Active USB repeater


An issue cropped up on Stack Exchange Hardware Recommendations, relating to USB repeaters, for situations where the USB device was situated at some distance from the USB host, i.e. a PC. Described below is an attempt to address the problem of signal loss over the long cables required.

The question

From Externally Powered USB Active Repeater, on Stack Exchange, Hardware recommendations

I have a Surface tablet. And I need to get it’s USB signal about 10′ away.

Here’s what I’ve tried:

  1. An active hub 5′ away, and that doesn’t work. Apparently the tablet doesn’t put enough power on the USB to supply the hub 5′ away.
  2. An active hub next to the tablet and another 5′ away. Same problem, cause again the laptop is the power source and doesn’t provide enough power to supply the hub 5′ away.
  3. I’ve switched both hubs for externally powered hubs, but it turns out that the external power was only for charging things when the hub wasn’t plugged into a source.

All I really need here is an externally powered active hub. I want it to use it’s own power not the source power to repeat the signal. (And I want to not have to buy 4 more hubs to achieve this.) Does such a thing exist?


It appears that this is also a problem for the Raspberry Pi. Where the power on the USB is insufficient to drive some devices/devices that are far away. It appears a significant enough issue that modifying a hub was considered a good solution:

As I understand it there are hubs that supplement the power over the USB correctly. Perhaps all I need here is the right powered hub.


My answer

It does seem a bit strange that you can’t get the signal over such a distance, and I am wondering if there is a fault with your Surface tablet.

Ten feet is approximately three meters (3.048m to be precise). Now, looking at the USB cabling specifications of Wikipedia:

The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. USB 3.0 cables contain twice as many wires as USB 2.x to support SuperSpeed data transmission, and are thus larger in diameter.

The USB 1.1 standard specifies that a standard cable can have a maximum length of 5 meters with devices operating at Full Speed (12 Mbit/s), and a maximum length of 3 meters with devices operating at Low Speed (1.5 Mbit/s).

USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns. The USB 2.0 specification requires that cable delay be less than 5.2 ns per meter (192 000 km/s, which is close to the maximum achievable transmission speed for standard copper wire).

The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires the maximum practical length is 3 meters (9.8 ft).

So, even if your Surface is using USB 1.1 at a Low Speed, ten feet of cable should work. However, I would assume that it would be using at least USB 2.0, so ten feet is well within the five meter limit.

What errors, if any, do you see being displayed? Are you getting an error such as,

USB Device Not Recognised

appearing on your Windows screen?

You might want to take a look at this video, Free USB Repeater (extender). The black hub that he is using is a $9.95 Dynex DXHUB23 4-Port USB Hub. Before using the hub, he also suggests using a USB 2.0 active repeater cable. These abound aplenty, just have a search on google. For example, for $14.95, on Amazon, there is the PTC Premium 33ft / 10 Meter USB 2.0 Active Repeater / Extension Cable.

If you really want to extend a long way (20m) then you could employ the USB 2.0 Hi-Speed Active Extension Repeater Cable (A-A M/F), 20M (65-ft.)

Also of interest is the article, USB Cable Length Limitations And How To Break Them, from

USB Cable Length Limitations

With the popularity of USB (it powers everything from our external hard drives to the charging of our mobile phones), we are constantly amassing USB cables. USB 1.0/1.1 has been almost entirely supplanted by USB 2.0. Since USB 3.0 / USB 3.1 has yet to become as ubiquitous as 2.0 that means that most people have several 2.0 cables lying around. The mistake that most individuals make is when they take a 10 foot cord that came with a device and then purchase a 10 foot extension to make a cable that is 20 feet in total length. This kind of setup will not work and requires the use of a special type of USB cable known as an active or repeater cable. But before we get to active cables or hubs, how long can a USB cable be?

