[aprssig] Error checking within APRS packets

[Stephen H. Smith]

On 6/26/2011 11:48 AM, Guido Trentalancia wrote:
>
> It should still be possible to do it with little risk. As long as there are 
> no (multiple) interactive connections ongoing, there should be no need (for 
> anything or anyone) to switch the stream. Or at least, this is my 
> understanding (consider I do not have such a TNC, as the Kenwood TM-D710 uses 
> 0x01 by default, and I have never used that function anyway). And usually for 
> APRS, one TNC is exclusively devoted to APRS itself.

Stream switching is very much used exclusively within APRS!

The Kantronics KPC-9612 and KAM dual-port TNCs contain two separate radio 
interfaces (modems) that input/output through a single shared RS-232 serial 
port.  They can also cross-digipeat between the two modems internally.  The 
vertical "pipe" character and the tilde are used to select between the two 
sides of either of these two devices.

The 9612 has been used in mixed 1200 baud/9600 baud networks or as a gateway 
between users on 144.39 and a backbone on 9600 baud UHF.  For decades, The KAM 
which operates 300 baud HF on one side and 1200 baud VHF on the other, has been 
used by nearly all HF-->VHF gateway stations.  The real-time all-US HF, and 
real-time local-area VHF, displays on my UIview APRS webserver are produced by 
the two sides of a single KAM.connected to a single serial port on the PC.

The existence of these two devices is one reason that the pipe symbol and tilde 
symbol in the APRS symbol chart (that otherwise uses all printable ASCII 
characters) have been kept unassigned all these decades.   Full APRS symbol set 
into here on my website:

. <http://WA8LMF.net/aprs/APRS_symbols.htm>

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Stephen H. Smith    wa8lmf (at) aol.com
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