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[aprssig] which digi

Scott Miller scott at opentrac.org
Thu Feb 26 20:51:09 UTC 2009


> If the digipeater callsign are LD5BE you can connect to this changing 
> your UNPROTO to LD5BE

So it's just an UNPROTO packet and not an APRS message?

> Send a "empty" packet and the TNC responds with a string.
> Put this string into a calculator and send the result back.
> If password is ok then you can change everything but the callsign.

So it's a challenge-response authentication scheme of some sort.  Are 
the details of the scheme public?

> Upgrading firmware over the air sounds spooky ;o) is there a fallback if 
>   it fails?

For the SR1, the firmware is sent in numbered blocks, one packet at a 
time.  As each block is received (and the packet checksum passes) it 
gets written to the appropriate place in temporary (in this case, 
external flash) memory.  Once every block has been sent, a 'write' 
command provides the start/end address and a master checksum.  If the 
checksum for the received firmware passes, it stops what it's doing, 
jumps to the bootloader, and writes everything to internal flash and resets.

You can take as long as you want to send all of the blocks, and it'll 
just set aside that 64k block of flash and not use it for anything else 
while it's keeping firmware there.

For a proper on-air format, there should be a way for the sending 
station to request a list of missing blocks.  So you'd sent each block 
once, expecting no acknowledgment, and then see which ones were missed. 
  You'd send all of the missing blocks and then query again, until all 
were received and the checksums matched.

Scott
N1VG




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