How to choose or make a USB hub. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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Today, the USB interface is most often used to connect peripheral devices to a computer. But sooner or later, the user discovers that all the USB ports on his computer are occupied by a mouse, keyboard, WEB camera and other devices, and there is nowhere to connect a newly purchased printer, TV tuner, USB oscilloscope or anything else. How do you connect the 127 devices promised by the USB specification to your computer?

So that more than one device can be connected to one USB port of a computer, hubs are used (English hub - the hub of a wheel into which all its spokes are inserted), also called concentrators. The hub has an “upstream” USB port that connects to a computer, and several “downstream” USB ports to which peripheral devices are connected. The USB specification allows for daisy-chaining of up to five hubs.

In stores selling computer peripherals, the assortment of USB hubs is quite large - for every taste, color and budget. It would seem that you can choose any one, the most attractive design with the required number of ports and for the minimum price. After all, an inexperienced user often imagines a hub as something like a device for connecting two TVs to one antenna - inside there is a pair of resistors or a miniature transformer.

However, in this case everything is much more complicated. I was convinced of this when I purchased two USB hubs, one for the digital interface to the transceiver, the second for connecting an external hard drive to a desktop PC. The first hub for four ports with the "DNS" logo was purchased in a regular store, the second - from an unknown manufacturer for seven ports - was ordered from a foreign online store.

Laboratory experiments showed that both hubs work without problems with a mouse, keyboard, USB-COM adapter and a sound card equipped with a USB interface. However, only the DNS hub works with an external hard drive and FLASH drive. When connecting such devices through an unnamed hub, the computer displays the message “USB device not detected.”

Additional experiments with the digital interface of the transceiver showed that the DNS hub works here without problems, but the use of an unnamed hub leads to the computer freezing every time the transmitter is turned on. When connecting the USB-COM adapter and an external sound card to the computer directly without a hub, everything worked without problems.

This situation interested me. I decided to find out how these two hubs differ. Why does one fully perform its functions, while the second, in principle, works, but not always and not with all devices?

Imagine my surprise when, after opening the cases, it turned out that both hubs were assembled on the same element base and according to absolutely identical circuits! Only the seven-port one has two identical USB hub controller chips installed in series: the upstream port of the second similar controller is connected to one of the four downstream ports of the first controller. Disabling the second controller by cutting the printed circuit conductors did not change the situation. To understand the reason, I had to get acquainted with the basics of the design and operation of the USB bus.

The first USB 1.0 specification was published in early 1996, and in the fall of 1998, the 1.1 specification appeared, eliminating the problems found in the first edition. The USB 1.1 specification defines two modes of information transfer: low-speed (LS - low-speed), operating at speeds up to 1,5 Mbit/s and full-speed (FS - Full-speed) with a maximum speed of 12 Mbit/s.

In the spring of 2000, the USB 2.0 specification was published, providing a 40-fold increase in bus bandwidth. In addition to the two previously available speed modes, a third has been introduced - high-speed HS (High-speed), capable of operating at speeds of up to 480 Mbit/s.

In 2008, a new standard appeared - USB 3.0 (Super Speed), according to which the transfer speed was increased to 5 Gbit/s. However, to achieve such speeds, it was necessary to seriously change the design of connectors and cables, while full compatibility with previous versions could not be achieved. This interface is advisable to use for communication with high-speed hard drives if frequent transfers of large files are required. But it is undoubtedly the future.

There is one subtle point associated with the "USB 2.0" logo. Although the maximum throughput of this interface is 480 Mbit/s, the specification also includes the possibility of its operation in LS and FS modes. Thus, 480 Mbit/s throughput can only be provided by devices capable of operating in HS mode.

USB developers recommend using the “USB 2.0” logo only for HS devices, but the market has its own laws and many manufacturers use this logo for FS devices that, in fact, satisfy only the USB 1.1 specification. In other words, the inscription on the packaging “USB 2.0” does not mean anything. Devices that fully comply with this specification must be labeled "USB 2.0 HI-SPEED" and clearly indicate 480 Mbps capability.

The signal transmitted over a communication line at a speed of 480 Mbit/s is a rectangular pulse, traveling at a frequency of up to 480 MHz. Anyone more or less knowledgeable in radio engineering understands that for undistorted transmission of rectangular pulses of such a frequency, when developing a printed circuit board, it is necessary to strictly adhere to the requirements for the characteristic impedance of transmission lines between microcircuits and connectors and its constancy along the entire length of the line.

