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;
}
On Fri, Apr 13, 2012 at 4:03 PM, Tim Caswell <[email protected]> wrote:
> Looks like you're missing the two's compliment part and the upper case
> part.
>
>
> On Fri, Apr 13, 2012 at 3:54 PM, Kevin O <[email protected]> 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.
>>
>> 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);
>> }
>>
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>
>
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