Help a fellow engineer with a USB problem

One of the great things about online technical articles is that they help engineers years after they're published. Case in point, an October 2002 article called "USB 2.0: Compliance and beyond." A reader e-mailed me about where to get Matlab scripts for USB compliance testing mentioned in the article. I directed him to the USB Implementors Forum Developers page. The reader asked another question and for this I ask your help. See the bold text at the end of this message.

If you have any insight into this issue, please e-mail me at m.rowe@tmworld.com. I'll forward your answers to the engineer.

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According to the USB 2.0 specification it definites that the transition time of a high-speed driver must not be less than the specified minimum allowable differential rise and fall time (THSR and THSF). Transition times are measured when driving a reference load of 45 to ground on D+ and D-. Figure 7-12 shows a recommended Transmitter Test Fixture for performing these measurements.

For a hub, or for a device with detachable cable, the 10% to 90% high-speed differential rise and fall times must be 500 ps or longer when measured at the A or B receptacles (respectively).

For a device with a captive cable assembly, it is a recommended design guideline that the 10% to 90% highspeed differential rise and fall times must be 500 ps or longer when measured at the point where the cable is attached to the device circuit board.

It is required that high-speed data transitions be monotonic over the minimum vertical openings specified in the preceding eye pattern templates.

But it does not define clearly about the high level and low level. Could you explain more clearly about the high level and low level about the rise time and fall time? And how can I get the high and low level voltage in the fact USB signal?