[aprssig] What is "TNC Channel Switching"?

[Stephen H. Smith]

On 4/11/2014 9:23 AM, Andrew Pavlin wrote:
> Greetings, all.
>
> I was slogging through the APRS spec, and came upon a mention that the ASCII characters '|' and '~' were reserved for TNC channel switching, and couldn't appear in any messages or even as a symbol code.
>
> What does this mean, and is it still applicable? Will I break the APRS-IS or some ancient TNC somewhere if I send those characters in an APRS message?
>
> Just curious.
>
> Andrew Pavlin, KA2DDO
> Sent from my Verizon Wireless BlackBerry
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There are some classic serial-port-interfaced TNCs that actually have two 
separate modems in a single box connected to a single serial port.

Notably the Kantronics "KAM" (Kantronics All Mode) which combines a 300-baud 
multimode HF modem and a 1200-baud VHF packet-only modem.   The Kantronics 9612 
combined a 1200-baud packet modem similar to a KPC3 and a 9600-baud packet 
modem into a single box.    In either case, the two modems can be connected to 
two separate radios and operated simultaneously via a single serial line 
to/from the PC.

Commands typed from the PC console were prefixed with either the "pipe" symbol 
or the tilde to indicated which of the two modems you wanted to send commands 
(or text to be transmitted) to.

In the heyday of conventional packet, numerous third-party programs were 
developed that "covered up" this clumsy command-line interface by providing 
split-screen interfaces and/or "Radio1"/"Radio2" function key mappings to the 
user that handled the "stream" switches for you.

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