modulo 256 checksum

[Kevin O]

I am working on a node integration to a home automation device. The device has an ASCII protocol that I am building a parser and a command generator for. 

All of the ASCII messages that I send need to be appended with modulo 256 checksum. Here is the exact verbiage from the documentation.

2-digit checksum: This is the hexadecimal two’s complement of the modulo-256 sum of the ASCII values of all characters in the message excluding the checksum itself and the CR-LF terminator at the end of the message. Permissible characters are ASCII 0-9 and upper case A-F. When all the characters are added to the Checksum, the value should equal 0.

I have an ASCII string of 09sw13100 that is supposed to result in hex B8. That is what I am using as my test.

Any ideas on how to implement this? I've tried the function below but it produces the wrong output.

function calcChecksum(asciiString) {

  var buf = new Buffer(asciiString);

  var sum = 0;

  for(var i=0; i<buf.length; i++) {

    sum = sum + buf[i];

  }

  sum = sum%256;

  return sum.toString(16);

} 

[Tim Caswell]

Looks like you're missing the two's compliment part and the upper case part.

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[Tim Caswell]

Try this:

function calcChecksum(string) {
  var buf = new Buffer(string);

  // Calculate the modulo 256 checksum
  var sum = 0;
  for (var i = 0, l = buf.length; i < l; i++) {
    sum = (sum + buf[i]) % 256;
  }
  // Find the compliment
  sum = 256 - sum;

  // Convert to a two byte uppercase hex value
  var chars = sum.toString(16).toUpperCase();
  if (chars.length ==1 ) chars = "0" + chars;
  return chars;

[Jann Horn]

On Fri, Apr 13, 2012 at 01:54:37PM -0700, Kevin O wrote:
> 2-digit checksum: This is the hexadecimal two’s complement of the
> > modulo-256 sum of

Well, you forgot to make it two's complement.

> Any ideas on how to implement this? I've tried the function below but it
> produces the wrong output.
>
> function calcChecksum(asciiString) {
> var buf = new Buffer(asciiString);
> var sum = 0;
> for(var i=0; i<buf.length; i++) {
> sum = sum + buf[i];
> }
> sum = sum%256;

Here, insert `sum = sum ^ 0xff`. That should work, I think.

> return sum.toString(16);
> }

[Kevin O]

Incredible! Thanks, Tim.

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[mscdex]

On Apr 13, 4:54 pm, Kevin O <kevinohar...@gmail.com> wrote:
> I have an ASCII string of *09sw13100* that is supposed to result in hex *B8.
> *That is what I am using as my test.

>
> Any ideas on how to implement this? I've tried the function below but it
> produces the wrong output.
>

> [...]

Try this:
function calcChecksum(str) {
var sum = 0, i = 0, len = str.length;
for (; i<len; ++i) {
sum += str.charCodeAt(i);
if (sum & 0x80)
sum = (0xff - sum + 1) * -1;
}
sum = sum % 256;
if (sum < 0)
sum *= -1;
return sum.toString(16);
}

console.log(calcChecksum('09sw13100'));
// output: b8

[Paddy Byers]

Or more simply:

  var buf = new Buffer(asciiString);

  var sum = 0;

  for(var i=0; i<buf.length; i++) {

    sum -= buf[i];

  }

  sum = sum&0xff;

  return sum.toString(16).toUpperCase();

Paddy

[Jorge]

function calcChecksum(asciiString) {
var sum= 0, i= asciiString.length;
while (i--) sum+= asciiString.charCodeAt(i);
sum= (256- (sum%= 256));
return (sum<16 ? '0' : '')+ sum.toString(16).toUpperCase();
}

calcChecksum('09sw13100')
-> "B8"
--
Jorge.

[Kevin Swiber]

On Fri, Apr 13, 2012 at 4:54 PM, Kevin O <kevino...@gmail.com> wrote:

I am working on a node integration to a home automation device. The device has an ASCII protocol that I am building a parser and a command generator for. 

This sounds pretty sweet, and it would be cool to see this as a module on npm.  Any plans to release it when it's ready?

--  

Kevin Swiber

Projects: https://github.com/kevinswiber
Twitter: @kevinswiber

[Jorge]

Yeah, cool, that's the best !

function calcChecksum(asciiString) {
var sum= 0;
var i= asciiString.length;
while (i--) sum-= asciiString.charCodeAt(i);
return ((sum&= 0xff) < 16 ? '0' : '')+ sum.toString(16).toUpperCase();

[Kevin O]

Actually yes. I have this "other" project that is taking priority though. ;)