Making the GPS/computer connection

Most mariners know that it is possible to connect a GPS receiver to a laptop or desktop computer. Making this hookup allows you to get GPS position information and display it on electronic chart software running on your computer.

The general parameters of the task are easy enough: you take the National Marine Electronics Association (NMEA) 0183 standard output from a GPS receiver and route it over a cable to the computer’s serial port. Electronic charting and navigation programs like the Cap’n, Navtrek, Maptech, Chart-View, and NavimaQ pick up the GPS position data from the serial port and make use of it for displaying your position and for calculating navigational information.

The practical wiring of this setup is not always as easy it might be, however. "If you understand a little bit about electronics, it’s not so hard," said Milt Baker, owner of Bluewater Books and Charts in Fort Lauderdale, Fla., which sells marine software. "But if you aren’t familiar with it you can get a bit confused."

In addition to the need for manually connecting wires by crimping or soldering them, there are a few other factors that can make this operation a bit tricky. One of these involves compatibility between GPS and computer signaling. Most GPS receivers output data using the NMEA 0183 standard. This 0183 standard is similar to, but not the same as, RS-232, the standard used for computer serial ports. The difference has to do with the voltage used and the way those voltages are applied to the various data wires (there is a great deal of arcane detail here about differential signaling, A/B data wires, voltage variations, etc., that we won’t get into). Luckily, there is a wide enough overlap in signaling voltages between the typical GPS and computer so that the two devices can talk to each other. If the two units aren’t able to talk to each other, sometimes a product called a level converter will solve the problem. "These little devices cost about $200," said Kelly Hulce, president of Valhalla Enterprises, a marine electronics system design and installation firm in Fort Lauderdale. "They are probably the single best thing you can do to interface marine electronics." If you have a GPS unit that has the capability to output RS-232 data directly, there’s no need for any type of converter box. However, not all GPS units can do this.

Also, to do a really bulletproof installation, you should employ something called an opto-isolator. This is a device that has tiny light-emitting diodes (LEDs) and light receptors on a chip. These complementary components are separated by an air space. An opto-isolator, wired between a GPS receiver and a computer, will convert signaling voltages (for example, high voltage for 1, low voltage for 0) into LED light pulses. The light receptor interprets the pulses: LED on means high voltage, LED off means low voltage. Thus, there is no electrical connection between the GPS and the computer. This prevents high voltages or voltage surges (like those from lightning strikes, for example) from traveling from one component to the other and causing damage. It also prevents data corruption when two devices are using different signaling voltages. "The NMEA standard is very specific," says Dennis Mills, president of Nautical Technologies, which sells the Cap’n program. "All listeners must be opto-isolated."

Another factor that can cause problems is that not all GPS units put out identical NMEA 0183 data. Some GPS products will vary somewhat in the type of data "sentence" they output. This can cause problems for the electronic charting software or other software that is attempting pick up the NMEA 0183 data and apply it. The good news is that navigation software programmers are aware of most of these interface problems and have written their software to successfully convert the less-than-standard 0183 data into data their program can use.

Most GPS receivers have a combination power/data cable that ends not with a connector but in six or seven loose wires. Two of the wires must be attached to a 12V power supply; the others (it can vary with model and manufacturer) are for wiring to a serial port connector that can plug into the back of the computer. The majority of laptops use a DB9, nine-pin male connector on their serial ports. Some older laptops require a 25-pin serial connector. But this isn’t a problem because inexpensive adapters are available that will convert a 9-pin connector to a 25-pin connector. For desktop units, the DB9 has also become a standard. Certainly most 486 or Pentium machines will use a DB9 connector. Evidently, even nine pins for this connector is overkill. "Pin number 2 is used for data out from the GPS, pin number 3 is data into the GPS for uploading waypoints, and pin number 5 is a ground," said Lyn Beckhorn, marine software manager at Captain Jack’s Software Source in Port Ludlow, Wash. "The other pins are blank. There’s nothing on them."

So the question remains of how to connect the GPS to the laptop. There are two ways to do it. You can take the loose ends of the data wire from the GPS receiver and attach a DB9 connector to them, or you can use the DB9 connector and cable (which also has loose wire ends) supplied by some software companies and connect it to the GPS data cable wire ends.

One thing to remember about all this wiring work: Make a temporary connection first and test to make sure it works. Once you’re sure that you’ve got it right, then make a permanent connection.

Attaching the GPS data wires to a DB9 female connector involves either soldering the wires to the plug or crimping them on using a crimping tool. If you already have a soldering iron, you might try the solder approach. However, you should know that soldering is more difficult than it looks. In the early 60s, during the hot years of the race to the moon, NASA did a study on the best ways to make electrical connections. The study showed that getting a good, reliable solder connection requires quite a bit of skill and plenty of practice. Crimping the connections, on the other hand, can be done using a special racheting tool that you can buy at your local electronics supply house. These tools generally cost from $25 to $50. The advantage to this approach is that it takes very little time to learn how to make a tight, reliable connection.

If you buy one of the major electronic charting programs like Navtrek, Maptech, the Cap’n, or ChartView, you’ll be supplied with a cable that has a female DB9 connector on one end and that ends in loose wires. So, the second approach to getting these two devices connected is to take these loose ends and join them to the loose ends of the GPS-supplied power/data cable. You can use butt connectors and a crimping tool to connect the two sets of wires and then cover the connectors with heat-shrink tubing. Or you can get your soldering iron out and solder the wires together (see comments on soldering above). The NavimaQ product, an electronic charting program for Macintosh owners that can use NOAA/BSB and Maptech charts, supplies an eight-pin Mini-DIN8 plug and cable for plugging into a Mac’s modem or printer serial ports. Like the cable and connectors mentioned above, this cable also ends in loose wires that need to be connected to the data wires from the GPS. Once connected to the laptop, the GPS will output a stream of data using the NMEA 0183 protocol. A GPS receiver outputs data on lat/long, course over the ground, speed over the ground, etc. An electronic charting program then takes that ASCII output and uses it for displaying vessel position as well as any navigational functions. (ASCII, by the way, stands for American Standard Code for Information Interchange. This data coding standard has 128 numerals, letters, symbols, and control codes, each represented by a seven-bit binary number. All e-mail messages, for example, are transmitted over the Internet in ASCII format.) How do you know if you’ve made the GPS-to-computer connection properly? You can use a Windows 95 utility called "hyperterminal" that allows you to look at what is happening at the serial port. "You have to set the parameters properly," said Tim Sullivan, marketing manager for Maptech. "It should be set for 4,800 baud, eight data bits, one stop bit and no parity."

If things are working properly, you will see a stream of incoming ASCII characters. The other way to know if you’ve done it right is to fire up your electronic chart software and start using it. If there is no GPS information available, the charting software will put up a window or somehow flag you that GPS position information is not available. The Maptech program has a menu command called "view I/O data." This will pop up a window and show you what is coming in. Maptech color-codes the data as a further aid in troubleshooting: black text means things are okay, red text means the GPS is not sending data properly. Nautical Software, which sells the ChartView program, has a plug-in to ChartView called GPS Transfer Wizard that aids users in getting their GPS connection set up properly. And if your particular brand of GPS allows you to upload waypoints from your computer to the GPS, the program gives you a way to control the movement of waypoint and route data between the two units. If all this seems difficult and if you question whether you’re up to it, there is another solution: a marine electronics store or a freelance marine electronics technician can do the installation for you.

By Ocean Navigator