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[aprssig] AX.25 'H' flag

Scott Miller scott at opentrac.org
Mon Nov 14 21:50:29 UTC 2005

> 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

I learned that when writing my OpenTracker TX code a couple of years ago. 
Makes a big difference.

> 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.

Yes, that's what it says, and it doesn't work that well.  But last time I 
grabbed a packet from the KPC-3 on the logic analyzer, there was a long 
stretch of nothing up until the first and only flag, which was immediately 
followed by data.  Even decoded a couple of bytes by hand off the screen to 
make sure I was really seeing the start of the frame.  I haven't really 
examined the problem closely yet, though.

Scott, N1VG

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