Data Acquisition

The idle states for low- and high-speed devices are opposite of each other. For the lowspeeddevice the idle state is the D+ line at 0 volts, and the D– a positive voltage. The idlestate for a high-speed device is such that the D+ is a positive voltage and the D– is0 volts. In most data communications, a positive voltage indicates a zero (0) condition,and a one (1), a minus voltage. In the USB system it is not possible to say this because ituses an encoding system called NRZI.The voltages used for differential balanced signaling are:• Maximum voltage transmitted +3.6 V DC• Minimum voltage transmitted +2.8 V DC• Minimum voltage needed to sense a transition +/– 2 V DC• Typical line voltage as seen from the receiver +/– 3 V DCFigure 10.8NRZI exampleThe USB uses non-return to zero inverted (NRZI) encoding scheme. In NRZI, a ‘1’ isdefined as ‘no change’ or ‘transition’ of voltage whereas a ‘0’ is a change or transition ofvoltage. A string of ‘0s’ would cause a clock-like data stream. The USB signaling systemuses the transition from one voltage to another to synchronize the receivers. A stream ofones therefore would mean no transitions. This would cause the receiver to losesynchronization. To overcome this problem the USB system uses a 6 of 7 bit stuffingtechnique. If six or more ones are to be transmitted in a row the transmitter stuffs in azero (a transition). If the receiver sees six ones in a row, it knows that the next transition(zero) is to be ignored.Devices like keyboards and mice need power to operate. This power is supplied by theUSB system through the cables and hubs. External hubs can be either self-powered orpowered off the bus. The voltage supplied by a USB hub is +5 V DC. The hubs must beable to supply a minimum of 100 mA and a maximum of 500 mA, through each port. Ifan external hub with four ports is powered off the bus, then it will divide the 500 mAsupplied between the ports. Four times 100 mA equals 400 mA. This leaves 100 mA to