Number 300 - May 2008

Understanding USB and Firewire
by Brian K. Lewis, Ph.D.* Sarasota Personal Computer Users Group, Inc.
Both USB and Firewire connections have changed since I last wrote about them in the Monitor (Dec 2005). There are also more changes on the horizon for both of them. So it is time to review again what they are, how they work and Ewhat they can be used to accomplish.    USB (universal serial bus) is now the most common method of connecting computers to many peripheral devices such as printers, scanners, video cameras, external drives, mice, keyboards, etc. USB started out working at a maximum speed of 12 megabits per second (Mb/s) and was revised in version 2.0 to a maximum of 480 Mb/s. In reality, USB never achieves these maximum values. In real time use the throughput is closer to 50% of the rated speed or less. To get a little better understanding of these speeds you need to understand what a megabit means. In computer terminology a bit stands for a zero or one in the binary language of computing. It takes 8 bits to equal one byte. For example, a computer will convert the decimal number five to a binary number containing 8 bits. So a mega bit is a million bits and a megabyte is a million bytes. Essentially, 480 megabits per second is the equivalent of 60 megabytes per second. A USB connection doesn't sound nearly as fast when expressed this way, does it?    The cable used for USB connections contains four wires, two for data communication and two for power. The transmitted power is 5 volts which is provided by the computer bus. The maximum length of a USB cable is 5 meters (16.4 ft). This allows for the maximum turn-around time of 1500 nanoseconds for the USB signal. This distance can be extended by means of "repeaters" such as powered hubs or active cables.    In most low-powered uses the devices connected to the USB port use the power from the computer bus. However, each computer port is limited to a maximum of 500 mA (milli-amperes) of current. If the device requires more power, then it must have its own power connection. Frequently, users find that their computer doesn't have enough USB ports for all the connections they wish to make. In theory, you can connect up to 127 devices through a USB chain. This chain is a star formation. That means you can connect a hub directly to the computer port. The hub has 4 to 7 ports. You can then connect another hub to one port and USB devices to the other ports. Here is where some users run into trouble. If the hub is powered by the computer, then each port on the hub can only provide a maximum of 100 mA. Any device which requires more power and is not self-powered will not work. The solution to this is to use a powered hub. In other words, a hub that can be connected to an electrical outlet. Powered hubs can provide the full 500 mA to each port. There is one more problem with hubs. If you plug in a low-speed device (USB spec. 1.1), then all the ports on that hub will revert to low speed mode. Any hi-speed device (USB spec. 2.0) plugged into the same hub, will not function at the higher speed. In some cases, the device will not work at all. For this reason, if you need a hub in order to obtain more USB ports, you should purchase a powered USB 2.0 hub that requires its own electrical connection.    When you plug in a USB device to either a hub or a computer port, there is an initial identification process referred to as the "configuration" step that occurs. During this configuration process the device can not draw more than 100 mA. If it does, the process will fail and it will appear that the device failed to work. The configuration process identifies the device, its drivers, and its power requirements. Only after this process is complete will the device be able to draw more than 100 mA current. Since this process is not instantaneous, some time must be allowed by the user before attempting to use the device. In addition, this configuration may not occur if the device is plugged into the computer before the computer is turned on and booted. In these cases, it may be better to connect the USB device after the computer is fully operational. However, most of the time, the computer will identify and configure the device during the bootup process.    There is a new USB specification in the works, USB 3.0. The initial information on this specification includes a speed of 4 Gb/sec (gigabits/sec) or 300 MB/sec (megabytes/sec). One of the prime movers behind this new specification is Intel. The need for	increased speed is related to the increases in the speed capability of USB peripherals such as SATA external drives. Another aim is to keep up with the speeds of flash chips in USB solid state drives (thumb drives, flash drives, etc.). In addition, Intel hopes to compete with Firewire in the world of audio and video, such as camcorders and external DVD drives, where Firewire has been preeminent. The complete specification is expected to be published later this year with USB 3.0 devices appearing early in 2009.    Firewire is the name that Apple gives to its version of the IEEE 1394 communications specification. However, the term really has been become the generic term for this type of connection rather than referring to IEEE 1394. Sony refers to their use as i.Link and Panasonic has its own name of DV. Firewire is a form of serial communication between devices like USB. Both Firewire and USB are capable of allowing devices to be "hot-plugged". That simply means that they can be added to or removed from the computer without shutting down the computer.    But there are many differences between the two systems. The speed of communication is one of the major differences. Firewire started at a speed of 100 Mb/s and was later increased to 400 Mb/s and then 800 Mb/s with a change in the cabling. Due to its design Firewire can come much closer to its maxi mum speed in real time conditions. So it is significantly faster than USB 2.0.    Another difference is that Firewire devices do not use the star or hub configuration. They are connected in a "daisy chain" and any one device can communicate directly with another device. For example, a camcorder or scanner can communicate directly with a printer without having to go through the computer. Firewire can connect up to 63 devices and the 800 Mb/s version can handle cables up to 100 meters in length between each device. That means that if you have 5 devices in your chain, the distance from one end to the other can be 400 meters.    In addition, power transmission with Firewire is much higher than with USB. The cables carry up to 30 volts and 1.5 amps. In this case, more devices can draw their power directly from the Firewire bus. One other difference is that of cost. Firewire cables have 6 to 9 connectors, depending on the version. The controllers for Firewire are more complex. In addition, Apple and other patent holders demand a royalty payment from manufacturers of Firewire devices. All of this increases the overall cost of Firewire compared to USB. This partly explains the success of USB compared to Firewire.    Firewire is not standing still in its development. Just like USB it has a faster version in the works. A new specification has been published for a 3.2 Gb/sec version. It is backwards compatible with the Firewire 800. Whether it will be compatible with Firewire 400 is not clear to me. Firewire 400 devices can be run in a Firewire 800 chain only with the help of special adapters for the cable connections. I would expect devices using this new specification to show up later this year.    For most uses, other than really high speed devices, USB is more than satisfactory. When you become involved with digital camcorders, external Blu-ray DVD drives, or external SATA hard drives, then you need to consider adding Firewire capability to your computer. Whichever you choose to use, they both have a significant advantage over the older serial/parallel port systems.    *Dr. Lewis is a former university and medical school professor of physiology. He has been working with personal computers for over thirty years, developing software and assembling systems. He can be reached at bwsail at yahoo.com.    Copyright 2008. This article is from the April 2008 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication.
Number 300 - May 2008