Why do we need to buffer USB signal if the cable is longer than 5m?
Is that because a signal voltage drop?
Is that because it drives currents?
Electronic – Why is USB cable maximum length is shorter than in RS232
cablesrs232usb
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Best Answer
Speed of transmission matters because USB is half-duplex: in order to transmit a response, the bus must be turned around and data transmitted in the other direction. So the host sends out data and waits for an acknowledgement or a response. All transfers are controlled by the host. The device then has a certain (fairly short) time in which to respond. This time is roughly the time taken for two signal trips along a 5m cable.
(I can't find references right this second, but the relevant spec documents are public)
Edit: thanks to psmears for finding this section
So my answer is only half-right: the round trip limit is for a worst-case chain of hubs and cables, for a total depth of 25m.
Dan Neely is also right that USB was always supposed to be the lowest-cost solution for "slow" peripherals like keyboards, mice, printers etc. If you wanted full duplex for more speed and more distance, 100baseT ethernet is the natural choice.