How to USB Hubs Work

Currently, for connecting peripheral devices to a computer often using a USB interface. Sooner or later, the user discovers that all ports are busy computer mouse, the keyboard, the WEB camera, etc. and newly acquired printer, TV tuner, USB oscilloscope or anything else connected nowhere. Where are the promised plug in the USB specification 127 devices?

To a single USB port can be connected more than one device, used USB hubs (eUSBHubs), also called concentrators. A hub converts a single uplink port (upstream port) in multiple downlink ports (downstream ports). USB architecture allows serial connection of up to 5 hubs.

In stores selling computer peripherals hubs choice is big enough for every taste, color and budget. It would seem, choose any, the most handsome design with the right amount of ports and for the minimum price. For inexperienced users often imagines hub something like device to connect two televisions to a single antenna – a pair of resistors and capacitors.

However, in this case it is more complicated. In this I was convinced, when it took two USB hub, a single digital interface to the transceiver and the second to connect an external hard drive to a stationary PC.

First Hub on 4 ports with “DNS” logo was purchased at a retail store, the second – 7 ports from the manufacturer “No Name”, I ordered a foreign online store.

Experiments in the laboratory have shown that both copies without any problems working with a mouse, keyboard, USB-COM converter and a USB sound card. However, external hard drives and flash drives is only a hub for DNS label. When you connect the flash drive or hard drive to the computer hub anonymous gave a message “USB device not defined.”

Additional experiments with digital interface transceiver shown that the first hub (DNS) here also works without problems, but the second (unnamed) causes the computer to hang each time the transmitter is switched on. And it’s not in accordance with the antenna transceiver, because in direct, without a hub, connect the USB-COM converter and an external sound card to the computer it worked without problems.

This situation intrigued me and I decided to find out what is the difference between these two USB hub. Why does one fully perform their functions, and the second in principle works, but not always and not with all devices.

Imagine my surprise when, after the opening of the case it was found that both the hub are assembled on the same element base and on exactly the same plan! Only a 7-port hub and two controllers were installed in series. I note immediately that after the experiment with disconnecting the second controller’s situation has not changed.

To understand the reason, I had to learn the basics of the theory of the USB bus. The first USB 1.0 specification was published at the beginning of 1996 and the fall of 1998 there was a 1.1 specification correcting problems found in the first edition.

USB 1.1 specification defines two data communication modes: a low speed LS (Low-speed) – 1,5 Mbit / sec full-FS (Full-speed) – 12 Mbit / sec.

In spring 2000 USB 2.0 version was published which provided a 40-fold increase in bus bandwidth. In addition to the two high-speed mode provided 1.1 specification was introduced third – high-speed HS (High-speed) – 480 Mbit / s.

With logo USB 2.0 connected one thin point. Throughput of the interface as described above, 480 Mbit / sec, but the specification is the possibility of functioning devices FS and LS mode. Thus, the actual throughput of 480 Mbit / s can only provide a device capable of operating in HS mode.

USB developers recommend the use of the logo “USB 2.0” only for the HS-devices, but in the marketing of its own laws and many manufacturers use the logo and for FS-devices are, in fact, USB 1.1 devices. In other words, the inscription on the package “USB 2.0” still about what does not speak. Real USB 2.0 devices are labeled “USB 2.0 HI-SPEED |” and a clear indication of the support bus 480 Mbit / s speed.

480 Mbit / s is a square wave with a frequency of 480 MHz. For anyone even slightly versed in electrical engineering expert it is clear that for the undistorted transmission of rectangular pulses with a high frequency in the development of PCB layout is necessary to strictly observe the requirements for consultation of the wave resistance of the transmission line.

The characteristic impedance of differential signal lines from the controller to the connector on the board should be 90 ohms +/- 10%. Lines must be held symmetrically, at least 5-fold, and the gap between them from other signal lines. Under them throughout the layer should be continuous foil – common. Areas in which these requirements are not feasible (for example, the connection point to the controller) must have a minimum length.

And, of course, have to comply with the usual requirements for the installation of high-frequency circuits – all wires must have a minimum length of the bypass capacitors as close as possible to the corresponding terminals of the controller, etc.

When looking at pictures of the PCB hubs can be seen that the mounting hub DNS under the brand name, these requirements are more or less respected.

Manufacturers also used the NO NAME hub-sided printed circuit board, therefore, the wave resistance of the line is very different from the standard 90 ohms and have a high sensitivity to electromagnetic interference

To test the hypothesis that the poor performance of the hub caused by ignoring the requirements of the USB specification for PCB layout, I have developed a version of the board. Compared with the prototype on board a few additional bypass capacitors are installed and, if possible, met the requirements of the installation. Board Size 75 x 60 mm.

The geometrical dimensions of the signal lines to obtain the desired wave resistance calculated in the TX-LINE program, which is included in the package Microwave Office of National Instruments Corporation. By itself, this program is free and available for download on the company’s website after registration. Just in case, I put it in the file, link to that page at the end.

The program does not require installation, work with it intuitively. We must go to the tab with the type of line – “Coupled MSLine”, select the material line – copper, enter the dielectric constant of fiberglass Dielectric Constant = 5,5 line and enter the parameters. If we assume a thickness of 1 mm fiber-glass, printed conductor width of 0.7 mm, the distance between them of 0.5 mm, copper foil thickness of 0.02 mm and the operating frequency of the 500 MHz impedance obtain about 93 ohms.

