[aprssig] What is "TNC Channel Switching"?

Heikki Hannikainen hessu at hes.iki.fi
Fri Apr 11 22:56:29 CDT 2014

On Fri, 11 Apr 2014, Rick Green wrote:

> 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?

I don't think it's necessary to treat them as reserved any more, when 
transmitted by other devices which are not using these TNCs. And in fact, 
they have not been treated as reserved for a while, they're widely used 
and do appear in packets today.

>  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??

They're sensitive to these characters when they get the packet off the 
serial line from the PC only.

The characters are already allocated for use in Mic-E type codes:


There's a very large fleet of Byonics trackers on the network worldwide, 
and they're not having any trouble due to | appearing in the type code.

I can see a lot of ~ in the raw packets, too, since the Mic-E protocol 
encodes some values with it, like this:


Here are the numbers for yesterday:

hessu at soul2:/data/aprsis-archive/2014$ grep '~' aprsis-20140411.log |wc -l
hessu at soul2:/data/aprsis-archive/2014$ grep '|' aprsis-20140411.log |wc -l

If a station using one of these TNCs tried to transmit these characters, 
that might cause a problem. I don't know if the TNCs require some pause 
before the switch charater for triggering, like some similar escape 
sequences do. Could someone with a TNC try it out?

When the Kantronics TNCs are used as stand-alone digipeaters, they treat 
these characters just fine, digipeating packets from Byonics trackers 
without blinking. Well, they do blink a led or two.

   - Hessu

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