I’ve just got hold of a Raspberry Pi, and my first thoughts were that it would be interesting to try to get a nice packet radio station working without the overhead of a full computer running it. I’ve experimented in the past with linux based systems but not progressed too far. I regret missing the popular period of packet, which seems to have died in the Internet age, but suspect that there might be some latent interest such as mine with a bit of encouragement.
Prepare your Raspberry
Basic setup following recipe from https://projects.drogon.net/raspberry-pi/initial-setup1/ I used Win32DiskImager and a Kingston 16GB ultimate 100X Class 10 card I would then advise tidying things up following https://projects.drogon.net/raspberry-pi/initial-setup2/ It’s then always good practice to update aptitude and upgrade any packages:
sudo apt-get update
sudo apt-get upgrade
The upgrade command will likely take some time as it recompiles all the upgraded packages, but you’ll be left feeling warm and fuzzy that everything is bang up to date.
Prepare the TNC
To start off with I’m using a hardware TNC and may progress to using soundmodem and a USB soundcard later.
I have a PacComm Tiny-2 Mk2, and I have previously made interface cables for a variety of radios, I also have RS232 cables to connect to PC. Google will help if you need it here.
To confirm everything is working, connect a PC to the TNC, then using a terminal emulator such as Putty check that the radio and TNC are working correctly. When the TNC is powered on you should see a stream of boot text, and you should be able to enter commands to set callsign etc. With the radio tuned to the 2m APRS frequency 144.800MHz you should hopefully see some APRS data packets coming in.
Raspeberry RS232 interface
The UART pins on the GPIO header are at TTL levels and need converting top be able to be used as an RS232 interface. The device for the job is a MAX3232CPE which is a variant of the MAX232 serial buffer which tolerates 3.3v inputs.
I found a neat shortcut to building my own interface circuit was to buy a ready-made board from eBay – part no JY-R2T it comes complete with 9pin D connector, activity LEDs, a header connector and a short cable that allows you to plug in to the relevant pins on the Raspberry – perfect!
Connections for my interface board to the GPIO header are:
(GPIO) (MAX3232 interface)
Pin 1 (3.3v) -> VCC
Pin 6 (0v) -> GND
Pin 8 (TxD) -> TXD
Pin 10 (RxD) -> RXD
The Debian image we installed will have the UART configured to provide a console on the serial port – we want to be able to use it for our own purposes. If you followed the second Drogon recipe above, we have already commented out the getty line in /etc/inittab, but we also need to change /boot/cmdline.txt – remove the parameters ‘console=ttyAMA0,115200 kgdboc=ttyAMA0,115200’ and reboot.
Finally, for testing the interface, let’s install a terminal emulator and connect to our TNC or a PC running Putty. For the PC a straight pin-pin male-female 9 pin d cable should work. Connecting the Raspberry to the TNC with my serial interface board required a crossover male – female cable where tx and rx lines cross.
sudo apt-get install minicom
minicom –b 9600 –o –D /dev/ttyAMA0
If all goes well, you should have the same console access to the TNC that you had with Putty under Windows.