Maximum length of USB 2.0 cable:

The 2.0 specification limits the length of a cable between USB 2.0 devices (Full Speed or Hi-Speed) to 5 meters (or about 16 feet and 5 inches). In other words, you cannot just connect a bunch of extension cables together (like taking a 6 foot cord and extending it with 4 other 6 foot extension cords) and run them 30 feet to another room. However, you can connect a 6 foot cable with a 10 foot extension cable for a total of 16 feet, which is below the maximum cable length for USB 2.0.

Maximum length of USB 3.0 / USB 3.1 cable:

The 3.0/3.1 specification does not specify a maximum cable length between USB 3.0/3.1 devices (SuperSpeed or SuperSpeed+), but there is a recommended length of 3 meters (or about 9 feet and 10 inches). However, the biggest limitation to the length of the cable is the quality of the cable. Results may vary, but with a high quality cable you should be able to go beyond 3 meters. However, to ensure you achieve the best results possible, use an active cable when going more than 10 feet (3 meters).

How to break the USB length limitations:

With the USB specs limiting the length of cables, is there a way to extend those limits? Yes! However, in order to go beyond these cable length limits (or recommended lengths) you need to use self-powered USB hubs or active (repeater) cables; both of which have their own limits as well. Other options such as USB over Ethernet or building your own USB bridge can extend the USB range further.

USB Hubs:

You can use extension cables and self-powered USB hubs connected together to extend the range of your USB device. However, it is important to remember when using 2.0 hubs and cables that the distance between each powered hub can be no more than 5 meters (16 feet and 5 inches). When using 3.0/3.1 hubs and cables, do not exceed the recommended length of 3 meters (9 feet and 10 inches) between hubs. Note: It is possible to use bus-powered USB hubs, but you will quickly run out of power as you extend your setup.

Active (Repeater) Extension Cables:

USB active extension cables contain electronics that regenerate the USB signal. Active cables are essentially 1 port USB hubs. You can use a regular USB cable in conjunction with an active cable as long as the regular cable is not more than 5 meters (16 feet and 5 inches) long for 2.0 devices and not more than 3 meters (9 feet and 10 inches) long for 3.0 devices. Note: Typically, active cables are bus-powered cables. To ensure you receive the full 500mA power of a USB port, consider purchasing an active cable that includes a separate power adapter.

USB Hub Limits and Maximum Length of Active Cables

Just like there is a limit on a regular (passive) USB cable, there is also a limit on how long an active cable can be and how many USB hubs you can use.

Maximum number of USB Hubs:

The USB 2.0/3.0/3.1 specifications call for only 7 tiers of devices to be connected. When you count the devices on each end (the host and the peripheral device), that only leaves 5 tiers available and a USB hub is considered 1 tier. Thus, only a maximum of 5 USB hubs can be used for a total maximum length of 30 meters (about 98 feet and 5 inches).

Maximum length of USB active (repeater) cable:

This number depends if you are using a regular cable with an active cable or not. If you are not using a regular cable, then the maximum active cable length for USB 2.0 is 30 meters (98 feet and 5 inches) and the maximum recommended length for USB 3.0/3.1 is 18 meters (about 59 feet). If you are using a regular cable (max length of 5 meters for 2.0 and max length of 3 meters for 3.0/3.1) with an active cable, then the maximum length for USB 2.0 is 25 meters (about 82 feet) and the maximum recommended length for USB 3.0/3.1 is 15 meters (about 49 feet).

Is there any way to go beyond the limit of active cables or hubs?

There are other ways you can extend a USB signal beyond the 30 meter limit. You can use USB over Ethernet to achieve distances up to 100 meters (about 328 feet). Additionally, you can build your own USB bridge to transmit data over different communication channels such as wireless methods.

What about USB 1.0/1.1?

USB 1.0/1.1 has been superseded by USB 2.0 and USB 3.0/3.1. In addition, USB 2.0 cables are backward compatible so they will work just fine with any USB 1.0/1.1 devices. However, in case you are using a 1.0/1.1 USB host and device, there are limits to the maximum length of the cable. The limit for USB 1.0/1.1 cable length is 3 meters (about 9 feet and 10 inches) and the maximum total length should not exceed 18 meters (about 59 feet).

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