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Advanced GPS Interfacing

Every Windows compatible computer has at least one connector for receiving and sending information in a serial mode (one character after another) to external devices, such as a modem or a GPS. The connector is usually called Com 1 (short for communications port No. 1).

To start with, you should have an unused serial port. Before you can use GPS data in the navigation and charting program, the cable from the GPS must be plugged into that port. If you only have one Com port on your laptop and if you're using it for an external mouse connection, then you'll need to either not use the external mouse or install a Com port PC card to obtain a Com port for the GPS connection.

The IBM ThinkPad notebook computers come from the factory with the Com 1 connector port disabled because IBM uses that port for infrared devices. The procedure for disabling the infrared serial device and enabling the 9-pin Com 1 port seems to vary from one ThinkPad model to another. So you'll need to read your manual, or call IBM.

Internal fax-modem cards can also interfere with the Com 1 connector. The fax-modem may have been set up to use Com 1. Your choices are to set it up for Com 2 or reconfigure your computer so that the serial connector on the back is Com 2. The manual should explain how to do this. In some cases you can solve the problem by removing the PC card modem from your computer when you want to receive GPS data.

The GPS Com port test using HyperTerminal
If you're not seeing a boat icon on your charting program but the navigation program is set up correctly to receive and display the GPS data, you can test the Com port to see if the GPS data is getting into your computer. Do this by using HyperTerminal in Windows 9x.

1. From the Start button menu, select Programs, then Accessories and double click on HyperTerminal.

2. To get started, double click on the Hypertrm.exe icon.

3. Enter a name, such as GPS Test.

4. When the phone-number window opens, select Direct to Com 1 in the "Connect Using:" box and then click on OK. Note: If you are using a different Com port, select Direct to Com (port number you are using, i.e., 2, 3 or 4.)

5. In the Port Settings window, select 4800 bits per second and click on OK. Note: The majority of GPS units use 4800 but there are some newer GPS units that can also use 9600. Check your GPS manual to be sure. The other default settings are correct and there is no need to change them.

6. If everything is set correctly, the GPS is locked on and the selected Com port is okay, then you will see GPS data being displayed in the HyperTerminal window. That means the problem lies in the ECS program settings. If you can't fix the problem, a quick call to the company's tech support should set you up.

7. If no data is displayed, check to ensure that the GPS is working and set up to transmit the NMEA data. Also recheck all the wire connections. Every GPS company uses different colors of wires for sending out and receiving data. Check your GPS manual to ensure that you are connecting the correct colored wires to the corresponding wires on the cable that came with your navigation program. If everything is set up and wired correctly, then your Com port may be out or disabled.

Using a multiplexer
If you want to import additional data into your computer, such as wind data and depth readings, you'll need either additional Com ports installed on your computer or, more realistically, a multiplexer.

A multiplexer can combine up to 4 NMEA-0183 listener channels into a single RS-232 output for connection to a computer's Com port. This will have complete opto-isolation (in accordance with NMEA requirements). If more than 4 inputs are required, then gang several multiplexers together.

Installation and wiring of the multiplexer is quite simple. The module can be mounted to any convenient surface with all connections made via a terminal strip. The unit is fully automatic once power is applied. It immediately begins acquiring data without user interaction. Simply connect the multiplexer output terminal to your Com port and connect all the inputs to the multiplexer's terminal strip. The multiplexer reads and stores the incoming data from each instrument. When the full message is received, the multiplexer automatically dumps it to the RS 232C output, even as it reads the other inputs. Complete messages are dumped as soon as they are received without any loss of input data.

Multiplexers can be purchased from the same company that sold you the ECS program and come with complete connecting instructions.

Jim Sexton is offline  
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