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The Limit Of USB Cable Length And Breakthrough Method
Oct 20, 2017

The Limit Of USB Cable Length And Breakthrough Method

The length of the cable is determined by its electrical characteristics, such as the length of the RS232 cable is limited to the maximum capacity of the data cable, that is, no more than 2500pF. The cable length of the RS485 is inversely proportional to its data transfer rate and is determined by electrical characteristics such as capacitance and reflection.

USB1.1 provides cable length of 3 meters, USB2.0 provides 5 meters. One of the reasons for constraining the cable length is cable delay. We know that the current travels almost at the speed of light on the cable, but it takes some time from this end to the other end. USB 1.1 defines the maximum delay time is 18 nsec, USB2.0 is specified as 26nsec. The speed of the current through the copper wire depends on the dielectric constant of the surrounding material. Although we can not accurately understand this parameter, but we can put the speed of 65% of the ordinary cable current flow rate. The maximum cable length for USB 1.1 is 3.51 meter, while the USB 2.0 cable length is 5.07 meter.

In fact, the quality of the USB cable is more important than the length. May feel cheaper cable has a better price, but the actual is not the case. Although these inexpensive cables may work on some devices, they are not necessarily working on all devices. This is because the USB specification allows the use of a variety of driver chips to receive chips and cables, and if your computer has an inexpensive driver chip (low speed and low output power), and just met a bad design USB peripheral interface, then expensive The cable may work, the cheap cable may fail to communicate. This often causes driver bugs or Windows problems.

The way to overcome the limitations of USB cable length is to use HUB. HUB is just a repeater for the repeater and the USB signal. And an independent power supply. We are optimistic that the USB cable length limit is limited to a cable, not the total length of the cable between the computer and the peripheral. If you use a HUB, the maximum length between the computer and the peripherals will reach 10 meters, that is, the host to HUB5 meters, HUB to the peripheral 5 meters.

The USB standard specifies that HUB allows cascading of five. If you use a 5 meter cable, the maximum distance between the host and the peripherals is 30 meters without any violation. In this case, both Ethernet and wireless data connections will be simpler (5 HUBs, 6 cables) than this.