911
string 00
hex value 0  1911
string 00
hex value 0  1911
string 00
hex value 0  1911
string 00
hex value 0  1911
checksum 1911
checksum 1911

checksum is 1911 hex

gcjr · May 7, 2022, 12:14pm

consider

void
calcByte (
    uint8_t    *buf,
    int         nByte )
{
    printf ("\n%s: %d\n", __func__, nByte);

    int  sum = 0;

    for (unsigned n = 4; n < nByte - 2; n++)  {
        sum += buf [n];
        sum &= 0xffff;
     // printf ("    %3d  0x%04x 0x%04x\n", n, sum, buf [n]);
    }
    printf (" 0x%04x\n", sum);
}

horace · May 7, 2022, 5:55pm

arduino version of code

#include <string.h>
#include <stdio.h>

// function to calculate checksum
// skip first 8 bytes header - checksum is last 4 bytes
unsigned short checksum(byte *data) {
  unsigned short checksum=0, hexvalue;;
  for(int i=8; i<strlen(data)-4; i+=2) {
    sscanf(&(data[i]), "%2x",&hexvalue);
    checksum+=hexvalue;
  }
  return checksum;
}
void setup(void) {
  Serial.begin(115200);
  byte data[]="AA55AAFF025D0EC50EC50EC40EC50EC50EC50EC50EC50EC60EC50"
  "EC50EC50EC50EC50EC40EC4000000000000000000000000000000000000000000000000000000000"
  "0000000000000003F003D09000026504100000389002CE55A0019001C00180017FFD8FFD8010100000000000000000000090EC6030EC40EC4100000007F007B02AD0000000000001911";
   Serial.print("checksum "); Serial.println(checksum(data), HEX);
}

void loop(void) {}

a run gives

checksum 1911

stepus · May 7, 2022, 8:23pm

Thanks! its working, i had a same code as yours but then i was doing
sum = (sum^0xFFFF);
sum +=sum+01;

i found it somewhere on internet i wasnt expecting its so easy only sum of all bytes

system · November 3, 2022, 8:23pm

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Calculating checksum - communication with new ANT BMS

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