Internetpräsenz Peter Lenhard

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navibe GM720 english

Experiences and repair

This is not a pofessional page, you can read my personal expieriences only. Who ever follows my footsteps does so at own risk!

System: Windows XP, connection: USB cable

Buying

I purchased the unit in november 2006 via eBay from techladen (german dealer, present only at ebay afaik). I am very satisfied wirth the dealer, he sent the unit very soon after my payment and the descripton at eBay was correct.

Technically very good

Under the technical point of view the buying was a bargain. It was the cheapest gps-mouse with SiRF III I have seen. The receiver works quite accurate, my route planner told me the position within 1 or 2 meters. I tested only once in the street I'm living, under good conditions.

Poorly equipped

You can't buy luxury for a bargain price. With the unit comes a Mini-CD with a driver and a handbook.

The driver (I downloaded the neweset one from navibe) installs a virtual com-port via USB.

The handbook is only slightly more than a technical data sheet.

The unit works very fine without any modifications and there is probably no reason for changes in the setup.

But if you want to make modifications (set the baud-rate to 4800 or so), you will need additional tools.

The virtual com-port

If you set the port-number manually in the drier, it will be remembered the next time you plug in the unit an the same usb port. But if you choose anothe usb port, the com-port-numver will be regognized as busy and the next one is assigned. Then you have to assign you favorite port number manually.

Becuase of a very similar technical description I tried to use the driver for holux 213, it worked fine and in the same manner on my equipment.

The baud-rate of the sirf chip is 9600 and cannot be changed here. If you choose another rate on your computer, there will be no communication.

If you need a baud-rate 4800, you have to use additional tools.

My experiences with sirfdemo.exe

As a curious hobbyist I looked for a configuration tool. In the forum gpspassion (listed in the links) you can read several postings about a tool programm named sirfdemo.exe. It is manufactured by sirf themselves as a tool for developers and partners and they do not give it to buyers directly. But you find a download link in the forum.

The programm (newest version I found: 3.81) has no actual documentation (last version found was 3.75), but luckily there are several postings from carl@sirf in the forum, a meber of sirf support. If you read the postings completeley, you will know enough.

The program reads data in nmea format and in sirf format.
Nmea data is shown in a text window only.
Sirf data gets examined and shown in several special windows. I sirf mode you can change some features of your sirf chip.

Be careful: This program is a tool for developers who know what they do. It is possible to send your sirf chip to a state where no communication and no return ist possible with one mouseclick only!

First trap (for everyone): Swith to User1 Protocol
If you click this menu entry, your sirf chip is set to an alternative communications protocol you manufacturer can develop. In the very most cases this is not done, and the sirf chip does not communicate at all.
The only way out is a factory reset.

As you can imagine I'm one of these heroes who first use a program and then read, when there is a problem. I set my unit to user1 protocol before I knew what it is. After more reading in the forum I realized I had to use screwdriver and soldering iron.

Second trap (BlueTooth only): baud-rate 57600 is set automatically
If you change to sirf mode via menu entry, sirfdemo (3.81) will additionally change the baud-rate to 57600. Most BlueTooth modems are not capable of this rate and it is not possible to communicate with the gps unit via BlueTooth any more, because the sirf chip uses 57600 baud. Some manufacturers do not perform such senseless baud-rate changes, in this case you have to set the baud-rate 38400 on your computer and can communicate with your sirf chip.
Otherwise you have to perform a factory reset, see below.

With BlueTooth you should change to sirf mode with a sirf command you enter manually (Transmit Serial Message...), see documentation. I have never tried this because my unit is usb only.

Factory reset

I performed the following steps sucessfully on my GM720, but you will surely loose your warranty.

You open the four screws (one under the warranty sticker) and open the case of your unit. The circuit board ist fixed with a fifth screw, it has to be removed. On the other side of the circuit board you can see a square sheet metal cover which has to be removed. You need a very small screw driver and a soldering iron for electronics. The cover is soldered to the small circuit board on three of its edges and has to be resoldered later, so be careful with it. Put the screwdriver under the cover near one edge and turn it carefully. There is a mechanical tension now and if you heaten the soldering at the edge, the cover will come up for 1 or 2 millimeters. Do the same with the two other soldered edges and the sheet metal cover will fall down.

You can see two small paper stickers, one of them covering a small soldered-in coin cell. Leave the paper where it is and put the screwdriver under the coin cell, so you produce a mechanical tension at the soldering point of the cell. Heat up one of these points on the circuit board and the cell looses contact. The batteriy-buffered RAM on the sirf chip will die out soon, best you short circuit the two contacts far the battery on the circuit board. You have reached your goal: factory reset.

Resolder the battery now and place the sheet metal cover on the small circuit board. On the placed cover, heat up the three solder points and they will connect to their metal contacts beneath.

I wasn't feeling well doing all this, but my unit works fine again.

Baud-rate 4800

If you change to nmea mode in sirfdemo, you have to specify the transmitted nmea sentences and the baud-rate. Be careful: The standard setting requires 9600 baud, otherwise the transmitting capacity is not sufficient and you get transmission faults. If you activate less nmea sentences (only RMC?), 4800 baud should be enaough.

You have to switch you computer to the same baud-rate, needless to say.