[aprssig] AX.25 'H' flag

Scott Miller scott at opentrac.org
Mon Nov 14 15:38:17 CST 2005

> Yep.  Can't get much closer to the on-the-air format than that.
> Well, you can, but I wouldn't recommend it (6803 at 0.79 MHz,
> Assembly language, XR-2211 into an interrupt pin).

Pretty close - mine's a 68HC908AP64 at 7.3728 MHz, C language, XR-2211 into 
a timer/counter module pin.  The timer module saves some processing overhead 
vs. a plain IRQ, and as a weird side effect my scheme winds up putting the 
recovered bit clock on an I/O pin, which I guess it a good thing because it 
gives me something to hook the logic analyzer up to for debugging.

I have yet to figure out why it has such a hard time locking on to my KPC-3. 
I can copy two OpenTrackers transmitting through the same radio, and I can 
copy the KPC-3 beacons coming back from the digi 20 miles away, inside the 
house with an HT, but I can't reliably decode them direct.

> Anything else is not per spec.

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.

Scott, N1VG

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