[aprssig] TNC Zip-Lan for training

Bob Bruninga bruninga at usna.edu
Tue Oct 6 22:25:22 EDT 2009

> I was wondering if multiple TNC's could 
> be connected together [without radios]
> using something like Bob's ZIP-LAN?
> http://www.aprs.org/txt/zip-lan.txt

Sure I do it in our labs for students (for PSK-31), but it will also work great for APRS and packet.  Ieven splice in some on-air audio so that real packets are thrown around as well.

For PSK-31 (no TNC, just soundcards) I connect all the LINE-OUTS through a 10k series resistor to a single 1K resistor to ground.  Then I connect that 1k resistor tie point to ALL of the PC line-inputs.  

I also feed HF audio into the same tie point (via a 10k series resistor) and now all the students can see 20 Meter PSK on all their PC's and can join in by transmitting to each other.  Their signals appear on everyone's waterfall just like all the HF signals... but there is no PTT and no Transmitter, so their signals stay in the lab.

No reason the same thing should not work for TNC's as long as you can adjsut  the TNC's audio HIGH enough.  Normally TNC's are designed with low level MIC audio and you need more like linelevel to drive all the othere TNC's.  But again, you can also feed in live APRS off-air audio so they can all se what a congested channel looks like.

Bob, WB4aPR


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