1 Jan, 2007

USB, 1394 (Firewire), and e-SATA

When IBM designed its first PC, it minimized any custom hardware. The box used industry standard off-the-shelf chips for each interface. The keyboard was connected to one type of chip, serial ports ran off a National Semiconductor serial chip, and the printer connected to a "parallel" port driven by yet a third chip.

Today all these specialized devices are supported as functions of the "Southbridge" chip on the mainboard. It is expensive to design all these different specialized functions into the chip and to provide connectors from that chip to all the different ports. Even the physical specialized connectors are expensive, and the parallel printer port takes up more space than any three other devices.

These "legacy" interfaces are obsolete. Today all their functions, and may additional hardware services, can be provided by a single standard device interface. Computers have been shipping with a USB port for five years. There are USB keyboards and mice. Printers and scanners come with a USB interface. A few vendors have shipped "legacy free" mainboards that replace all the old connectors with USB ports, but consumers have expressed some resistance to change. The old inefficient interfaces remain because people seem to want them.


Every modern computer mainboard has four USB 2 ports on the back. Typically the mainboard will have two additional "headers" onto which you can plug a cable that feeds two additional pairs of USB ports. A case will have one pair of USB ports on the front, and mainboards often come with a bracket that you can screw onto the end of an unoccupied card slot to generate two more USB ports in the back.

All the devices (except video) that used to have individual ports on the back of the computer can now be connected to a USB port. The keyboard and mouse can connect to either USB or the round "PS/2" port. Printers now connect directly to USB instead of the massive obsolete "parallel port". If anyone needs a modem any more, an external serial port can also be connected through USB.

If you have an old computer without USB 2, you can buy a PCI card with 4 or 5 USB ports for $27.

The modern USB 2.0 port supports three specific transfer speeds:

Unfortunately, a buyer has to be careful of the terminology. Technically, USB 2.0 is a specification of the physical and electrical interface specification. A few vendors claim to have USB 2.0 ports but do not support high speed devices. So make sure that a device claims "high speed" or 480 Mbs before buying it.

Converting to bytes, USB appears to be 60 megabytes per second, but in practice it does not transfer data that fast. There is overhead, and in practice a USB disk is unable to operate faster than 10-16 megabytes per second.

That is much faster than is necessary for DVD, HD DVD, Blu-Ray, and standard or high definition TV tuners. Although the video feed to monitors requires massive bandwidth, HDTV is compressed down to around a megabyte per second.

With an external adapter (typically built into the cable) a USB port can be connected to old parallel printers or modems. For around $60 several vendors produce a box that connects to one USB port on a laptop and provides legacy connectors for the keyboard and mouse (PS/2), modem (9 pin), parallel printer (25 pin), and even a 100 megabit Ethernet port. This allows the laptop vendor to avoid obsolete connectors that most of the time serve no useful purpose.


FireWire is the informal name for a standard technically known as IEEE 1394. It was developed by Apple as a low cost alternative to SCSI for disk-speed external devices. The original FireWire specification runs at 400 megabits per second and a second generation doubles that to 800 megabits.

Firewire is more efficient than USB. A Firewire 400 Mb/s transfers data faster than the nominally faster USB 480 Mb/s.

However, Firewire has not caught on. It is available on some but not all mainboards. It is much less common than USB for most devices. Even Apple has transferred its attention to USB.

External SATA (eSATA)

So if you are looking for a simple and inexpensive way to connect external disks to your computer, the current best technology is eSATA. This is a slightly more rugged version of the simple SATA cables that connect hard disks inside the computer.

Like the internal SATA, it is available in 150 and 300 Megabyte per second speeds. Since desktop disks typically transfer at a maximum of 40 megabytes/sec, and the very fastest Raptor disks only do 80 megabytes/sec, 150 is a perfectly fine speed. In fact, since the slower 150 MB/s is slightly more robust against connection problems, the author forces all his own SATA external disks to run at 150 even when they are nominally rated at 300.

eSATA allows you to run external disks at the same speed and with the same performance as internal disks. If you have a small case, then external disks may be a requirement. Even with a large case they can be useful for backup or archival. While USB will continue to be useful for the wide range of slower speed devices, eSATA is establishing itself as the preferred external disk connection.

External Enclosures

For $30 one can purchase an external  ATA or Serial ATA device enclosure. They come in various sizes to hold a 5 1/4 inch CD or DVD reader or writer, a standard 3 1/2 inch IDE hard disk, or a 2 1/2 inch low profile "laptop" disk.

The external enclosure can connect to the PC over a USB, Firewire, or eSATA cable. Converting a SATA disk to an eSATA cable is the simplest, but the electronics to do any other conversion is fairly trivial and a small part of the overall cost of any enclosure.

The 2 1/2 inch laptop disk is smaller and more expensive that the standard 3 1/2 inch disk. However, its power requirements are low enough that the disk and small enclosure can be powered by the USB port on many (but not all) laptop computers. The enclosure is small enough to fit in a pocket and provides convenient supplemental storage for the typically limited disk space in laptop computers. It is also a convenient way to carry data between desktop systems.

If you need to upgrade to a standard 3 1/2 inch disk, then you need an external power source anyway. At that point the higher speed of the eSATA connection is more attractive than USB.

Copyright 1998, 2007 PCLT -- Introduction to PC Hardware -- H. Gilbert