[aprssig] ARPS physical layer

John Hansen john at coastalchip.com
Thu Sep 16 10:17:36 EDT 2010

You may find it helpful to take a look at a paper I did for the TAPR Digital
Communications Conference:

*PIC-et Radio: How to Send AX.25 UI Frames Using Inexpensive PIC
Microprocessors* <http://www.tnc-x.com/dcc.doc>


It goes over most of this stuff in some detail.  The bytes you have before
the address are the 7E flags.  In addition, you'll find that many TNC's idle
during the TXDelay period using 00's instead of 7E's.  With NRZI it does not
matter which state you start in.  Zero's are represented by a change of
state, it doesn't matter whether they are low to high or high to low.
There is no data scrambler with 1200 baud packet, but there is with 9600
baud packet.

Almost every developer I've ever talked about this (including me) has said
at one time or another that this or that feature "seems to be a poor
choice."  I've occasionally run across items that I didn't think really
conformed to the standard. You pretty much have to get over that if you want
to play in this sand box.  The standard is what it is and at this point it
has become so institutionalized that it's not going to change.

John W2FS

On Thu, Sep 16, 2010 at 4:44 AM, 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<http://www.eskimo.com/%7Earcher>
> >
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