[aprssig] ARPS physical layer

Wes Johnston, AI4PX wes at ai4px.com
Thu Sep 16 07:38:38 EDT 2010

You also need to be aware than KPC tncs and TAPR clone tncs treat the
"has been digipeated" flag differently.  I can't remember which is
which, but one of them marks only the LAST digipeater field, the other
marks EACH digipeater.  Your code needs to simply look for the last
digipeater marked as digipeated.

God help those who do not help themselves.

On Thu, Sep 16, 2010 at 04:44, Derek Love <DLove at app-tech.co.uk> wrote:
> Thanks for the info guys, maybe I should be a bit more specific.
> I have the APRS 2.0 spec, I know about HDLC, NRZI, byte reversal, bit
> stuffing, CRC/FCS generation and stop/start (7E) flags.
> However, I come from a PMR / Data radio environment, so my first questions
> are:
> What's the preamble sequence and length?
> What's the sync sequence and length?
> The preamble is usually a 1100 sequence of 16 or 24 bits to allow the RF
> sections to settle down and the receiving modem to set its levels up. The
> Sync sequence is usually 16 or 24 bits carefully chosen so they don't
> correlate well with Gaussian noise so that false detects are minimised.
> So far as I can see, there is no mention at all of a preamble and the only
> sync sequence I can see is the 7E start flag, which seems to be a very poor
> choice?
> I can't see any specification for which state the NRZI coding process should
> be started with either......
> I have also found a reference to a data scrambler somewhere, but no
> indication if it's part of the standard or not.
> Derek Love
> -----Original Message-----
> From: Curt, WE7U [mailto:curt.we7u at gmail.com]
> Sent: 15 September 2010 22:48
> To: TAPR APRS Mailing List
> Cc: Derek Love
> Subject: Re: [aprssig] ARPS physical layer
> There's a byte-wise CRC16 algorithm that was published in an IEEE
> magazine in the late 70's or early 80's that I used when I wrote
> assembly for decoding packets in '84 or '85.  Worth checking out if
> you're writing code.
> --
> Curt, WE7U.                         <http://www.eskimo.com/~archer>
>    APRS:  Where it's at!                    <http://www.xastir.org>
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