USB: Troubleshooting and Guidelines
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In simple terms, USB (Universal Serial Bus) describes a way to connect a hardware device (or peripheral) to a controlling device (usually a computer). The term USB also includes the way that the device(s) and controller communicate. USB connectivity is the standard for a wide range of devices and periperals.
This document provides a checklist for troubleshooting problems with USB devices, hubs or connections, and also gives an overview of best-practices when using USB connections between a PC and peripheral devices (such as keyboards, mice, flash drives, cameras, etc.) and between a PC and multiseat devices (devices that have built-in video card and USB hub capability).
Userful recommends using high-quality, USB-certified products where possible; USB-certified products have undergone rigorous compliance testing and are qualified to display the certified USB logos of the USB Implementers Forum (www.usb.org).
Troubleshooting USB Problems
With so many devices plugged in together, things can quickly get complicated, and troubleshooting can be confusing. Following the simple steps below can solve many problems with USB performance (each area is discussed in more detail in the "USB Guide" section, below):
- Check your cables.
- Use only high-quality, "Hi-speed USB" certified cables
- Ensure cables are not more than 5 metres (16 feet) long.
- Avoid daisy-chaining cables and hubs.
- Use powered hubs where possible.
- This is especially important if using devices with a high power requirement.
- Spread out demand over the computer's buses.
- Connect cables to your PC through as many 'banks' of USB ports as possible to avoid overloading the USB bus.
- Keep USB cables away from sources of electromagnetic interference.
- Do not bundle USB cables with power cables.
- Keep cables away from power supplies, microwave ovens, etc.
- Check to see if the Linux system is using USB 1.1 or USB 2.0.
- USB 2.0 devices demand lots of power and bandwidth and perform best under the USB 2.0 specification. However:
- In some cases, using the USB 1.1m specification prevents overloading the USB system and allows a multistation system to operate more stably.
USB Multiseat Devices
There are some special considerations when using multiseat devices (with USB-connected video):
- USB multiseat devices only work with USB 2.0 ports.
- It is necessary to configure the host computer so that USB ports are not disabled when the host computer enters sleep mode.
- Success when using USB stations with notebook and netbook computers may vary since many notebooks have power management schemes that can interfere with or limit the amount of power allowed to USB ports.
According to the USB Implementers Forum, the maximum length of a USB 2.0 cable is 5 metres (just over 16 feet). This limit is imposed by the electrical design of USB signalling. Cable quality is very important. Only use high-quality USB 2.0 compliant cables. (Note: 'compliant' is different than 'compatible'. Good quality cables often say "Hi-Speed Certified".)
We recommend cable specifications of at least 24 AWG (power) and 28 AWG (signal). Low quality USB cables can cause excessive voltage drops that can create system instability issues or can cause a USB multiseat device to be disabled by the Linux Kernel. This is especially important if you are using demanding USB devices like mass storage devices (flash drives, optical drives, hard drives, cameras), multimedia devices (audio, microphone, webcam),etc.
Userful does not recommend daisy chaining USB hubs or USB multiseat devices. But if you absolutely must, be sure to use powered USB hubs when daisy chaining USB stations. The USB protocol specifies a maximum of 5 hubs ("hops") per USB port. If you need to use extension cables, ensure that the distance between the USB device and the computer/powered USB hub is not greater than 5 m (15 ft) to prevent voltage drops.
Hubs: Powered or Unpowered
Powered USB hubs may be used to provide additional connectivity for peripherals such as USB keys, digital cameras and audio devices.
If the hub is self powered (i.e., comes with an AC power adaptor), then any USB device may be attached to it. However if the hub is bus powered, then only low power (100 mA max) devices can be attached to it. A bus-powered hub should not be connected directly to another bus-powered hub.
Devices plugged into a USB hub associated with a station will also become associated with that station. Userful strongly recommends using a dedicated powered USB hub for each station where users may attach multiple USB peripherals (e.g., if users may plug in USB keys, digital cameras, audio headsets, etc.). In this manner, devices are automatically assigned to each station.
USB devices can either be self-powered (e.g., USB CD drives, powered USB hubs) or bus-powered (e.g., audio headsets, mice, keyboards). Self-powered devices include an AC power adapter. Because each USB port on the central computer only provides a power supply up to 500 mA for its devices, connecting multiple bus-powered devices to an unpowered hub (one without its own power supply) can easily exceed this maximum power consumption.
Typically USB ports only provide sufficient power to support a single keyboard and mouse and one other low-powered device such as a USB audio headset. To attach additional high-power devices such as a USB floppy drive, you will need to connect them directly to the computer box, or to a powered USB hub.'
Most recent computer models have more than one set of USB connections (called "ports"). Typically each set of USB ports in a given location share a "bus" -- the bus is a system that transfers data between the USB ports and the CPU.
