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USB cable length

Introduction

The USB interface was developed for connecting peripherals like printers and scanners to computers. Since the USB 2.0 release also higher speed devices like external hard disks are often equipped with an USB interface. Especially for those fast devices, cable length and quality can be critical for proper operation of the devices. This document will guide you through the USB rules to determine what the maximum cable length is for your specific application.

USB cable length defined by the specification

With many interfaces the maximum cable length is determined by specific electrical properties of the communication system. For RS-232 for example the cable length was limited by the maximum capacitance of the data cable. This value was defined as 2500 pF. Every data cable with a capacity with that value or lower would be within the length limits. With differential interfaces like RS-485 the maximum cable length is almost inverse proportional to the data rate. The slower the bits fly over the line, the longer the cable can be. This is also because of electrical properties like capacitance and reflections.

The developers of USB did it the other way around. They thought about the use of USB as a desktop interface and said: "Well, 4 meters is quite a length for a cable between a computer and a peripheral, lets limit the cable length somewhere in that range." Because of this approach, the slow USB 1.1 specification defines a maximum cable length of 3 meter, and the faster USB 2.0 specification allows 5 meter cable length. Probably the engineers realized that 3 meter was not that far in an office environment and they therefore increased the maximum length somewhat in the new release of the specification.

With this in mind you can understand that those 3 and 5 meter for USB cable lengths are hardly the maximum in all cases because they were not caused by electrical characteristics of the cable or the driver/receiver chips, but by some bureaucrat behind a desk. So you might assume that using a little longer cable would still work. Unfortunately this is not the case. After the engineers made their decision about the maximum cable length, they used it directly in the specification to limit parameters which have physical properties. One of those properties is the cable delay. As you might know electricity travels almost by the speed of light through a cable, but it still takes some time to travel through a cable from one side to the other. This maximum delay time has been defined as 18 nsec for the low-speed USB 1.1 connections and 26 nsec for the high-speed USB 2.0 connections. The speed of electricity through a copper wire depends on the dielectric coefficient of the surrounding material. Although we don't know this parameter exactly for every USB cable out there, a speed of 65% of the speed of light is quite a realistic guess for a general cable. In that case we get 5.07 meter as maximum cable length for the high speed USB 2.0 connection and 3.51 meter for low speed USB 1.1.

This maximum time delay is fixed in the interface, so even with the highest quality cable made from the best materials which you buy in the most expensive computer shop you wouldn't get a significantly longer maximum USB cable length than with your cheap Chinese copy cable bought from an on-line store. This is because the speed of electricity through a copper cable is defined by simple physics laws which even the most expensive cables cannot change.

You might think that buying the cheapest cable would give the best price-to-quality ratio, but from personal experience I have to warn you. Although these cables may function with some devices, cheaper cables do not always function with all devices. This is because the USB specifications allow quite a large range in properties of driver chips, receiver chips and cables. If you have a cheap driver chip in your computer which is relatively slow and has low output power and want to connect it to peripheral which has also a poorly designed USB interface which operates just within the limits of the specification, an expensive cable might work, where over a cheap cable the communication will fail. Although the maximum distance between your devices will not vary dependent on the quality of the USB cable, the reliability of the connection certainly will. For equipment you rely on like external hard disks, I would never take the risk of using a bargain cable. I once had a problem with an Epson scanner which would hang during the scan process in one out of ten pages on average. After I replaced the short USB cable with a longer cable I had bought from a specialized store, I never had scan problems anymore. The problem wasn't in the cable length but the cable quality. My estimation is that what people often call driver bugs or Windows problems are in fact many times just problems caused by cheap cables in or outside the computer.

How to overcome the USB length limit

Current computers are often equipped with 6 or 8 USB ports where each device connects directly with its own cable to the computer. When USB was just released, most computers were only equipped with 2 USB slots and if you wanted to connect more devices to your computer you had to use an USB hub. A hub is really nothing more than a repeater and splitter of USB signals, often with its own power supply connection which sits in between the computer and one of more peripherals. The nice thing is that the maximum USB cable length only applies to each cable segment in the system, not to the total cable length between your computer and the peripheral. If you use one USB hub, the maximum distance between the computer and the device doubles to 10 meter, because 5 meter cable is allowed from the computer to the hub, and a second 5 meter from the hub to the device.

And this is not the end yet. The USB standard allows up to 5 hubs between a computer and another device. If we use cables of 5 meter each, this gives a total cable length between the computer and the device of 30 meter, without breaking any rules. You have to carefuly think though if in such a situation a simple ethernet based local area network or wireless data connection wouldn't have been easier and cheaper to set up than a USB network with five hubs ans six connection cables.

Please keep in mind that the extension to 30 meter only works for devices which communicate high-speed according to the USB 2.0 specification. USB 1.1 devices are limited to 6 times 3 meter which is still 18 meter.
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