MotoAtlas

Using your Motorola Oncore GPS with DeLorme's StreetAtlas

The Motorola GPS normally uses a binary communication protocol but does have an NMEA mode. However, their NMEA mode isn't perfect. I'm not sure what messages Street Atlas 4.0 sends to get the position information - [Do you know this?] but the Motorola GPS doesn't respond to them.

To get the GPS to work, you must tell the GPS to send location messages manually. I used PCPLUS to do this. Here are the important sequences:

• Connect the GPS to the com port you're using and power it up. Using PCPLUS (I couldn't get Window's HyperTerm to work correctly), set the communication protocol to 9600, 8N1.

  If the GPS was last in a mode where it was regularly reporting positions, it will still be in that mode and you may get packets right from the beginning. These will probably be in binary which means that you're likely to hear your computer's bell going crazy. Not to worry. You can send messages while it's spewing out garbage and it will accept them. This holds true when you're sending the NMEA messages later on as well - though at that point the messages will be somewhat readable and not look (as much) like garbage.

• To tell the GPS to go to NMEA mode send the following bytes:

  64,64,67,105,1,43

  I set up a macro to do this and used the ALT+Dial-the-number-on-the- keypad trick to get the nonASCII characters.

  Then send a CR and LF. This is critical as the Motorola GPS doesn't accept any messages unless it sees the CR/LF. A CR is the Enter key and a LF is control-J. Or you could set up PCPLUS to send a CR/LF every time you hit the Enter key.

• Once you've told the GPS to go to NMEA mode it will change its baud rate to 4800. You need to do the same thing in PCPLUS - so the new settings are 4800, 8N1.

• Now tell the GPS to start outputting position messages. The following three commands send position messages every two seconds:

  $PMOTG,GGA,0002*02 CR/LF
  $PMOTG,RMC,0002*1F CR/LF
  $PMOTG,GSA,0002*16 CR/LF

• By now the GPS should be kicking out three different position messages every two seconds. You're ready to close out of PCPLUS and start of Street Atlas. Windows won't let you share serial ports so you'll only be able to use one of these applications at a time. ["Here's a nickel, kid. Go buy yourself a real operating system like Linux."]

• The Motorola GPS will remember these settings when you power it off, so you may only need to jump through the hoops once. More likely, however you use your Motorola GPS for many other things than Street Atlas. In which case you will need to return it to Motorola Binary Mode. The GPS software that Motorola supplies will take care of this during its power up sequence. Alternately, you can send the following message:

  $PMOTG,FOR,0*2A CR/LF

  It will continue to kick out the messages that you requested while it was in NMEA mode - but now they'll be in Motorola Binary Format.

More about that NMEA format

Suppose you want to change the report rate, or use one of the other NMEA messages. The only thing stopping you is the sumcheck - the last two digits of the NMEA message.

This program is designed to calculate the checksum on an NMEA message. The following is an example of a Motorola Oncore NMEA command message:

$PMOTG,GGA,0002*02

followed by a CR/LF.

All data under NMEA is sent as sentences. A sentence starts with a "$", a two letter "talker ID", and a three letter "sentence ID". Then follows a number of fields giving data. Finally a "*" precedes a two hex-digit checksum. The checksum is calculated as the exclusive-or of all the characters between the $ and the *.

This program takes any sequence of characters and provides a checksum for that sequence of characters. So, to calculate the checksum for the above message, you would type:

nmeachecksum "PMOTG,GGA,0002"

and the software would return "02".

The Code

/*****
 nmeachecksum: calculates exclusive-or checksum of provided string.
 Code released into public domain.
 (c)1997 Rudy Moore
 *****/

main(int argv, char *argc[])
{
        int i;
        char sumcheck=0;

        if (argv != 2) {
                printf("Usage: %s <message string>\n\n",argc[0]);
                printf("This program calculates the exclusive-or checksum of the provided string.\n");
                exit(1);
                }

        for( i=0; i<strlen(argc[1]); i++ ) {
                sumcheck = sumcheck ^ argc[1][i];
                }

        printf("sumcheck: %.2X\n",sumcheck);
        printf("complete message: $%s*%.2X\n",argc[1],sumcheck);
}

Rudy Moore