29. February 2016 · Comments Off on Adding time-out timer (TOT) and notional assemblies to the VAPN design · Topic: Design · Tags:

Got to get in one leap-year posting so why not show some more incremental thoughts on the high level block diagram of the proposed Virginia Packet Network.

Time-out timers (TOT)

One thing I learned from others’ designs is the smart move to place a hardware timeout timer circuit between the TNC and the radio.  It’s one thing if you are operating the radio personally, but another when the system is remote and autonomous; You certainly don’t want to explain your days-long carrier to anyone.  For something like packet, an individual transmission is short so a runaway transmitter is obvious within seconds.  Something akin to a 555 resetting timer circuit makes good sense (the uses of the 555 seem never ending).  I don’t consider the Raspberry-Pi to be high-reliability hardware whatsoever so designing in a fail-safe TOT is prudent.

Hardware hiccups, software crashes or freezes leading to situations like this week long Winlink transmission reported by KYPN.  Favorite quote:

[. . .] how the heck does a sysop not notice something like this going on for a freaking week?

I’m prepared to go to ridiculous lengths to not be “that guy” who loses control over their transmissions.

The diagram below shows this circuit block attached to the TNC block of each frequency subsystem.


2016-02-29 VAPN System Line Drawing

2016-02-29 VAPN System Line Drawing


  • Rearranged the frequencies at the top to finally be from low to high;
  • Adjusted the frequency values to better reflect latest intentions;
  • Added time out timers to the design;
  • Added notional analog interface circuitry with the time out timer block;
  • Dashed lines added around likely assemblies: In this case each frequency will have one rack box with the radio, TOT/TNC combo and power supply (not shown);
  • Cosmetic improvements.

Of all the hopes in this project, the modular approach of having one box per frequency is very appealing.  One box with an RF connector, AC power jack, and Ethernet port… how nice… how modular.

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