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GPS receiver for Kenwood TM-D710E

| 14 Comments

My mobile rig, a Kenwood TM-D710E, needed a GPS receiver for APRS use. The GPS-710 wasn’t readily available and, besides, is quite expensive. I therefore decided to build my own.

I found a Royaltek RGM-3600 GPS receiver on eBay for less than £20. This is a very good SiRF III based receiver. The magnetic case made a nice fridge magnet as only the bare module was needed. Although the listing didn’t mention it, this turned out to be the RS-232 version of the module. If it had been the TTL version, I would have needed an additional MAX232 level converter. The module pinout is available here.

Bob WB4APR (yes, that Bob) has very helpfully provided the pinout of the RJ45 (okay, 8P8C) connector on the TM-D710 display head. It shows that pin 6 is ground and pin 7 is +10V. This is a handy place to leech a few tens of milliwatts for the GPS receiver.

I decided that the easiest way to get access to the +10V pin is by repurposing a RJ45 Y adaptor. Apparently these splitters are sometimes used in ISDN installations. I bought mine from Maplin for £5.99.


The Y adaptor is very simple but the fact it’s already wired makes things easier.


The male end of the adaptor isn’t ideal for soldering but a little bit of work with a craft knife exposes the contacts enough to solder wires into them. Because the result is a bit fragile, I decided to cover the solder joints with epoxy. There are better adaptors that would be easier to tap into.


The GPS requires a 5V power supply. I wanted to make sure the system draws as little power as possible, so I used a Recom R78 series switching DC-DC converter from Rapid. The R78 is a small self-contained device that’s pin compatible with the 78xx linear regulators so it’s trivially easy to use. I decided to add an optional input capacitor and blocking diode for extra protection. The whole assembly was then secured onto the Y adaptor with epoxy to minimise mechanical stress.


All internals of the GPS receiver. The 2.5mm stereo jack is wired according to Kenwood’s instructions (page APRS-2).


The schematic.


I happened to have a suitable plastic box for the parts. The rectangular cutout is made with a craft knife. All parts are installed in the box with double-sided foam tape.


I made two short (~10cm) cables to connect the GPS to the display head, a 2.5mm stereo and a RJ45. Both are wired straight through. I’m planning to replace the jack lead with one with 90 degrees angled jacks for neatness. The box is attached to the back of the display head with velcro. The fact that the receiver antenna sits vertically does not seem to matter.


The GPS sits neatly behind the display head.

The +10V is supplied even when the radio is off. In the off state the rig seems to draw about 40 mA of current from the battery. Some of this is for the standby state of radio and some, probably most, is for the GPS. Overall that’s a negligible amount so I can just leave the GPS on all the time. This is good as the module seems to default to 19200 baud if left unpowered for a long time. The Kenwood only supports baud rates up to 9600, so I had to change the baud rate using the serial terminal on my computer and a suitable interface cable.

14 Comments

  1. Hi,

    I have one of those same GPS receivers and also found it’s stuck i 19200 baud when you first get it. I have it talking to my computer and I was wondering if you could share the command to set it to 4800 baud please? I intend to put the receiver inside my D710 head. When it’s done I can send you a picture to show you were to make the connections.

    Thanks and 73,

    Sander W1SOP

  2. Thanks for your comment. The NMEA command is $PSRF100, as documented here.

    I’m pretty sure the right string and checksum are:
    $PSRF100,1,4800,8,1,0*0E
    …although I recommend you check with the documentation before trying.

    My GPS receiver has once reverted back to 19200 after being unpowered for a while. With that in mind, I would suggest that if you integrate the device into the D710 head, leave the RS232 pins accessible in case you need to reconfigure it.

  3. Thanks for getting back to me so quickly. I had not considered sirf as a source of documentation oddly enough. Unfortunately the command seems to have no effect so it’s possible RoyalTek did something to the firmware. I hope they respond to my request for help. The docs say that the settings are stored in memory backed sram so depending on how long that battery lasts I can see how this would be an issue. Thanks for the warning. Save a few dollars with a cheap gps unit and spend hours making it work. It’s the ham way! :)

    73,

    Sander W1SOP

  4. The fact that it’s talking to your computer is encouraging. Have you checked whether your GPS is in the binary mode instead of NMEA mode? In SiRF mode the NMEA commands will not work.

    SiRFDemo is a good tool for debugging the GPS. You can also use it to switch between NMEA and binary. It’s easy to not-quite-brick-the-device-but-scare-yourself with it, so read the documentation.

    I wonder if one could set the default baud rate to 4800 by flashing the Royaltek unit with some generic, non-Royaltek SiRF firmware. I wouldn’t want to be the first to try it though…

    It’s likely the memory backup battery was the reason my GPS lost the settings when I first tried it. It had probably been powered off for quite some time. Now that the battery is full it should probably last for quite a bit longer.

  5. Yeah, I had found the sirfdemo software before reading your most recent reply. I tried setting the speed to 9600 a few times and finally it stuck. Since both rx and tx will be soldered in parallel to the jack on the headunit I will be able to reprogram it that way later. I’m trying to avoid removing the PCB from the headunit but that would make it really easy to solder onto the pads where the jack is connected. From the top side I can solder either to the smd resistors or remove some coating from the two tiny traces I need to connect to. The rx line is particularly cramped.

    As an aside I noticed SiRF has been bought and that apparently was an opportunity to wipe all support files from their web site. No sirfdemo software there. I got it from the same site as your link.

    I got two of the GPS units at about $17 each so I’m not so worried about bricking one :) Maybe royaltek has its own programming software. Still hoping for a reply from them.

    73,

    Sander W1SOP

  6. Hello!
    Values ​​that have the capacitor and resistor R78 which are welded to 5v 0.5a? Can you tell me what are the connections?
    thanks

  7. Thanks for the question. I just went by the Recom datasheet (first page). So, 3.3-10uF capacitor between Vin and GND. There is no resistor.

  8. Thanks, actually is a diode. Could you please tell more graphically (drawing), I am a newbie in electronics, the wiring diagram between GPS + RJ45 + R78 + diode + capacitor + Jack 2.5.
    thanks

  9. It’s not a diode; I think you’re looking at the wrong page of the datasheet. The standard application circuit is on page 1 of the datasheet.

    I’ve added a simple schematic to the post. It’s really very simple. I’m still not 100% sure if I got the 2.5mm jack Tx/Rx polarity right though; do double-check.

  10. My apologies; it turns out I did add a diode in the circuit too, although it’s optional. I’ll update the circuit diagram appropriately, but you can just leave it out.

  11. Thank you … try to build it .

  12. Hi!
    I need your email to send you some pictures and I You will report on the GPS that I have sent to Taiwan, via Ebay. In the Ebay website described him as a RoyalTek RGM 3600 (I bought the same link you have on the page), but I think it is not the same model.
    Santiago (smatiascm@jerez.es)

  13. Off the subject, and please pardon the triviality!!!! How did you mount the mic to the AC vent?

  14. I used a vehicle-specific Brodit clip. They are super sturdy, easy to fit and easy to remove. The Kenwood’s stock bracket is attached to the Brodit clip with screws.

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