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[aprssig] AX.25 'H' flag

Curt, WE7U archer at eskimo.com
Mon Nov 14 21:45:23 UTC 2005


On Mon, 14 Nov 2005, Scott Miller wrote:

> Yeah, but that's hard to enforce when you've got the stray 'or' in the spec
> to confuse things.  Kind of like how the V2 spec had the wrong description
> of bit stuffing.  If everyone built their TNCs to spec, none of them would
> ever work.

Also, some TNC's send flag characters as the preamble, others send
0's.  0's in NRZI encoding give you more clock transitions making it
easier for the PLL on the other end to lock to the signal.  I
remember in my 6803 code I saw some TNC's using one method, some
using the other.  I _think_ the spec said to use flag characters,
which is non-optimum.

You get a nice pretty square wave out of all 0's, NRZI-encoded.
Very easy to sync to.

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
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