usb

When was the first time we saw a computer? If you’re below 30 or even 40 years, doesn’t matter in which part of the world you lived in at the time, you owned or saw one. Perhaps, for better or worse, a computer might be one of the very few things that became gradually more important or relevant in our lives. Present day, computing is more than ever merged with communication, though they have developed separately. Thanks to USB cables we can connect phones, keyboards, scanners and other kind of devices in a cheap, easy and secure way.

What can be achieved with USB?

  • Easy to connect devices to our PC.
  • It’s a low cost solution that supports transfer rates up to 12MB/s
  • It can support voice, audio and other types of files.
  • Works for different configurations and it can enhance the capacities for more connections.
  • Its versatility allows different uses for communication between devices (example: acquiring microcontroller data via serial communication through USB cables).

Can a USB cable be categorized?

Yes, it depends mainly on the transfer rate of the universal serial bus, a feature that also categorizes the applications it may have.

Low-Speed: 10-100 kb/s. The applications for this transfer rate goes from keyboard, mouse, game peripherals and virtual reality peripherals to monitor configuration.

Medium-Speed: 500 kb/s-10 Mb/s. Used for audio, integrated services digital networks (ISDN), phones and others.

High-Speed: 25-500 MB/s. Used for video and external disks. This speed guarantees latency and a high bandwith, without losing simplicity.

USB diagram and electrical characteristics

usb-02

The USB uses a differential output driver to drive the USB data signal onto the USB cable. The static output swing of the driver in its low state must be below 0.3V with a 1.5k ohm load to 3.6V and in its high state mus be above 2.8V with a 15 k ohm load to ground. With the different transfer rates for USB cables, some of these values can vary to achieve that goal. The signal that goes through an USB cable haves the following waveform.

usb-03

The USB cable must be able to withstand short-circuits of D+ and D- to Vbus, GND, other line or the cable shield at the connector. The transmit phase consists of a symmetrical signal that toggles between drive high and drive low.

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