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

Stephen H. Smith wa8lmf2 at aol.com
Wed Jan 25 17:26:19 CST 2017

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.
> 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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