GPS for Dummies.pdf - Engineering Surveyor

Chapter 9: Interfacing a GPS Receiver to a Computer157Setting up communications between a GPS receiver and a PC is a one-timeprocess. After you get everything working, no worries about the next time.COM portsA COM port is a computer serial port that’s used to connect a mouse, modem,or other device, like a GPS receiver. (COM stands for communication, and theports are called serial ports because they receive data serially, one character ata time.) COM ports typically have a small oval, D-shaped connector (with ninepins) and are located on the back of your computer. (If you have other devicesplugged into your serial ports, you’ll need to unplug them so your GPS receivercable has a port to plug in to. You can plug the other devices back in whenyou’re through transferring data with your GPS receiver.)Your PC might have one or two physical serial ports that you can plug devicesinto, but Windows allows you to assign a COM port number to each device.These numbers usually range from 1 to 4 but might go as high as 256 if a USBadapter is used, (which I talk about in the upcoming section, “USB ports”). Inmost cases, you won’t need to use Windows to reassign any of the port numbers.Just know that you have numbered COM ports and that you need toassign one to your GPS receiver, which I talk about next.To get more detailed information about COM ports and Windows, includinghow to change settings with Windows Device Manager, head to the Microsoftsupport Web site at http://support.microsoft.com and search theKnowledge Base for com port. Or pick up a copy of PCs For Dummies, 9thEdition (Wiley) by Dan Gookin, which has an excellent chapter that clearlyexplains everything you need to know about the subject.The program you’re using to interface with the GPS receiver is where you needto specify which COM port number the receiver is connected to. Programs typicallyuse a drop-down list box that shows all the COM ports; just select onefrom there. (Some programs have an autoselect feature that tries to establishcommunications on all available COM ports until the port with the GPSreceiver is found.)COM ports have properties that establish how the communication betweenthe two devices will occur. Generally, both of the devices need to have thesame settings. The COM port properties include Baud rate: Baud is the speed at which the port communicates withother devices. This number is in bits per second (bps): the bigger thenumber, the faster the speed. Data bits: This is the number of data bits that are transferred for eachcharacter, typically 7 or 8.