The characteristic impedance of the two-wire differential signal line on the board should be 90 ohms ± 10%. The line must be symmetrical, and the distance between it and other printed conductors on the board must be at least five times the distance between the line conductors. Under them, on the back side of the board, there should be a continuous layer of foil throughout - a screen (common wire). Sections of the line where these requirements are not met (for example, approaches to microcircuit pins or connector contacts) must be of a minimum length.

Typical errors when tracing such communication lines are shown in Fig. 1, where 1 is the screen break below the line; 2 - branch from the line conductor; 3 - non-parallelism of conductors and change in the gap between them; 4 - extraneous conductor next to the line.

Ris.1

And, of course, you need to comply with the usual requirements for the installation of high-frequency circuits. All conductors must be of a minimum length, and blocking capacitors should be located as close as possible to the corresponding pins of the microcircuits.

When looking at the photographs of printed circuit boards of purchased hubs, it is clear that in the DNS hub (Fig. 2) these requirements are more or less met. The developers of the unnamed hub (Fig. 3) used a single-sided printed circuit board in it, so the characteristic impedance of the communication lines is very different from the standard 90 Ohms and there is a high sensitivity to electromagnetic interference.

Fig. 2

Fig. 3

Both hubs use the same FE1.1s USB hub controller chips. The website of their manufacturer jfd-ic.com is, unfortunately, only available in Chinese. A possible connection diagram for this microcircuit is shown in Fig. 4. It differs from the standard one in the absence of LED indicators of active ports and an additional non-volatile memory chip. More details on the characteristics and features of the FE1.1s chip can be found in [1] (in English).

Fig. 4

To test the assumption that the poor performance of the hub is caused by ignoring the requirements of the USB specification for the PCB topology, I developed my own version of the board. A drawing of printed conductors on its conventionally upper side is shown in Fig. 5. The foil on the bottom side is completely preserved, with the exception of the countersink holes for the leads of parts that are not connected to the common wire. The location of parts on both sides of the board is shown in Fig. 6. Pieces of tinned wire, soldered on both sides of the board, are inserted into the via holes (they are shown filled in).

Fig. 5

Fig. 6

The geometric dimensions of the signal lines to obtain the required characteristic impedance were calculated using the TX-LINE program [2]. It is free and available for download after registering on the site. The program does not require installation, working with it is intuitive.

By launching the program and going to the tab of coupled microstrip lines (coupled MS line), you should select the material of the line conductors - copper (copper), enter the dielectric constant of fiberglass equal to 5,5, and the dimensions of the line. With a fiberglass thickness of 1 mm, a width of printed conductors of 0,7 mm, a distance between them of 0,5 mm and a foil thickness of 0,02 mm, we obtain a wave impedance of about 500 Ohms at a frequency of 93 MHz.

All passive elements intended for surface mounting are of standard size 1206 or 0805. Oxide capacitors C1, C3, C5, quartz resonator ZQ1 and external power connector XS5 are mounted on the side of solid foil, the remaining elements are mounted on the side of printed conductors.

If the hub will be used only as a passive one (all devices connected to it will receive power from the computer), then the VD1 diode can be replaced with a jumper. When connecting devices that consume more than 500 mA to the hub, the power from the computer will not be enough. In this case, the jumper should be removed and, without installing the VD1 diode, connect a stabilized voltage source of 5 V of the required power to the XS5 connector.

To operate the hub in both passive and active modes without soldering, a Schottky barrier diode VD1 must be installed in it. It will prevent voltage from the external power supply from entering the computer's USB port.

In principle, to reduce the thickness of the board, all parts can be placed on the side of the printed conductors, but without metallization of the holes this complicates installation. If necessary, you can change the dimensions of the board and the location of the uSb connectors by slightly adjusting the pattern of the printed conductors.

I removed the FE1.1 s chip from my seven-port hub, but it can also be purchased separately on the Internet. This is one of the few USB hub controllers produced in the SSOP-28 package with a pin pitch of 0,64 mm. A board for such a case can easily be made by thermally transferring a design onto foil.

Testing the manufactured hub, I found that the influence of electromagnetic radiation had completely disappeared, two of its four ports work great with a FLASH drive and a USB hard drive, but the other two only work with a mouse.

