[aprssig] D700/D710 data port

Stephen H. Smith wa8lmf2 at aol.com
Mon Mar 31 16:02:16 EDT 2008

Scott Miller wrote:
> Since I've had several requests, I'm considering building a Garmin 
> protocol converter for the D700/D710 after all.  I don't have one 
> myself, though, and it's not clear from the manual what the data port 
> does when it's in APRS mode.
> I could have it act as just a dumb GPS data translator, using only the 
> GPS jack, but that wouldn't support waypoint symbols and comment text.
> So what exactly does the TNC's DB9 do when the radio is busy receiving 
> APRS data?  Does it still function as a normal TNC, spitting out every 
> packet it receives in monitor mode?  I think I've heard enough bad 
> things about KISS mode on the D700 to want to avoid that.
> What about incoming APRS messages?  Do they get any special treatment? 
> Can outgoing messages be sent through the data port?

[ The following observations apply to the TH-D7/D700.  I understand that 
the D710 has expanded capability to operate in standalone mode and still 
pass data to external devices, that the earlier radios don't have.  
Presumably, any protocol converter should cater to the lowest common 
denominator to accommodate the greatest range of radios. (Or have a 
jumper or software command option to switch between D700 and D710 mode.  

In stand-alone "APRS" mode, nothing comes out of the main D700 serial 
port.    It does listen for radio control commands in proprietary 
Kenwood format, but doesn't respond to packet/APRS-type data sent to 
it.   All interaction and display is via the front panel buttons and 

When the radio is placed into "PACKET" mode, the serial ports starts 
acting like a typical TNC connected to a normal radio,   with a command 
set similar to  (but not identical to)  a TNC2.  When the TNC is in 
command mode (switched to from CONVersational with the typical "^C") it  
accepts the Kenwood radio control commands in addition to standard TNC 
commands.     [ The Kenwood radio control commands include turning the 
TNC ON/OFF, switching between "APRS" and "PACKET", setting the band, 
frequency, PL codes, dimming the front panel, etc. ]

Further, in PACKET mode, the internal firmware of the radio forwards 
NMEA data heard from the GPS attached to the dedicated GPS port out the 
main serial port, along with received off-air data.   The raw NMEA GPS 
data is repackaged with a distinctive prefix that allows applications at 
the other end of the serial cable to separate locally-generated GPS data 
from off-air RX packets in the same serial stream.   The D700's two 
serial ports can operate at different baud rates with the GPS port 
running at generic 4800 NMEA while the main port typically runs at 
9600.  The baud rate conversion is completely automatic. 

In the "PACKET" mode, the TNC does not use GPS data to originate beacons 
itself, as does a KPC3+ with a GPS on it's second receive-only serial 
port.    Rather it is up to the application at the other end of the 
serial cable to extract (prefixed) raw NMEA GPS data, reform it into 
proper APRS format (ideally Mic-E), command the TNC into CONV mode, send 
the string back to the TNC and then "^C" the TNC back to command mode.

The Kenwood GPS data forwarding only relays a SINGLE NMEA string of your 
choice out the main serial port, determined by the TNC command 
"GPSTEXT".    This forces you to choose between either  
LAT/LONG/VELOCITY or LAT/LONG/ALTITUDE (depending on whether you forward 
GPGLL or GPRMC) in beacons generated by external programs.  Oddly, when 
running in stand-alone "APRS" mode, the Kenwoods parse multiple strings 
and generate Mic-E packets that contain both velocity and 
altitude.         Since you would be getting data directly from the GPS 
, not via the Kenwood pass-through, you should be able to de BETTER than 
normal PC/programs connected to the serial port; ie. 
externally-generated beacons that include LAT/LONG, VELOCITY and 

P.S.  I have a Garmin Nuvi 680 (4.3" wide QVGA screen), a D700 and a 
TH-D7 just waiting to beta-test your creation......


Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
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