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 Updated  29Mar.2011

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Kenwood TH-D7 
NMEA bug fix
"The NMEA Potator"

TH-D7 with GPS

BACKGROUND

PROJECT DESCRIPTION

MOUNTED INSIDE THE TH-D7

This project is not a modification of your TH-D7, or, at least, is not an easy one. It is a circuit that, externally or internally, reshapes the NMEA sentences from a non-compatible GPS to be correctly understood by older versions of this popular Kenwood handheld transceiver.

BACKGROUND

When early TH-D7 came out (it was 1999), it was a great success because of their innovative concept of features packed inside a radio. A proof of that is the large number of units sold and that this model, together with the TM-D700 mobile, is still in production. What suddenly made these radios different from the others was the immediate APRS capability simply connecting them to a GPS unit. At that time, all GPS units were compliant to the NMEA 183.2, but soon the new NMEA 183.3 started to be adopted by all GPS manufacturers: to support the increased accuracy, 4 or more decimal digits appeared in the position data, while Kenwood radios were designed to read fixed-lenght data with 3 decimal digits. After that many TH-D7 owners discovered that newest GPS units did not work with their radio, Kenwood installed a new firmware in the CPU to support the NMEA 183.3, fixing some other bugs and adding new functionalities. This made happy the new buyers, while to the owners of older s/n was given the possibility to send the radio at their expenses to authorized service labs for an upgrade. Useless to say, we preferred to find our own solution for our little HTs...

 

PROJECT DESCRIPTION

The TH-D7 uses two NMEA sentences out of the many sent by the GPS unit every second: the $GPGGA and the $GPRMC. As said in the BACKGROUND section, 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. Let's look more in detail to the following examples of NMEA sentences as sent, respectively, by a non-compatible GPS and a compatible one:

NMEA 183.3 GPS sends:

$GPRMC,192944,V,3733.8310,N,01504.5017,E,20.0,270.8,081205,2.0,E,S*23
$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,S*23
$GPGGA,192958,
3733.807,N,01504.422,E,8,09,2.0,335.4,M,38.3,M,,*4A

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, so why not to snip the extra digit before it reaches the TH-D7? Our "potator" (ital-english term for "snipper")  project is based on this idea, simple but needing a minimum of hardware able to:

  • receive the NMEA strings

  •  

  • identify the $GPGGA and $GPRMC sentences

  •  

  • recognize the length of the Lat/Long fields inside those sentences

  •  

  • remove the least significant decimal digits exceeding the 3 expected by the radio

  •  

  • recalculate the checksum

  •  

  • reformat both sentences and send them to the radio

 

A suitable microcontroller to perform these tasks is the PIC16F628, because of the Hardware UART that allows pipeline data RX/processing/TX with minimal code-writing efforts. It has to be paired with an RS-232 level interface to correctly communicate between the GPS and the radio, unless the microcontroller is put inside the radio. A LED blinks differently to show whether or not the NMEA sentences are being snipped, and stays off if no NMEA data are received.

The schematic is the following:


(click on picture to open in new frame)

The code has been written by Andrea, and correct operation has been verified with the GPS units that formerly didn't work with our radios: Garmin E-TREX, Garmin E-MAP, Garmin 65, Magellan 315. There should be no reasons why other NMEA183.3 units will not work with our project. The .hex code to allow you to program the micro by yourself can be found in the downloads section.

The circuit can be put in a small plastic box and powered by a 9V battery, still keeping the portability of the system GPS+potator+THD7.

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MOUNTED INSIDE THE TH-D7

The original project was not intended to be put inside the radio, but to stay externally and process the data flowing from the GPS to the TH-D7. Nevertheless, having a couple of SMD versions of the microcontroller and a lot of courage, Johnny has managed to definitively implant the circuit inside the radios and forget about incompatible GPS units.

The miniaturized SMD PCB version is shaped to fit inside the hole corresponding to the speaker magnet. In this area we have about 4mm of thickness: enough if used smartly as shown in the picture. The level shifter is not needed because the data flow is captured after the RS-232 interface already mounted inside the TH-D7: dataIN, dataOUT and the 5V supply are taken from the bottom side of the Logic unit.

The implant is a risky operation, and even if we have been successful in doing it in the two radios we own, it is not recommended to do the same with yours.

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The projects presented in these pages are our own design and have been tested and verified by ourselves at the best we can. However, they might be inspired by concepts, ideas, solutions coming from known-art or free resources on the Web. We provide them as  reference designs to skilled hobbyists and technicians  who are willing to reproduce them for non-commercial use. Your results might be different from ours and we cannot be considered responsible for that. Similarly, we are not responsible for any damage or injury you might incur while building, assembling or using the equipments, projects or ideas presented in these pages. The firmware embedded in our projects is our property unless differently stated and, when available in the Download Area, it is license-free only for non-commercial purposes.  

Hotwater 2007-2011