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I understand the max length guidelines for USB 1.1 is 3M and USB 2.0 is 5M.

If I have a USB 2.0 computer going into a USB 2.0 hub, the theoretical max length is 5M (PC to hub). What if a USB 1.1 device is plugged into that hub? Would the max length between computer and hub now be 3M (per USB 1.1) or 5M (per USB 2.0)?

Thanks,

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  • \$\begingroup\$ Surprisingly simple, yet interesting question. My uneducated guess would be 3m, but I don't know for sure and I'm curious. Waiting for the others to reply :) \$\endgroup\$
    – Ilya
    Feb 11 at 21:03
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    \$\begingroup\$ the hub-PC connection is still USB 2.0 \$\endgroup\$ Feb 11 at 21:07
  • \$\begingroup\$ The hub is "talking" to PC using USB2.0 protocol regardless of what is plugged into it. \$\endgroup\$
    – Eugene Sh.
    Feb 11 at 21:35
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    \$\begingroup\$ meter is m ... mega is M \$\endgroup\$
    – jsotola
    Feb 12 at 0:10
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As the hub is capable of multiple devices being plugged in, and it handles each device at the speed the device supports, so the connection to the PC operates at USB2.0 all times.

Which means that the limitation of USB2.0 still apply, thus the cable length is still 5 m and not 3 m.

If you wonder why low speed devices (1.5 MBit/s) have a lower cable length than higher speed devices (which is a bit counter intuitive), it's because the cable delay specification was raised from 18 ns to 26 ns with higher speed devices (even in USB1.1). And with a bit of rounding this translates to 3 m and 5 m.

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What if a USB 1.1 device is plugged into that hub?

If a USB 1.1 (Low-speed or Full-speed) device is plugged into USB2.0 hub, the slow signals for LS/FS devices are not flowing directly from PC host; they are TRANSLATED by the hub. Communication between PC and hub, however, occurs at HS rate (480 Mbps) using HS protocol. How it is done?

The LS/FS devices receive their corresponding traffic from the hub. USB 2.0 hubs all have special built-in communication processors called "Transaction Translators" (TT). The TT operates under a special class of HS traffic/commands called "split transactions". LS/FS transactions are "splt" into several parts on host's HS end. Short fast packets (at HS rate, "start split") are stored in the TT. The TT buffers the information, and then unfolds/transalates the command/data into lower speed protocol.

In very simplified terms, while the slow LS/FS data are in progress, the host "pings" TT with short packets to ensure the completion of slow downsream transactions, and "completes the split" when the low-end transaction finishes. This does not take much of bus time, so the hub's upstream bus is not that busy, and can service other HS devices or other TTs in other hubs, and several LS/FS devices at once. The split transaction protocol is fairly complicated: USB 2.0 specifications have about 150 pages explaining how this protocol should work.

That's why the 3-m limit does not apply to this smart topology, and the cable length can be at least 5m (see discussion of real cable limits here).

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