Ok, so you now have a bag-of-parts, and an article with a schematic. You could build the TNC in one step and check it out at the end. And it is perfectly fine to do it that way. Bob's article has a great set of instructions for how to do that.

These instructions, or STEPS, are for those that want to savor the journey. That is, add a few parts and play with it a little. Add a few more and watch it do a little more. You get the idea. The added benefit of doing it this was is that you will not let problems pile up to be discovered at the end of the build.

Remember, however, this is not intended to replace Bob's instructions. Just to augment them. You still need to read his article and understand it before gong forward.





STEP 4 (Updated July 26, 2005)




What do I do now?

  1. Your 16F88 has been loaded with "bloader"; the boot loader program noted in Bob's article. It has a neat little program in it that turns the red LED on and off for you to to let you know it is there. It will only flash the LED if there is no other program loaded (in addition to the boot loader).
  2. Remember there are CMOS parts on the prototype board and they are sensitive to static electricity. Take care not to zap anything while handling the board.
  3. So go ahead and plug power into the prototype board and verify the red LED flashes!
  4. Remember, once you download any other program (hex file) into the 16F88 the LED will no longer flash. But the boot loader is still there and everything is working fine.


The neat thing about the Olimex board is that you can add two wires and actually get the board to talk to "Screamer" running on your PC!

  1. Just add the the 2 wires shown in RED below. Don't be confused by the schematic and the Spark Fun board being slightly different. These connections will work for you!

  1. Now go to the ARRL site and download the software for the project. You don't need to subscribe to QEX to download it. Here is the link if you have trouble locating it: http://www.arrl.org/qexfiles/  You will need to unzip the file to a directory. This zip bundle will have the screamer program in it.
  2. Dig out your old serial cable and connect the prototype board up to a serial port on your computer. Power up the prototype board.
  3. When you start screamer it will ask for a hex file to load. Browse to the hex file Bob provided in the zip bundle. Select "Download" with the mouse and follow Bob's instructions in the article about power cycling the prototype board. Screamer should download the TNC program (hex file) for you. Not that you can do anything with it yet but you can check out screamer and the boot loader this way.
  4. And there is more!  With only 2 wires added to the prototype board you can see Bob's power-on message and get the software to respond to all of Bob's TNC commands. Isn't that amazing! Don't believe me? Make sure Screamer is NOT running. Open up your favorite terminal emulation program (use Hyperterminal if you don't have a favorite) . Follow Bob's instructions and set the options for 9600 baud, 8 bits, no parity. You may need to experiment with the end of line treatment if you  hit the "Enter" key and nothing happens.
  5. Once you get the terminal emulator program sorted out power cycle the prototype board again. You should see Bob's power-on message. Now you can issue any of Bob's commands and the software will respond. Even use the "perm" command to write your settings to EE memory. They will still be there when you turn the board off and on again. Since the discrete components don't actually "handshake" with the software, it (the software) doesn't even know they are not there!
  6. Not a bad return for only adding 2 wires!


  1. Now we can have some real fun! Lets add a few more parts to the board so we can actually receive some packet. Check out the green lines in the diagram below.

  1. Yep, by adding about 7 parts you will be ready to receive packet.
  2. The only hard part about this step is to wire up a cable to connect to your radio. I am using my 2 meter transceiver and it has a nice auxiliary plug on the back I can get the audio and ground from. The audio from that connector is independent of the speaker audio volume. That makes it nice since I can turn the speaker audio down all the way and not affect the signal going to the TNC!.  At any rate, you need to think about how you want to connect your TNC to your radio at this point.
  3. Now you can tune your radio to 144.39 (the APRS frequency) and listen for the bursts of packet. The sound is an acquired taste;  fortunately you do not need to have the volume very high. If everything is connected you should see the RCV LED come on steady when you hear a good burst of packet through the speaker.
  4. Make sure you have Hyperterm set-up per Bob's instructions and connected to your TNC. Not every packet will print but you should see them start to appear on the terminal emulator's screen.
  5. If this does not happen for you we need to do a little debugging. If it does - congratulations!


You are almost there now! Just a few more components to wire-in and you can transmit as well as receive.

There are logically two separate parts to the transmission circuitry; the part that keys the transmitter ( which includes Q1 ) and the resister ladder ( R1 - R4 & associated components ) that is used to construct the audio sine wave. Both of these are shown in blue below.

If you are using a GPS with this unit don't forget the shorting strap (included in your parts bag) across pins 4 & 5.

What about all the other stuff? Well it already came wired wired up on your prototype board. What a deal!

That's about it. If I have forgotten anything drop me a line and I will add it.