[aprssig] What is "TNC Channel Switching"? (bug free)

Robert Bruninga bruninga at usna.edu
Fri Apr 11 10:07:16 CDT 2014


> modern software could attempt to send such packets in Command or
Converse mode on a KPC?
> Nothing modern. KISS mode wouldn't have a problem.

Sure, every program is perfect and has 20/20 clear vision as to what all
the possible states an attached piece of hardware can be in.  And said
hardware is also perfect and never ends up in a state that the programmer
could not forsee with clarity  always provides alternative workarounds.

That is why there are no more bugs ever found in modern software.  Nothing
has a RESET button and ctrl-alt-delete is obsolete.

Bob, WB4APR
------------------------------------------------------


Andrew, KA2DDO
Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: Rick Green <rtg at aapsc.com>
Sender: aprssig-bounces at tapr.org
Date: Fri, 11 Apr 2014 10:44:37
To: TAPR APRS Mailing List<aprssig at tapr.org>
Reply-To: TAPR APRS Mailing List <aprssig at tapr.org>
Subject: Re: [aprssig] What is "TNC Channel Switching"?

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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