.. APRS with a what kind of pie?!

So I’ve been using Xastir (linux) to provide a sound-card modem for packet radio with quite a bit of success, however, it is kind of hackish and inconvenient to lug around a netbook for the sole purpose of APRS tracking.

TNC-X dedicated hardware TNC

TNC-X dedicated hardware TNC

To solve the problem of the kludgy netbook I tried a TNC-X which is a hardware TNC. I was quite happy to assemble the kit but quickly noticed a number of disappointments. With what appeared to be such a well thought out design, how in the heck did the author overlook galvanic isolation of the PTT circuitry?! Well, sure enough … the PTT no longer works so until I bother digging around in the circuit, I have a receive-only TNC. Total crap!

I got to thinking that perhaps I could just build my own damn TNC using an Arduino. I looked for some Bell 202 Modem ICs and keep coming up dry (or looking at a WAY over-priced MX614 IC). I know that it is possible to use the processing power of the Arduino to do “zero-crossing” frequency decoding, however, I am not sure how much I like that thought. Perhaps I’ll look at this sort of project as an academic endeavour.

Raspberry Pi development board

Raspberry Pi development board

Then the solution leaped out at me! I was staring blankly at a Raspberry Pi development board. If you haven’t heard of these yet, you surely will soon enough. The embedded ARM processor will happily run Linux thus allowing for a ton of flexibility – for free! In a form-factor little larger than a credit card, and at a price point of $35 you get the following:

  • 700MHz processor
  • 256MB ram
  • HDMI & composite video outputs
  • 2 x USB ports
  • Ethernet connectivity
  • SD Card (“hard drive”)
  • A ton of GPIO pins for hardware development
Raspberry Pi form factor

Raspberry Pi form factor

So, pop in a USB soundcard and it looks like I have a very capable soundcard modem TNC which can perform a myriad functions. Oh, and it draws a measly 3.5W (full load – ie. network, etc)

Holy crap! So, $35 for the embedded computer, $15 for a USB soundcard and $10 for an 8GB SD card. That is a $60 TNC-extraordinaire!

I’ll report back with the details soon enough!

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3 Responses to .. APRS with a what kind of pie?!

  1. Hi James
    I’m doing some radio related projects with the Arduino Uno. A Friend and I have been processing National Weather Service SAME alerting. We are using the XR2211 you may want to take a look at that ic from EXAR. You will pick and capacitor which I’ve been using .1uf poly film. and the proper resistors for mark & Space. It has a carrier detection outputs and data out. download a pdf on it and check it out. The XR2206 is the sister encoder chip to the xr2206. They sell for about $5.00 each. We are using a simple pll routine which samples each bit 8 times.

    I’m interested in APRS and packet radio and would like to see a cheaper TNC which could be used to pass APRS to the internet etc. Or used for emergency services.

    I hope the information I provided helps some. If you need a further help let me know.
    look me up on QRZ.com

  2. James says:

    I’ve seen a couple of examples of this in action and I knew it was possible given my previous exposure to embedded micro-controllers (the ARM in particular). I had just never considered using the RaPi as a TNC / iGate when I first received it – I had slightly more advanced plans for it.

    I’ll likely make an entire step-by-step how-to article on configuring soundmodem, and how to install and configure xastir, etc. I think that perhaps the only area where I will have to brush up on my scripting skills is in operating in a so-called “headless” mode.

    Thanks for the post, Bob!

  3. Bob VE3MPG says:

    James, here’s a link that crossed my browser today:

    It may give you a few ideas; no doubt that Raspberry Pi is a versatile platform for many digital projects. Nice article as usual.

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