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[aprssig] APRS spec 101 STOP

Steve Noskowicz noskosteve at yahoo.com
Thu Jul 17 04:50:07 UTC 2008


Ben, Ben, Ben...
P L E A S E    S T O P..  I only wanted to know the type.

Besides, I don't do C...or is that Greek?

Thanks anyway
-- 
73, Steve, K9DCI


--- On Wed, 7/16/08, Ben Jackson <ben at ben.com> wrote:

> From: Ben Jackson <ben at ben.com>
> Subject: Re: [aprssig] APRS spec 101
> To: "TAPR APRS Mailing List" <aprssig at lists.tapr.org>
> Date: Wednesday, July 16, 2008, 4:20 PM
> In case it's useful, here's my implementation (AVR,
> but should be portable)
> from the HF beacon I worked on for a while.  These are just
> the key lines
> abstracted out of the code:
> 
> 	uint16_t ax25fcsval;
> 
> 	/* flag(s) not part of fcs */
> 
> 	/* reset */
> 	x25fcsval = 0xffff;
> 
> 	/* all data bits in transmit (lsb first) order. */
> 	/* does not include "stuffing" bits */
> 	foreach bit {
> 		xor = ax25fcsval & 1;
> 		ax25fcsval = (ax25fcsval >> 1);
> 		if (bit ^ xor) ax25fcsval ^= 0x8408;
> 	}
> 
> 	/* xmit (lsb first) */
> 	uint16_t fcs = ax25fcsval ^ 0xffff;	/* latch fcs here */
> 	ax25byte(fcs & 0xff);
> 	ax25byte(fcs >> 8);
> 
> 	/* flag not part of fcs */
> 
> -- 



      




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