I got my Unsigned.io MicroModems working under Linux.

By default they talk KISS protocol.

I attached one to an USB port of my Linux PC and identified it as "ttyUSB2" - look at serial port names ad watch out for the latest one inserted:

ls -al /dev/serial/by-id/

(will look like usb-FTDI_...-if00-port0 -> ../../ttyUSB2 - that is, it's currently the "ttyUSB2" port).

I installed the common ax25-apps and ax25-tools packages (almost any Linux distribution ships them).

1. Create the /etc/ax25/axports configuration file containing only:

# portname mycallsign portspeed paclen window description aprs       KJ4HDR     9600      256    7      my APRS shack

2. "Attach" the port to the portname:

sudo kissattach /dev/ttyUSB2 aprs

3. Listen for packets and show on the terminal:

sudo axlisten -c -a

The Linux kernel interface has a number of benefits, including the use of the KISS modem as a real network interface (i.e.: you can route TCP/IP traffic from/to the "ax0" interface, while sending/receving APRS data on another "ax1" interface, and so on).

---

If you only need APRS without fancy stuff (multiple modems, digipeating, etc), there is no need to use the Linux kernel driver: you just have to read and parse KISS packets coming from the modem serial port (the MicroModem). I use the Device TNC KISS library to extract KISS packets and the Ham Finnish APRS Parser library to parse the encapsulated APRS data (the guys at aprs.fi did a pretty good job).

Example source:

#!/usr/bin/perl

# requires CPAN libraries:
# cpan install Device::TNC::KISS Ham::APRS::FAP

use Ham::APRS::FAP qw(parseaprs kiss_to_tnc2);
use Device::TNC::KISS;

my %config = (
  'port' => '/dev/ttyUSB2',
  'baudrate' => 9600,
  'warn_malformed_kiss' => 0,
  'raw_log' => '/dev/null'
);

my $micromodem = new Device::TNC::KISS(%config);

while(my $kisspkt = $micromodem->read_kiss_frame())
{
  # kiss_to_tnc2 expects a kiss packet without leading/ending 0xc0 byte:
  $kisspkt =~ s/^\xc0//;
  $kisspkt =~ s/\xc0$//;
  $pkt = kiss_to_tnc2($kisspkt);
  %data = ();
  $aprs = parseaprs($pkt, \%data);
  if ( $aprs == 1 )
  {
    print "aprs: ok\n";
    while ( ($key, $value) = each(%data) )
    {
      if($key eq 'capabilities' or $key eq 'wx' or $key eq 'digipeaters')
      {
        if($key eq 'digipeaters')
        {
          while(my($kk,$vv) = each $value)
          {
            while(my($kkk,$vvv) = each $vv)
            {
              print "digi_$kk" . "_$kkk: $vvv\n";
            }
          }
        }
        else
        {
          while(my($kk,$vv) = each $value)
          {
            print "$key" . "_$kk: $vv\n";
          }
        }
      }
      else
      {
        print "$key: $value\n";
      }
    }
  }
  else
  {
    print "aprs: error\n";
    print "resultcode: $data{resultcode}\n";
    print "resultmsg: $data{resultmsg}\n";
  }
  print "\n";
}

# ---

Note that most of this Perl script deals with correctly printing arrays and arrays of arrays (station capabilities, wheater stations, digipeaters in the path).

A coworked donated me an old Yaesu FT-23R radio. I attached it to the MicroModem; the MicroModem to a serial port terminal (I used minicom configured with 9600/N/8/1), tuned the radio to 144.8 MHz (local channel for APRS) and got KISS-encapsulated APRS packets on the terminal.

Note that raw KISS APRS data appear as ugly binary-encoded chunks followed by some human readable fields:

One of my old hobby projects was sampling 24 hours of true radio APRS traffic and sending a daily summary to a blog. I used to read plaintext APRS packets from an old Kenwood TH-D7 radio (which features an RS232 TNC and a number of APRS functions). Back then, when using "parseaprs" function of the Ham::APRS::FAP I needed to set "isax25" parameter to "false", because the TH-D7 outputs data in "TNC2" mode instead of "KISS":

To work with the Yaesu and the Micromodem, I just had let the "isax25" to its default value "true".

My home server (running 24/365) is a 12-year-old Pentium M: it needs less than 1% CPU time to decode some thousands of KISS APRS packets per day.

I also got the TH-D7 correctly getting and decoding APRS packets sent with the MicroModem, without having to tune any parameter. MicroModem just worked out of the box.

I got also a second MicroModem, I am planning some contraption to install on my bike.

My scripts are available on github.