The foil serves as a shield on the opposite side of the board.Holes for countersunk mounting parts. The selected color transitions are inserted through pigtails, soldered on both sides of the board.

All passive SMD components sizes of 1206 or 0805. Capacitors C6-C8 tantalum. Resistor R1 2,7K +/- 1%. Rosette XS6 USB mini-BF, XS1-XS4 – USB-AF. The crystal ZQ1 12 MHz. The capacitors C1-C3, ZQ1 quartz resonator and the external power connector mounted XS5 unit side parts other elements – from the printed conductors.

S1 jumper set if the HUB will be used as a passive, ie, all connected devices will be powered by the computer. If HUB will be used with devices that consume more than 500 mA current, the power of the computer is not enough. In this case, the jumper must be removed and connected to the connector XS5 stabilized power supply of 5 V with the necessary power.

If you can operate a hub in passive and active mode, instead of the jumper to install the diode VD1 c Schottky barrier with an allowable current of at least 1 A, for example, SS24 to avoid applying a voltage from an external power supply to the computer’s USB port.

In principle, in order to reduce the thickness of the board, all the details can be placed on the part of the printed conductors, but without metallization of holes, this complicates the installation. Slightly adjusting the drawing board, you can change its size and location of the USB ports to meet specific needs.

After testing a mounted board, I discovered that two of the four ports work perfectly with flash and USB hard disk drive, and the other two – only with a mouse. The new mystery … But the influence of electromagnetic radiation is completely gone.

I had vypayat second controller 7-port hub and replace them first on a makeshift board. Now fully earned three of the four ports. And in high speed ​​mode stopped working port, which is the first controller to function without problems.

The Data Sheet on FE1.1s said that all controllers are tested before sale. Obviously, the rejected items are sent not into the garbage, and to the unnamed producers. Alternatively, the controller has some undocumented options. Anyway, the full-fledged version of the three USB 2.0 ports, in principle, I accept.

I draw your attention but the fact that almost all cheap hubs, where there is a socket for connecting an external power supply, do not have any isolation between the external and internal power supply. Those power pins at all connectors simply connected. As a result, there is a chance to destroy the computer’s USB port, turn ON voltage from an external power supply connected to the hub.

Therefore, if the external power supply connection is expected to purchase a USB hub, you need to open the cabinet and the cut line path from the rising power connector port (the one that goes to the computer). To preserve the possibility of use of a hub in the passive mode at this place can be soldered diode as shown in the circuit diagram. To reduce the voltage drop necessary to use a Schottky barrier diode with a current of at least 1 A.

I want to draw attention to one more important detail – USB cable. According to the USB 2.0 specification of the connection cable must be screened. When buying it is sometimes difficult to determine the cable screen or not. The only thing that may indicate the presence of the screen – is marking USB 2.0 on the cable. An indirect sign is the presence of ferrite latches on the ends of the cable.

However, no marking or the presence of the latches do not say anything about the quality of the screen. Well-cable it should be foil wrapped around the conductor, which is worn on top of the copper wire “stocking”. Often, manufacturers reduce the price of production, and instead of a full screen use multiple coppered steel veins.

If possible, the screen quality can be estimated by measuring the resistance of the multimeter between the metal connector housings on both ends of the cable. If the resistance is close to zero – in a cable full copper shield. If the resistance of 3-4 ohms and more – the screen is, but it is made of steel wires. Such a cable is typically thinner, but can lead to failures when operating in EMI.For example, if the cable near put cellular phone.

If the meter shows infinity, then the cable is not shielded and for working in high speed ​​mode is not suitable. In any case, the connector housing must not be connected with any of the contacts. No separate soldering, splicing, screening or replacement connector in the cable are not allowed.

The surest criterion of choice – a transparent outer sheath of the cable, which is clearly visible through high-quality braided shield. And if this has on both ends of the ferrite latch, such cables can easily be attributed to the category of PRO.

Summing up, formulate main criteria for choosing a quality USB 2.0 hub for a high-speed interface.

• Acquire a USB hub is better in retail stores, in advance stipulating the possibility of a refund or exchange for a different model.

• The packaging and logo package must be “USB 2.0 Hi Speed” and a clear indication of the support rate of 480 Mbit / s.

• Immediately after the purchase, as well as possible to it, you should test the operation of all hub ports with high-speed device such as a USB 2.0 flash drive.

• When connecting devices use the connection cables is planned to be a hub or a hub to a computer, the better to give preference to models hubs, which all connectors are mounted in the housing on the board since protruding “tails” with connectors will almost certainly do not have screens. As a result, one end of the screen of the connected cable will be hanging in the air, which can lead to malfunctions in the High Speed ​​mode.

• If the hub with an external power supply to be used, be prepared for the fact that certain required completion hub, as described above.

• No protection against overload is not cheap hubs that there may be written on the packaging. It is assumed that it is in your computer’s USB ports. Complete hub with overload protection – this is another price category.

• Get high-quality shielded cable with the inscription HIGH SPEED on it, possibly with a transparent outer shell.

• If none of the models sold hubs does not suit you – do yourself a USB hub, as I described above.

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