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The USB 1.1 spec gives well-defined \$V/I\$ characteristics for full-speed output drivers, which can be used to find the series damping resistance, \$R_S\$. The low-speed output schematics also reference \$R_S\$, but it's not entirely clear if it's the same (since low-speed characteristics are typically less strict than full-speed). And while the spec states:

In the rest of the chapter, references to the buffer assume a buffer with the series impedance unless stated otherwise.

Does that mean it applies to both low-speed and full-speed, or just full-speed? If it's the latter case, then what's the valid range of \$R_S\$ for low-speed output drivers?

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I think it applies both to low-speed and full-speed. Because usually, a USB 1.1 controller should support both low-speed and full-speed mode. And in device enumeration stage, we will work at low-speed mode, we can't change the termination resistor dynamically. So, if the termination resistor is not must have ones in low speed, it should work with them. But without it, your full-speed may won't work.

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