Support \ Specs - USB

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USB

The USB, or universal serial bus has replaced the RS232 and parallel communications in a lot of situations. USB is now the most used interface to connect devices like printer, mice and scanners to personal computers. Many people ask if there is a simple way to convert RS232 to USB. Afterall, they are both serial interfaces. The answer is a little disappointing for most: No, RS232 and USB can't be connected by just soldering the right connector to the cable, but there are RS232 to USB adapters sold for this purpose.

Let's look at USB in some more detail. The name of USB tells us in fact a lot. Universal should't need to have a lot of explanation. The definition of USB tries to address a lot of uses of the interface. Not only communication with modems as was the case with RS232, but with all kinds of devices. The second character in USB stands for serial. This is what confuses a lot of people. Serial is not a family of interchangable communication interfaces. It just tells us that every bit of information is sent in a specific time slot and that no two items of information can be send at one moment. This is not only the case for USB and RS232. Ethernet networks also communicate in a serial way. Actually most communications—even high-speed—are performed in a serial way. Parallel is the minority for the simple reason that you need more lines and thus more expensive material to transfer the data. We see parallel interfaces mostly used at short distances, like connecting a harddisk to the mainboard. The last character in USB stands for bus. This tells us something about the higher architecture allowed. USB is not designed primarly for 1:1 communication like RS232 or the parallel printer interface, but it is a bus architecture where you can attach more than two devices.

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USB requires a shielded cable containing 4 wires.

Two of these, D+ and D-, form a twisted pair responsible for carrying a differential data signal, as well as some single-ended signal states. (For low speed the data lines may not be twisted.)

The signals on these two wires are referenced to the (third) GND wire.

The fourth wire is called VBUS, and carries a nominal 5V supply, which may be used by a device for power.