I had to remove the second controller from the seven-port hub and replace the first one on a homemade board with it. Now three of the four ports are fully operational. Moreover, the port that functioned without problems with the first controller stopped working in HS mode.

The documentation for the FE1.1 s chip says that all its copies undergo final inspection after manufacturing. Obviously, defective copies are sent not to the trash, but to nameless manufacturers. Or the controller has some undocumented design options. One way or another, the option with three full USB 2.0 ports suited me.

Please note that almost all cheap hubs with a connector for connecting an external power supply do not have any isolation between the external and internal power supply circuits. The power contacts of all connectors are simply connected to each other. As a result, there is a chance to damage the computer's USB port by applying voltage to it from an external power supply connected to the hub.

If you plan to connect an external power supply to the purchased hub, you need to open the hub case and cut the conductor coming from pin 1 of the upstream port connector (the one that is connected to the computer). To maintain the ability to use the hub in passive mode, a diode can be soldered into this place similar to VD1 in the diagram in Fig. 4. It must be with a Schottky barrier (to reduce voltage drop) and with a permissible forward current of at least 1 A.

According to the USB 2.0 specification, the connecting cable must be shielded. When purchasing a cable, however, it can be difficult to determine whether it has a screen or not. The only thing that may indicate the presence of a screen is the “USB 2.0 High Speed” marking on the cable. An indirect sign is the noise-suppressing ferrite “latches” at its ends.

However, neither the markings nor the latches say anything about the quality of the screen. A good cable should have foil wrapped around the wire harness, with a braided copper "stocking" placed over it. Manufacturers often reduce the cost of production by using several copper-plated steel cores instead of a full screen.

The quality of the shield can be assessed by measuring the resistance between the metal housings of the connectors at both ends of the cable. If it is close to zero, the cable has a full copper screen. If the resistance is 3...4 Ohms or more, there is a screen, but it is made of steel wires. This cable is usually thinner, but using it in an environment with electromagnetic interference may cause your computer to malfunction. For example, when there is a cell phone next to the cable or an amateur transceiver is working nearby.

If the resistance between the connector housings is infinite, it means that the cable is not shielded and is unsuitable for operation in High Speed ​​mode. In any case, the connector body should not be connected to any of its pins. No independent soldering, splicing of wires, shielding or replacement of connectors in the cable is permitted.

The most reliable selection criterion is the transparent outer sheath of the cable, through which the high-quality shielding braid is clearly visible. And if there are ferrite latches at both ends, then such a cable can safely be classified as PRO.

To summarize what has been said, I will formulate the main criteria for choosing a USB 2.0 hub for high-speed information exchange:

- it is better to purchase a hub in a retail store, stipulating in advance the possibility of returning it or exchanging it for another model;
- the packaging and housing of the hub must bear the “USB 2.0 High Speed” logo and a clear indication of the ability to operate at a speed of 480 Mbit/s;
- immediately after purchase (and, if possible, before it), you should check the operation of all hub ports with a high-speed device, for example, a USB 2.0 FLASH drive;
- if you plan to use connecting cables to connect devices to a hub or a hub to a computer, it is better to give preference to those hubs in which all the connectors are mounted on the board, since the protruding “tails” with connectors almost certainly do not have screens. As a result, one end of the cable shield will not be connected anywhere, which may lead to failures when operating in High Speed ​​mode;
- if you plan to use a hub with an external power supply, be prepared for the fact that the hub will need to be modified as described above;
- there is no overload protection in cheap hubs, no matter what is written on the packaging. It is assumed that it is available in the USB ports of the computer. A full-fledged hub with overload protection is a completely different price category;
- Buy a high quality shielded cable with "High Speed" written on it, if possible with a transparent outer sheath.

If none of the sold hub models suits you, make it yourself as described above.

PCB file in Sprint Layout 6.0 format: ftp://ftp.radio.ru/pub/2014/11/hub.zip

Literature

1. FE1.1s USB 2.0 High Speed ​​4-Port Hub Controller. - URL: jfd-ic.com/Documents/FEI.1 s%2oData%20Sheet%20(Rev.%201.0).pdf.
2. TX-LINE: Transmission Line Calculator. - URL: awrcorp.com/products/optional-products/tx-line-transmission-line-calculator.

Author: N. Khlyupin

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