[aprssig] 2 Port Digitpeater was: APRS UHF freq? (9600 baud)

[Stephen H. Smith]

On 1/25/2017 11:40 AM, John Langner WB2OSZ wrote:
>> Has anybody actually succeeded in putting two KISS TNC's "back to
>> back" to form an independently operating (no computer between them)
>> two-way "bridge" to pass traffic bi-directionally?
>> Lots of suggesting that this could work, but has it been done?
> In theory it should work but it's not a proper solution to the problem.
> Here is why.
>
<SNIP>
> If you were to simply retransmit what you hear on one frequency onto some
> other frequency, that would be OK if only one person was doing it.  However,
> if you had two stations like this, that could hear each other, the same
> packet could go bouncing back and forth forever.
>

I have successfully done the back-to-back TNC approach in one very specific 
application where the looping issue will never occur.

That is at a Mic-E-enabled voice repeater, where a APRS posit is transmitted 
each time you unkey from a voice transmission.    I placed two TNCs wired 
back-to-back at the voice repeater site. One had it's receive audio input 
connected to the repeater's receive audio.    The other TNC was connected to a 
transceiver on 144.39. Only the TX audio, PTT and RX COR lines were connected - 
no RX audio connection.

The net result is a no-path-decrementing one-way digi.   When voice users would 
transmit tail-gate APRS bursts on the repeater uplink, they would get 
retransmitted on 144.39 without any loss in the path.   The repeater-input TNC 
had it's carrier detect wired to the repeater controller to mute the repeat 
audio during the packet burst.   The squelch/COR connection on the 144.39 TNC 
would inhibit the retransmission until the APRS channel was clear.  Since the 
voice repeater had a clear line-of-site shot at an APRS digipeater 7 miles 
away, I was able to use an Icom IC-02 handheld inside the building (no 
additional antennas on the tower) as the 144.39 transmitter.

I could have done this with a single TNC set up as a digipeater, with the RX 
side tied to the repeater audio and the TX side connected to the 144.39 radio, 
but I wanted positive carrier detect on both sides (to mute the repeater repeat 
audio, and to hold off transmit on 144.39.  Further, I wanted the MIc-E setup 
to be transparent to any path the voice user had setup.   (A normal digi setup 
would have consumed one hop of the user's path.)

-------------------------------------------------------------------------------
Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
EchoLink:  Node #  14400  [Think bottom of the 2-meter band]
Home Page:          http://wa8lmf.net

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