Re: OT: Some advice on perl: read byte to hex string

[Welly Hartanto <ms.linuz@gmail.com>]

Dave Carrigan wrote:

On Tuesday 19 September 2006 23:29, Welly Hartanto wrote:

  

for ($i=0; $i < (length($gotit)); $i++) {
    my $c = substr($gotit, $i, 1);
    $c1 = $c & 0xF0;                          #get the high nibble
    $c1 = $c1 >> 4;                           #
    $c1 = $c1 & 0x0F;                         #
    

$c is not a byte; it is a variable that contains a 1-byte string. So doing 
bitwise operations like & on it won't do what you expect. What it will do is 
convert $c to a numeric value (if possible) and then perform the &. For most 
of the data you are probably reading, it's converting it to 0 because it's 
not numeric to begin with. The rest of the time, it would convert it to some 
number between 1 and 9.

Presumably, what you want is the value of the byte from 0 to 255. So, you 
first need to convert $c to that value using the ord function.

$c1 = ((ord($c) & 0xf0) >> 4) & 0x0f;

  

    $str1 = hex($c1);                         #convert to hex
    

I don't know what this hex function is supposed to do. Most likely, you want

$str1 = sprintf("%02x", $c1);

  

    $c2 = $c & 0x0F;                          #get the low nibble
    $str2 = hec($c2);                         #convert to hex
    

$str2 = sprintf("%02x", ord($c) & 0x0f);
  

Thank you for all your suggestions.
It's solved by using :

for ($i=0; $i < (length($gotit)); $i++) {
    my $c = substr($gotit, $i, 1);
    $str1 = sprintf("%01x", ((ord($c) & 0xf0) >> 4) & 0x0f);
    $str2 = sprintf("%01x", ord($c) & 0x0f);
    print $str1.$str2."\t";
    print "\n";
}

At least for converting the data.
But another problem is it's only show the first 8 bytes only.
The data sent from the device is 11 bytes, so now 3 bytes are
missing.
I think it's something about the buffer (?)

::welly hartanto::

Below is the flow on how to read the data :

The thing is the data sometimes will form in a negative byte.
That's why some bitwise and AND operation exist.
To produce the right output I should :
1. Seperate the first 4 bit ( with & 0xF0 and 4 bitwise) .
2. And get the value back with & 0x0F.
3. Store the value ( name it "first").
4. The rest 4 bit with the & 0x0F.
5. Store the value as "second".
6. Make both value formed as hex string then concate them as firstsecond.

For example, the first byte which will be sent from the card reader device
will always 01111110(bin), which is 126(dec) or 7E(hex).

1. The & 0xF0
    01111110    <--- original value
    11110000
    -------- &
    01110000

2. The >> 4
    01110000 >> 4 = 00000111

3. The & 0xF0
    00000111
    00001111
    -------- &
    00000111

4. The value is 7(dec) and 7(hex)
5. The last 4bit with & 0xF0

    01111110
    00001111
    -------- &
    00001110

6. The value is 14(dec) or E(hex)
7. So the result will be 7E ( after concate with the first value ).