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post #1 of Old 10-07-1999 Thread Starter
Jim Sexton
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GPS Interfacing

Advanced GPS Interfacing

One of the most vexing problems for new users of charting software is connecting the GPS to the computer so they can communicate. Wouldn't it be great if all you needed to do was connect the blue and the green wire from your GPS to the blue and the green wire from the serial interface cable. Then turn on the computer and see your boat being plotted on the correct computer chart?

But problems occur because of the many GPS manufacturers who interpret the voluntary NMEA 0183 standards in different ways, or because of the myriad types of computers and the way their serial (Com) ports are configured.

In most cases, getting the GPS to "talk" to the computer is fairly simple. But when the two don't, here are some pointers. You'll probably need to first set up the GPS correctly and then find out which wires are used to transmit data. Next, determine which wires on the serial interface cable are used to receive data. This may sound a lot more complicated than it really is. Use the GPS manual and the instructions for the navigation software cable interfacing to guide you. And if you need more help, a quick phone call or visit to their web site can usually clear things up.

Most GPS units have I/O terminals and a cable for connecting to other devices. To transmit information from the GPS to the computer, connect the GPS-signal-ground wire (SG) to the serial-port-ground wire (NMEA B line). Then connect the GPS-transmit-data (TXD) to the serial-port-receive wire (NMEA A line). If you will be uploading data, such as waypoints, from the navigation program into the GPS, you will need to connect the GPS-receive-data (RXD) wire to the serial-transmit wire. Not all navigation programs or GPS units allow you to upload data. These wires are identified by color code in the user's guide.

The navigation and electronic charting programs have a serial interface cable. Most include the cable with the software, though some charge a little extra for it. Software companies also offer (for an additional charge) an optional opto-isolated cable, which I recommend. It isolates your computer from a direct electrical connection and provides a layer of protection.

Next make sure that your GPS unit is set up to transmit the NMEA 0183 standard sentence of RMC or GLL, VTG and DTG. Check your GPS operating manual for instructions on how to do this. If the "talker" (the GPS) isn't speaking the same language as the "listener" (the computer), they will not be able to communicate. The navigation software program will be looking for the correct NMEA sentences on a Com port.

If you are using one of the serial ports for a mouse and the other for a modem, or if your notebook has only one serial port and it's already being used, then you have a problem. But it's easily solved. A serial port PC card will add an extra port and an A/B switch will allow you to use a serial port for the GPS or modem. You may also want to consider using an internal modem.

Read more . . .Advanced GPS Interfacing

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