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[aprssig] AX.25 'H' flag

Wes Johnston aprs at kd4rdb.com
Mon Nov 14 18:44:47 UTC 2005

I know (understand) what you are saying.... and I darned sure know the
difference between the ascii representation of the ax.25 header when you
monitor a TNC in CONV/CMD mode... and the raw ax.25 header.  ;-)

But I used a pk88 tnc and and kpc3 during the testing of my aprs digi
software back in 1998, and I *think* it was the paccomm treated the
h-flag incorrectly.  (it's been 8 years, so I can't remember which was
which)  I had hooked two TNCs together in an audio loop so that they
talked to each other... one was in conv mode so that I could generate
packets by an attached TTY terminal, and the other TNC was in kiss mode
so that I could capture the raw frames and decode (ahem decypher them).
 I also captured KISS frames that bounced around my town, which was a
mix match of paccomm and KPC tncs.

Just to be sure.... I am talking about the KISS frames I captured by
monitoring the transmission of packets on air.

In the end, we can say how it's *supposed* to be in the spec all we want
( and I agree that each digipeater should set each bit in sequence), but
I just wanted to make sure that Scott knows that not all TNC's meet the
spec, and he'll have to work around them.

kc4pl has my pk88 TNC on loan... maybe I need to dig it out to recreate
this h flag problem again??  I'll have to see if I can dig out the
source code for my kiss mode digi, maybe this is in the comments....

Wes  - beating head on wall.... ;-) <bang> <bang> <bang>

Curt, WE7U wrote:
> On Mon, 14 Nov 2005, Wes Johnston wrote:
>>I am indeed talking about the kiss/ax.25 spec...  I used the CONV mode
>>method of H bit indication... so a little mixing of terms here... but I
>>did see a difference in the way the kpc3's and pacomms dealt with H bits.
> It's that mixing of terms that is causing the confusion.  If you're
> in CONV mode watching the TNC's ASCII-text version of the headers,
> all bets are off.  In that case you absolutely have to use your
> method of checking from the right end of the header to find the
> first '*'.  Unfortunately that header info DOES NOT indicate that
> all of the 'H' bits have been set from the left up to that point.
> They have though.
>>I think Bob said it would be chaotic... and he's right... but it's what
>>I saw.  Unfortunately, I don't remember which TNC filled up the h bits,
>>and which on just set the current bit.
> All of them fill up the 'H' bits, else nothing would work right.
> It's just the "CONV" version of the header which is whacko here.
> There are several ways to represent the header info in "CONV" mode.
> If you were looking at the KISS frames, you'd see all of the 'H'
> bits filled up through the last one used.  Every time.
> --
> Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
> "Lotto:    A tax on people who are bad at math." -- unknown
> "Windows:  Microsoft's tax on computer illiterates." -- WE7U
> "The world DOES revolve around me:  I picked the coordinate system!"

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