Data Acquisition

Cable lengths are very important (and short) for the USB system. All cables, even ifthey come out of repeater hubs, must be counted in the total length of the cables.The controller chips for the host hub usually reside on the motherboard inside the PC,(although the hub could be a PCB in a PCI slot). The host controller does the parallel toserial and serial to parallel conversion from the PCI bus to the USB connectors.Sometimes a pre-processor is used to improve efficiency of the USB system. This hostcontroller and connector combination is called the root hub or host hub. The host Hubs’function is to pass the information to and from the PCI bus to the data lines (+D and –D)on the USB socket. The host controller can control the speed at which the USB operates.It also connects power lines (+5 V and ground) to a USB device via the USB cable. Theexternal USB device may be another USB hub or a USB type device like a printer.Figure 10.2Host hub block diagramThe root hub has complete control over the USB ports. This control would include:• Initialization and configuration• Enabling and disabling the ports• Recognizing the speed of devices• Recognizing that a device has been connected• Getting information from the application software• Creating a packet and then frame• Sending the information on to the bus• Waiting and recognizing a response• Error correction• Recognizing that a device has been disconnected• Using the port as a repeater