[aprssig] What is "TNC Channel Switching"?

Rick Green rtg at aapsc.com
Fri Apr 11 09:44:37 CDT 2014


On Fri, 11 Apr 2014, Stephen H. Smith wrote:

> The Kantronics "stream switch" characters were a way of accommodating 
> multiple logical channels in the 7-bit ASCII world that predated KISS, by 
> using two typeable characters that were not used much (at least in American 
> English and if you weren't a Unix programmer!).

That doesn't answer the OP's real question.  Is it necessary to treat 
these characters as 'reserved', and prohibit their use within APRS 
packets?
   To answer this, I feel we need to know the answers to two other 
questions:
   Were the Kantronics dual-TNCs sensitive to these characters in received 
data on the radio side?  Would they cut off the current stream and switch 
the received data to the other radio in mid-packet??
   Did the Kantronics command language provide any way of 'escaping' these 
characters so that they could be embedded within packets?  It was 
mentioned that in practice they were 'prefixed' to a command to indicate 
which stream this command is to apply.  Is it possible that the TNC was 
ONLY sensitive to a stream switch character immediately after a ctrl-C as 
it enters command mode, or immediately after a carriage return if already 
in command mode?

If it can be shown that the Kantronics TNCs are only sensitive to these 
characters on the serial side, and as a prefix to a command, then there 
is no reason to prohibit their use within the protocol itself.

-- 
Rick Green, N8BJX

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