External GPS for TM-D710

I’ve been running my TM-D710 in the car for a while using a Garmin GPSMap 60CSX which works well but does add clutter to the dashboard and is just another device to remove when leaving the car in a public place. I could upgrade to the newer TM-D710G but I can’t justify the £550 price tag just to get a built in gps so I have decided to add a gps unit to my existing unit.

There are a few posts on the internet from others who have gone down the route of adding an internal GPS unit to the D710 but the small serial output unit that most use is no longer available and other suitable units are very hard to find. There are also 2 other issues that bother me with this method.

  1. The GPS antenna is usually installed sideways due to limited space inside the head unit. This gives it a view through the front windscreen of the car when mounted and needs the head unit of the TM-D710 to be installed in such a way that it can ‘see’ the sky.
  2. Installing the GPS internally prevents the possibility of using an external GPS unit like the GPSMap 60CSX to display received stations and navigate using APRS waypoints. This is a function that I would like to keep and be able to use in the future.

With this in mind, I decided to install an external GPS receiver that could utilise an external antenna for a better view of the sky and have the ability to remove so that the original GPS input on the head unit could still be used. I think the best way to do this is to add a new 3.5mm TRRS jack to the head unit and install a GPS ‘Puck’ that has a RS232 serial output on the roof of the car. In normal use, the GPS puck could be plugged into the back of the TM-D710 and left installed. If I wanted to use the GPSMap 60-CSX then I would just need to disconnect the GPS puck and plug in the Garmin.

I found this compact RS232 serial GPS…

https://www.aliexpress.com/item/DB9-Female-interface-connector-RS-232-UBLOX-7020-chipset-GPS-Receiver-with-Module-and-Antenna-BT/32796084876.html

Here’s the unit once it arrived.

Naturally, the first thing I did was open it up to see what’s inside.

I’m going to install the unit onto the roof of the car so I added some silicone to the lip of the case before I assembled the unit.

The last job for the GPS Puck was to remove the serial plug and install a TRRS plug.

Next, I had to strip down the TM-D710 head unit and install a TRRS socket. I used a socket on a short flying lead and drilled a small hole in the back of the head unit case to pass it through.

The new hole in the back of the head unit just above the screw hole on the left.

The serial output from the GPS Puck is fed into the head unit via the input pin on the existing GPS socket. The PCB needs to be removed from the case so the wire can be soldered on the front side.

The GPS Puck needs 12v to operate. This can be picked up from the beck side of the head PCB. 12V+ from the leg of IC6 and Ground from the capacitor close by.

The TRRS socket is on a short lead so the GPS puck can be easily disconnected.

The only thing left to do is reassemble the head unit and test.

All in all, a very easy mod to add GPS to an older TM-D710 without sacrificing the ability to add a more traditional GPS unit via the original connector.

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