> ---- Rick T <[email protected]> wrote:
> >
> > #define STX 0x02
> > #define ETX 0x03
>
> it is a function of type integer called calculate_lrc. There is an
> input pointer to a string called packet.
>
> > short int calculate_lrc(char *packet)
> > {
>
> define variables
> > char *lrc_ptr;
> > short int lrc = 0;
>
> assign the lrc_ptr (pointer) to the first character of the packet
> string
>
> > lrc_ptr = packet;
> >
>
> If the first character is a STX (start transmission x02, return with a
> failure value of -1
> > if( ! (lrc_ptr = strchr (lrc_ptr, STX )) )
> > return (-1);
>
The strchr function in ANSI C does this:
The strchr() function returns a pointer to the first occurrence of the
character c in the string s.
So, I take this to mean that if STX does not occur anywhere within the string
pointed to by lrc_ptr, then the condition is met and -1 is returned.
> Add one to the pointer
>
> > ++lrc_ptr;
>
> start a do while loop
> > do {
>
> assign the lrc variable to the current pointer address plus one
> character
>
> > lrc ^= *lrc_ptr++;
This statement is actually doing this:
lrc is assigned the value of
(lrc BitwiseXOR whatever is stored at the _current_ address of lrc_ptr).
Because the ++ is after lrc_ptr, it is a post-increment.
So, the ++ increments the pointer _after_ it is referenced in the statment.
You could break that statement into 2 parts to see what is really happening:
lrc ^= *lrc_ptr;
lrc_ptr ++;
>
> check to see if the next character is the end of transmission
> character ETX (x03)
>
> > } while (*lrc_ptr != ETX);
>
> drop out the do while loop when the current character pointed to by
> the lrc_ptr is ETX
>
> assign the lrc variable (perhaps "last received character" with the
> char
> pointed to by the current pointer position
I think this is actually performing a Longitudinal Redundancy check (see
http://en.wikipedia.org/wiki/Longitudinal_redundancy_check). It states:
Set LRC = 0
For each character c in the string
do
Set LRC = LRC XOR c
end do
An 8-bit LRC such as this is equivalent to a cyclic redundancy check using the
polynomial x8+1, but the independence of the bit streams is less clear when
looked at that way.
Many protocols use such an XOR-based longitudinal redundancy check character,
including the IEC 62056-21 standard for electrical meter reading, and the
ACCESS.bus protocol.
>
> > lrc ^= *lrc_ptr;
>
Once again, ^= means lrc = (lrc BitwiseXOR whatever is stored at the
_current_ address of lrc_ptr).
> return the character
>
> > return lrc;
>
> > }
>
> So my guess after 25+ years of fooling with this sort of thing is
> that this function takes a string from the comm port (or other
> UART device), lops off the control codes, and sends the last
> character in the string to the caller. I am rusty at this and without
>
> context, it is tricky to give you an exact answer.
It took me a while to step through this too. I miss C, I haven't been able to
use it in a project in ages.
> Of course, I could be completely out to lunch too. No pun intended
> as I swallow my burger.
> I miss the old days are reading teletype and paper tape codes. I
> remember writing C code to do something like this when I was
> communicating
> with a teletype device back when microcomputers came out. It was like
>
> magic.
> Got any COBOL you want translated. ;) Good luck, Roger
> --
> http://www.linkedin.com/pub/roger-austin/8/a4/60
> http://twitter.com/RogerTheGeek
> http://www.misshunt.com/ Home of the Clean/Dirty Magnet
> http://cfinnc.com/ ColdFusion Conference in North Carolina Oct 17-18
Best of luck too. I started pumping out my CF example, but I won't have time
to doublecheck it for accuracy yet.
One caveat to this whole thing is that in the code above, the C code is
treating each character as a char (1 byte). I have _no_ idea what CF will
treat that character in the string as....1 byte, 2 bytes, etc.
Are you able to say what application you will be using this for?
Allen Souliere
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