Introduction to InputOutput Ports
Introduction to Input/Output PortsThis chapter covers the primary peripheral input/output ports on a modern PC system. This includes a discussion of both the so-called "legacy" serial and parallel ports that have been standard on PCs since the beginning, as well as a discussion of the more current Universal Serial Bus (USB), which is replacing both serial and parallel ports, and IEEE 1394 (i.LINK or FireWire) interfaces. (IEEE stands for the Institute of Electrical and Electronic Engineers.) Although SCSI and IDE are also I/O interfaces, they are mainly used as internal interfaces. IDE is covered in Chapter 7, "The ATA/IDE Interface," even though desktop PCs today rarely implement SCSI. If you want to learn more about this architecture, please refer to my new book, Upgrading and Repairing Severs. Currently, the two most popular high-speed serial-bus architecture families for desktop and portable PCs are Universal Serial Bus (USB) and IEEE 1394, which is also called i.LINK or FireWire. Each interface type is available in two versions: USB 1.1 and USB 2.0; IEEE 1394a and IEEE 1394b (also called FireWire 800). The USB and IEEE 1394 port families are high-speed communications ports that far outstrip the capabilities of older standard serial and parallel ports. They can also be used as an alternative to SCSI for high-speed external peripheral connections. In addition to performance, these newer ports offer I/O device consolidation, which means that all types of external peripherals can connect to these ports. Why Serial?As mentioned in the previous section, the technology behind both USB and IEEE 1394 is serial in nature. The current trend in high-performance peripheral bus design is to use a serial architecture, in which 1 bit at a time is sent down a wire. Because parallel architecture (used by SCSI, ATA, and LPT ports) uses 8, 16, or more wires to send bits simultaneously, the parallel bus is actually much faster at the same clock speed. However, increasing the clock speed of a serial connection is much easier than increasing that of a parallel connection. Parallel connections in general suffer from several problems, the biggest being signal skew and jitter. Skew and jitter are the reasons high-speed parallel buses such as SCSI (small computer systems interface) are limited to short distances of 3 meters or less. The problem is that, although the 8 or 16 bits of data are fired from the transmitter at the same time, by the time they reach the receiver, propagation delays have conspired to allow some bits to arrive before the others. The longer the cable, the longer the time between the arrival of the first and last bits at the other end! This signal skew, as it is called, prevents you from running a high-speed transfer rate or a longer cableor both. Jitter is the tendency for the signal to reach its target voltage and float above and below for a short period of time. With a serial bus, the data is sent 1 bit at a time. Because there is no worry about when each bit will arrive, the clocking rate can be increased dramatically. For example, the top transfer rate possible with EPP/ECP parallel ports is 2.77MBps, whereas IEEE 1394a ports (which use high-speed serial technology) support transfer rates as high as 400Mbps (about 50MBps)25 times faster than parallel ports. USB 2.0 supports transfer rates of 480Mbps (about 60MBps), which is about 30 times faster than parallel ports, and the new IEEE 1394b (FireWire 800) ports reach transfer rates as high as 800Mbps (or about 100MBps), which is about 50 times faster than parallel ports! At high clock rates, parallel signals tend to interfere with each other. Serial again has an advantage because, with only one or two signal wires, crosstalk and interference between the wires in the cable are negligible. In general, parallel cabling is more expensive than serial cabling. Besides the many additional wires needed to carry the multiple bits in parallel, the cable also must be specially constructed to prevent crosstalk and interference between adjacent data lines. This is one reason external SCSI cables are so expensive. Serial cabling, by comparison, is very inexpensive. For one thing, it has significantly fewer wires. Furthermore, the shielding requirements are far simpler, even at very high speeds. Because of this, transmitting serial data reliably over longer distances is also easier, which is why parallel interfaces have shorter recommended cable lengths than do serial interfaces. For these reasonsin addition to the need for Plug and Play external peripheral interfaces and the elimination of the physical port crowding on portable computersthese high-performance serial buses were developed. USB is a standard feature on virtually all PCs today; is used for most general-purpose, high-speed external interfacing; and is the most compatible, widely available, and fastest general-purpose external interface. In addition, IEEE 1394 (more commonly known as FireWire), although mainly used in certain niche marketssuch as connecting DV (digital video) camcordersis also spreading into other high-bandwidth uses, such as high-resolution scanners, external hard drives, and networking. Comparing IEEE 1394 and USBAlthough both USB and IEEE 1394 are discussed in detail in the following sections, it's helpful to first compare them. Because of the similarity in both the form and function of USB and 1394 ports, there has been some confusion about the differences between them. Table 15.1 summarizes the differences between these technologies.
Because the overall performance and physical specifications are similar, the main difference between USB and 1394 is popularity. The bottom line is that USB is by far the most popular external interface for PCs, eclipsing all others by comparison. This is primarily because Intel developed most of USB and has placed built-in USB support in all its motherboard chipsets and motherboards since 1996. Few motherboard chipsets integrate 1394a or 1394b; in most cases, it has to be added as an extra-cost chip to the motherboard. The cost of the additional 1394 circuitry (and a $0.25 royalty paid to Apple Computer per system) and the fact that all motherboards already have USB, have limited the popularity of 1394 (FireWire) in the PC marketplace. Even with the overwhelming popularity of USB, a market for 1394 still exists. Perhaps the main reason 1394 will survive in conjunction with the USB 2.0 interface is that USB began as a PC-centric technology, whereas 1394 is not. In other words, USB and Hi-Speed USB initially required a PC as the host, whereas 1394 can connect two devices directly without a PC between them. As such, 1394 could be used to directly connect a DV camcorder to a DV-VCR for dubbing tapes or editing. Even this has changed, however, with the introduction of a supplement to the USB 2.0 specification called USB On-The-Go. USB On-The-Go, introduced in December 2001, enables the same device-to-device connections as was capable in 1394 (FireWire) and essentially nullifies the primary advantage 1394 had over USB. Because of the popularity and capabilities of USB, I recommend seeking out only USB peripherals over their 1394 (FireWire) counterparts where possible. Because both USB 2.0 and 1394a (FireWire) offer relatively close to the same overall capabilities and performance, you should make your choice based on which devices you intend to connect. If the digital video camera you want to connect has only a 1394 (FireWire/i.LINK) connection, you will need to add a 1394 FireWire card to your system, if such a connection isn't already present on your motherboard. Most general-purpose PC storage, I/O peripherals, and other devices are USB, whereas only video and some storage devices usually have 1394 connections. However, many devices now offer both USB 2.0 and 1394 interfaces to enable use with the widest range of computers. |
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