Nuts & Volts - May 2004

interconnection of the USB system to the computer. The controller could be implemented using hardware, software, or firmware. It initiates all data transfers through the system and communicates with the USB devices using a polled bus.

The controller, on a scheduled basis, sends out a token packet that has the information about the USB device address, type of data, and direction of transaction. All USB devices on the system will receive this packet and decode the address part of the packet. The device that is being addressed will start the transaction; data is transferred either from the device to the host or from the host to the device. To be certain that the data transfer was a success, the destination responds with a handshake packet. This is similar to networking between two computers.

The last thing you need to set up a USB system is the physical USB device — for example, a USB scanner. This USB physical device includes the function or device, USB logical device, and USB bus interface. USB physical devices furnish additional functionality to the host. The types of functionality provided to the host vary for each USB device, but all USB logical devices provide the same basic interface to the host. This is to let the host manage all USB devices in the same manner. USB provides an appealing solution for connecting different devices to a computer without any difficulties. With USB, you can expand your system’s hardware without the limits on port availability.

USB allows devices from different vendors to interoperate in an open architecture system. The USB specifications allow vendors or developers enough room for product versatility without the burden of carrying obsolete interfaces.

USB’s future looks good and we are seeing many new products with USB features. USB is here to stay — unless something better comes along. For more information, go to www.usb.org NV

Figure 6. USB host/device in layered form.

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