thd7gps
Source code and project on github
https://github.com/in3aqk/thd7gps
This is a Arduino firmware able to convert NMEA 183.3 strings to NMEA 183.2. It's necessary to let the old Kenwood TH-D7 hamradio rtx with unmodified firmware to work with new Nmea GPS.
The TH-D7 uses two NMEA sentences out of the many sent by the GPS unit every second: the $GPGGA and the $GPRMC.
Early firmware version did not recognize the commas separating the various fields inside these sentences, using instead a fixed-length method that could lead to wrong data interpretation.
For Example:
NMEA 183.3 GPS sends:
$GPRMC,192944,V,3733.8310,N,01504.5017,E,20.0,270.8,081205,2.0,E,S23 $GPGGA,192958,3733.8070,N,01504.4222,E,8,09,2.0,335.4,M,38.3,M,,4A
NMEA 183.2 GPS sends:
$GPRMC,192944,V,3733.831,N,01504.501,E,20.0,270.8,081205,2.0,E,S23 $GPGGA,192958,3733.807,N,01504.422,E,8,09,2.0,335.4,M,38.3,M,,4A
Pay attention the checksum on these examble string are probably wrong.
The 4 decimal digits in the Lat and Long fields instead of the 3 expected by the TH-D7 are the source of the incompatibility. The arduino code strips the extra digit and resent it to the TH-D7 in NMEA 183.2 format
The hardware in base on
- Arduino nano
- GPS whit nmea output
- Max 232 level converter
Hardware and firmware is able to:
- receive the NMEA strings with a software serial port
- identify the $GPGGA and $GPRMC sentences
- remove the least significant decimal digits exceeding the 3 expected by the radio
- recalculate the checksum
- transmit the nmea string to the radio with a software serial port
Reference
http://www.qsl.net/n6qab/n6qabgps1.htm
(c) 2017 Paolo Mattiolo In3aqk