A typical computer will have a group of USB ports on the back panel and a group on the front panel. Note that:
- each of these groups of ports has their own bus
- typically, the bus for the back panel USB ports is higher-powered than the bus for the front-panel ports. A higher-powered bus can handle more devices, and more demanding devices.
Extra USB ports can be added to most computers. There are two types of USB expansions:
- USB headers - consist of a plate that fits into the holes in a computer's back panel and cables to plug into unused USB header pins on the motherboard. Note that often the ports on the front panel of a computer tower are connected to the motherboard in this manner; if USB ports are added by installing a header, the front panel ports and the expansion ports may share a single bus.
- USB PCI expansion cards - consist of a card with built-in ports that fits into one of the PCI expansion slots. Such a card will use the PCI local bus.
For optimal performance from USB devices, spread the demand between all the available buses.
For example, the back of the computer may have 4 ports, the front of the computer may have 2 ports, and there may be a USB PCI expansion card in one of the slots. The USB cables should be spread out over all 3 of these different buses (or at least the 2 buses on the back of the computer). Doing this distributes the load over more buses and can help a system run more smoothly.
Like all devices that use electrical signals, USB is susceptible to Electro-Magnetic Interference (EMI). EMI may cause signal degradation that will appear as static on USB audio devices or unreliable operation of other devices. Causes of EMI signal degradation include:
- low-quality USB cables; or trying to use Original USB certified cable with Hi-Speed USB devices.
- bundling USB cables with power cables
- daisy chaining of cables
Try to keep USB cables from touching or being too near to high EMI sources, such as power cable, power supplies or CRT monitors.
Data Transfer Speeds & USB Specifications
USB allows hardware (devices) to communicate with the host computer (controller) through a two-way transfer of data.
Different USB devices are designed to transfer data at different maximum rates:
- Low-speed USB devices (most keyboards, mice and joysticks): 1.5 Megabits/second (Mb/s)
- Full-speed USB devices: 12 Mb/s
- High-speed USB devices: 480 Mb/s
- Super-speed USB devices: 5 Gigabits/second (Gb/s)
The computer/controller part of a USB connection also has a maximum data transfer rate defined by which USB specification is hard-coded into the motherboard:
- USB 1.1: The original USB specification, released in 1998.
- Allows a maximum data transfer rate of 12 Mb/s.
- Low- and full-speed USB devices should work optimally with a system using the USB 1.1 specification.
- High-speed USB devices will usually work, but their performance may be noticeably slower due to the slower data transfer rate. However, some instances of high-speed USB on a USB 1.1 controller may cause system instability.
- May be called "Original USB" or simply "USB".
- USB 2.0: Adopted in 2001.
- Allows a maximum data transfer rate of 480 Mb/s.
- High-speed USB devices should work optimally with a system using the 2.0 specification.
- Low- and full-speed USB devices will work under the 2.0 specification, but will not have faster performance than under USB 1.1.
- Devices have to go through rigorous testing to receive the "Hi-Speed USB" title.
Computers that use the USB 2.0 specification usually allow a user to change the setting to USB 1.1 by making modifications in the BIOS. Consider the following when deciding to set a system to use the USB 2.0 or 1.1 specification:
- Hi-Speed devices (USB 2.0 devices) demand more power and bandwidth, but if there aren't too many stations and USB devices, they will perform better if USB 2.0 is turned on.
- On the other hand, if a large number of high-speed devices are being used, setting the computer to use USB 1.1 can prevent overloading the USB system and may allow a multistation system to operate more stably.
Changing USB Specification in the BIOS
If a large number of Hi-speed USB devices are being used, setting the computer to use USB 1.1 can prevent overloading the USB system and may allow a multistation system to operate more stably; the tradeoff is that Hi-speed devices will be slowed.
Different computers have different BIOS and therefore the following instructions are guidelines only.
- Press the ‘Delete’ or ‘F10’ key during the first few moments of boot-up on the computer to enter the BIOS configuration screen. (This will vary depending on the make of the motherboard; determine the correct key to press for a given PC by examining the first screens to appear after switching it on.)
- In the BIOS configuration screens, navigate using the arrow keys. Press ‘Enter’ to select options and ‘Esc’ to exit.
- The particular location of USB controller options in the BIOS depends on the computer. Typically they will be found under the "Advanced" menu under "USB Options", "USB Controller Options" or "Chipset". It may be necessary to explore the BIOS to find the correct option.
- Change the USB Controller Speed from "Hi-Speed" (480 Mbp/s) to "Full-Speed" (12 Mbp/s).
- Press 'F10' to save your changes and exit the BIOS configuration screen. Press ‘Enter’ to confirm the action. The computer will restart.
|Note: Legacy USB support does not change the USB specification; it simply allows a low- or full-speed device to operate even if plugged into a hi-speed port. Legacy USB Support should be "Enabled".|