Data Acquisition

though the external hub is a repeater it cannot extend the overall length of the system.This is because of the timing requirements of the USB standard.The external hub is an intelligent device that can control communication lines andpower lines on its USB ports. It is a bi-directional repeater for information coming fromthe host hub and from USB devices. It talks to and even acts like an external USB deviceto the host hub. It plays an integral part in the configuration of devices at startup. There isno physical limit to the number of hubs.The USB system supports every peripheral that can be currently connected to a PC. It hasalso been adapted to devices that are not usually considered peripherals. This wouldinclude data acquisition devices such as digital I/O modules and analog input and outputmodules. All USB devices must be intelligent devices. Smart devices obviously cost morethan the old dumb RS-232 and RS-485 connected devices. With this cost the user getsmore functions, ease of use and the ability to connect more devices to the PC. With theold non-USB system the computer was limited to a few devices. The USB system allows127 devices to be connected to the PC at the same time. There are two types of USBdevices, low-speed and high-speed.Low-speed devices are not only limited in their speed but also in features. These devicescould be keyboards, mice and digital joysticks. Since these devices put out small amountsof information they are polled less and are slower than other devices. When the USB busis being accessed by high-speed devices, the low-speed device communication isdisabled. Turning off the low-speed device ports at the root or external hubs, disables thelow-speed devices. The hubs re-enable the low-speed ports after receiving a specialpreamble packet.High-speed devices like printers, CD-ROMs and speakers need the speed of the 12 Mbpsbus, to transfer the large amount of data required for these devices. All high-speeddevices see all traffic on the bus. They are never disabled like the low-speed ones. Whena device like a microphone is ‘connected’ to the speakers, most of the traffic, andtherefore packets, will be used by the audio system. Other traffic like keyboard andmouse functions will have to wait. The host hub controller driver decides who has to waitand how long.The host hub controller hardware and software drivers, control all transactions. The hosthub controller hardware does the physical connections from the PCI bus to the USBconnectors. It enables and initializes the host ports one at a time and determines the speedand direction of data transfer on both host ports. The host controller in conjunction withthe host hub software driver determines the frame contents, prioritization of the devices,and how many frames are needed for a particular transfer.