Hi There, One extra little bit is that USB is a "master/slave" system. One end is the master, the other the slave. In most cases you can see this as master ends are (usually) the flat four pin whilst slaves are (usually) the square one. Using this relationship you can get away with just two-twisted-pairs rather than the original 25 pins and now 9 pins of a conventional RS232 serial connection.
With devices connected to a PC (or Mac) -- the PC is the master device in charge of all data transfers, the USB peripheral device is the slave - doing what it's told. That works fine for connecting printers, scanners, mice, keyboards or mp3 players to a computer. The USB on the Garmin in question is, or course, a slave which connects to a computer. But that's why you can't get USB peripherals to talk to each other directly -- they're both slaves. So, just because you have a USB connection on a GPS and, for example, a USB connection on a radio -- they are unlikely to be able to communicate because neither has the master logic. Similarly you can't use a USB serial convertor to go between a USB GPS and the serial port on your radio -- the convertor's USB is designed to operate as a slave to a computer! Of course, firewire (aka IEEE 1394, i.Link etc) is not master/slave -- it's "computer" to "computer" -- which is why you can connect any firewire device to any other. Neither device is in charge of any data transfer so Mac's (or PCs) can be networked together just using firewire leads, cameras can transfer pictures to firewire drives, scanners can transfer data straight to a printer without a PC being needed and so forth. It also allows up to 63 devices to be chained together -- but that's another story. If only the chips were cheaper, it would be a far better way of connecting our kit together. Hope this helps. -- Regards Andy, G8TQH http://www.rickham.net/
