Upgrade your old TNC to a USB interface

With the current resurgence of Packet Radio I decided to dust off my old Packet Radio Terminal Node Controller (TNC). There were untold thousands of these made in the late 80's and early 90's and to this day can be found at junk sales and hamfests for very modest prices.

One of the issues facing us today is that these modems typicaly used an RS232 serial port to communicate with the host computer. Take a look at your computer. Do you have an RS232 port? Most likely not. They have gone the way of the printer port and the dinosaur. Everything has moved over to USB.

The upgrade is very easy requiring only a USB-TTL interface commonly used to program the Arduino and other like microprocessors. The interface connects to the TTL facilities on the rear of the TNC.

The wiring is trivial. Using some female DuPont connector wires connect the 2 devices together as follows...

USB                             TNC

GND -------------------- GND

TX   -------------------- TX

RX  --------------------  RX

Set up your terminal program to reflect the port and speed settings reqired by your TNC then turn it on.

Express Tracker as a KISS TNC

"AX.25 style Packet Radio is dead".That's what the old timers will tell you. They'll tell you it gave way to the Internet because the speeds were better and you didn't have to modify your rig. They may be right in that statement but when you're in a Red Cross shelter with no network access Packet Radio may be your only way to move that pile of ICS213 forms that have just arrived at your station.

In my search for a modern Terminal Node Controller (fancy term for data modem) I've looked at all sorts of designs. Readers of my blog will be familiar with my various attempts to find something that will connect to the USB port of a modern PC/Laptop for OEM type uses.

I _think_ I've found it. Say "hello" to the Express Tracker from RPC-Electronics. It's a small translucent box that's not quite as big as a packet of cigarettes. It can do APRS tracking when fitted with a GPS RX, act as a APRS Digipeater and most importantly for my needs has a _full_ KISS implementation.

KISS? What's that? I'm not gonna go into that here but suffice to say that it's a way of using a radio modem without all the overhead of a Terminal Node Controller getting in the way. Go here for more about KISS. KISS is interesting to me (and hopefully you too) because it presents a standard interface that many software packages can talk to.

So in my scenario I foresee our local ham club deployed at Red Cross shelters across the town after a severe storm. The local OEM has requested that all traffic be submitted digitally via ICS213 and similar forms. The problem is that there is little to no power to run email and fax machines etc. Enter Packet Radio. It'll run from your laptop and use the same power supply you are running your station from.

I'm not going into how you would operate your chosen package or why this one is better than that one. What I will do is show you how to add your Express Tracker to your Windows and Linux PC's. Mac users _may_ be able to follow the Linux instructions.

OK, Windows first. We are going to use AGWPE to attach to the ET. It's all trivial really. Plug in the USB cable and install the drivers if necessary. Then fire up AGWPE and configure your TNC as below. Your port number may be different to mine. Click OK and then restart AGWPE.

So now you have AGWPE running your ET. Fire up your favourite packet application such as Outpost Packet Manager (pictured below) and configure it to use AGWPE. That's it. You're now on Packet Radio using your Express Tracker.

Here's a few shots of a mail swapping session I had between OPM (Windows) and my real TNC;

And this is OPM talking to JNOS on Linux;

Linux is much simpler (isn't it always?). The modem comes out of the box pre-configured for KISS @ 19200bps. Simply plug it in, find out what the serial port is called (/dev/ttyACM0 on my machine) and then either kissattach /dev/ttyACM0 $PORTNAME $IPADDRESS to run the ET under the native AX.25 tools or add the following to your autoexec.nos file ...

# ---------- Express Tracker specific ----------

#

# We are using a USB<>Serial interface which forces hardware handshake. KISS spec says no handshake of any type so we have to turn it off

shell stty --file /dev/ttyACM0 -crtscts # nip out to the OS and turn off the RTS/CTS on the USB adapter

pause 1 # wait a second for the command to complete

attach asy ttyACM0 - ax25 etrack 1024 256 19200 # attach the Express Tracker to JNOS as a KISS device @ 19k2bps

# That's all. Carry on with the config

#

And that's how it's done!

$100 APRS Tracker Mk2

Way back (2001) before they invented Internet 2.0 I built an APRS tracker for less than $100. It was a collection of stuff slapped together into a box and sported a TinyTrak tracker, GPS receiver and a 2 watt transmitter. In its day it was quite an achievement. Most APRS implementations would run over $150 and require an old TNC and a full FM transceiver. GPS's in those days were also big and ugly too. To this day I still get requests for the radio I used.

Well, over the years it has had updates and repairs and even had a 10W linear amplifier added to improve its range but yet it was still a pile of components slapped together into a somewhat large box. It finally gave up the ghost and forced me to go a different route. I briefly looked at creating my own tracker but abandoned it due to time constraints and other projects.

That was then. This is now. I bought a new car recently and am in the throws of moving my radio gear into it. Whilst I have a "black box" radio that will do APRS very well I still want to run my own bespoke APRS tracker. So that led me back to the $100 APRS Tracker.

Looking around the shack I found an old Motorola SM50 left over from a D-STAR HotSpot project. All I needed now was the tracker and a GPS receiver. Then I remembered that I had looked at this project a few years back. At that time I was playing with Ublox NEO-6 GPS modules for use in NTP time servers. These are sub $10 GPS receivers for adding to your projects. They are trivial to use requiring just 3.3v and an antenna. As you can see from the below picture I was able to integrate one into a tracker prototype (it's that white square blob).

But it didn't work. Or rather, it worked badly. Whilst it would talk to the config software when attached to the PC and indeed would send a beacon it wouldn't send any position data. Now I remember why I abandoned it. Having a little more time now I took another look. 

Turns out that the problem was trivial. In its old form (pictured above) the GPS was spitting out position data at 9600bd but the PIC microcontroller was expecting the data at 4800bd. DUH!! The fix was to ground one of the config pins on the GPS thus forcing it into 4800bd mode. In writing this page and getting the files etc together I've come across some auto detecting firmware that makes this problem go away. And I can tell you that it works wonderfully! If only I had bothered to look a little harder the first time around.

So let's do some accounting ...

• Motorola SM50 FM transceiver    $30  ebay
• Tracker PCB                                  $8    oshpark.com
• Ublox NEO-6 GPS RX module     $12  ebay
• PIC 16F628 microcontroller          $3    ebay
• SMA antenna connector                 $1    hamfest
• Motorola pre-wired 16 pin plug     $4    ebay
• GPS "hockey puck" antenna          $5     ebay
• 1/4 wave 2 meter antenna               $8    ebay
• Mag mount for 2M antenna            $14  ebay

That's a grand total of $85. And if you already have some spare 2M antennas kicking around you can shave another $22 of that for a potential $63.

So I've parked all the board stuff on oshpark.com for you to order. You'll get 3 boards for $22. The PCB will allow you to use any current Ublox NEO-x GPS modules. They'll spit out 9600bd data so you'll have to use 9600bd compatible PIC APRS code. Luckily I've linked to that below. You can also change the PIC for a slightly better one which will have more facilities. 

Eagle files for download ...

APRSTracker_GPS_9600bd.sch

APRSTracker_GPS_9600bd.brd

PIC microcontroller code for download ...

kf163tracker-2.0.2.iso It's a bootable Linux disk. Contains firmware, manuals and config software.

Thanks to DK7IN for his original idea and PE1ICQ for his expansion of this idea and his excellent code.