The universal serial bus (USB) is the latest development in common use for external devices. Most modern PCs contain two external USB ports, and you can attach more than two USB devices by using a USB hub, which effectively turns one USB port into several. Devices available for USB include mice, keyboards, digital cameras, scanners, printers, and modems.
USB is capable of much higher speeds than conventional serial ports—12Mbps, as compared to 115kbps for most conventional serial ports. USB's speed is close to that of an EPP/ECP parallel port (about 16Mbps). USB is an appealing connection method for another reason: It's possible to connect up to 127 devices to a single USB controller. In practice, of course, 127 devices would most likely overload USB's bandwidth, unless most or all of these devices were very slow or seldom used. Nonetheless, with the explosion of external devices like scanners, digital cameras, and so on, USBs' appeal is substantial.
The Linux 2.2.x kernel series picked up USB support very late in its lifetime. Fortunately, the Linux developers have been working on substantially expanding USB support, and by the 2.4.x kernel will have substantial USB support. If you want to use a USB device, you may want to check the main Linux USB development page at http://www.linux-usb.org. Where applicable, I also describe Linux's support for specific USB devices in appropriate chapters of this book.
SCSI functionality in x86 motherboards is rare. When present, it's implemented by including a separate SCSI chipset on the motherboard; the SCSI features aren't implemented in the main motherboard chipset. Most boards that include SCSI features use chipsets from Adaptec (http://www.adaptec.com) or Symbios Logic (http://www.symbios.com). Both companies' products are well supported in Linux. In most cases you won't have problems with SCSI support built in to a motherboard. You should check to be sure that the specific SCSI chipset used on the motherboard is supported by Linux, however.
In order to boot from a SCSI host adapter, whether that adapter is on a separate board or built in to the motherboard, the SCSI adapter must have its own boot BIOS. In the case of SCSI adapters built in to the motherboard, this BIOS is generally integrated into the motherboard's main BIOS, although the SCSI BIOS may have its own user interface separate from that of the main BIOS. If you intend to boot from a SCSI hard disk attached to a host adapter on the motherboard, you should check to be sure that the motherboard includes an appropriate SCSI BIOS (most do). It might also be worth investigating BIOS upgrade procedures. In most cases, upgrading either the SCSI or the main motherboard BIOS on a computer with an integrated SCSI host adapter entails upgrading both the SCSI and the core motherboard BIOS. Because upgrading a motherboard BIOS is such a dangerous proposition, I recommend not doing so unless you're reasonably certain the upgrade will fix a problem you're experiencing.
One drawback to having a SCSI adapter on the motherboard is that there's often no external SCSI connector. Therefore, you can only connect internal SCSI devices. You can get around this limitation by using an internal/external adapter plug (shown in Figure 2.15). You mount the adapter in the computer's case, and then plug the last connector on an internal SCSI cable into the internal plug. The external plug then serves as a continuation of the SCSI chain for external devices. There are several different types of internal/external SCSI adapter plugs, providing access for a variety of internal and external cable types.
Like SCSI adapters, Ethernet network ports are rare on motherboards, but some motherboards contain this feature. In most cases, the Ethernet functionality is provided by a separate Ethernet chipset on the motherboard. In some cases this functionality is built in to the main chipset. Whatever the case, you should check to be sure that Linux drivers exist for the Ethernet chipset in use. Fortunately, Linux supports the vast majority of Ethernet devices, although the Ethernet features built in to some SiS chipsets is unsupported by Linux at the moment. You can read more about Linux's Ethernet support the "Ethernet Adapters" section of Chapter 16.
A few motherboards, mostly used on low-cost pre-built PCs, include modem functionality. These modems are generally of the "Windows-only" variety, meaning that no Linux drivers are available for them. Unless you're convinced that the modem is usable under Linux, I recommend you avoid such motherboards. Although you can probably use another modem, the motherboard may lack a second serial port, and the unused circuitry could conceivably interfere with any modem you might add to the system.
Video adapter support is common on many low-end motherboards. Placing the video adapter on the motherboard can reduce the complexity of the assembled computer by eliminating an expansion card. Many motherboard manufacturers produce at least one or two models that include video and audio functions on the motherboard and reduce the number of available slots by two or so as compared to their other models, thus reducing the size of the motherboard. Some motherboards that feature integrated video do so by using a main motherboard chipset that incorporates video functionality directly. Others add a video chipset to the motherboard that would otherwise appear on a separate video card.
Although convenient in many ways, adding the video support onto the motherboard does have its drawbacks. For one thing, such designs generally take some of the main system memory and give it to the video functions. For instance, on a 64MB system, you might end up with
Part i only 60MB of main memory and 4MB of video memory. (Most such motherboards let you use the BIOS to set the amount of video memory. Typical values range from 2MB to 16MB.) An integrated design also robs you of flexibility. It's harder to upgrade to a new video card in the future, because motherboards with integrated video typically have fewer available expansion slots and usually no AGP slots.
If you're considering a motherboard with integrated video support, you should be absolutely certain that Linux (or, more precisely, XFree86) supports the video chipset in use. You can learn more about such support from the main XFree86 Web site, http://www.xfree86.org. SuSE (http://www.suse.com) is a Linux distributor that works on developing new XFree86 drivers, so you may want to check its Web site, as well, for the latest information.
Like video features, audio features often appear on low-end motherboards in order to save space and reduce total system cost. Audio functions are usually, but not always, implemented by way of a secondary chipset, so the section titled "Sound Card Chipsets" in Chapter 10, "Sound Cards," may be of interest in evaluating Linux compatibility of such motherboards.
Integrated audio generally doesn't rob you of any system memory. This feature is therefore less detrimental than is integrated video in this respect. Integrated audio still suffers from the drawback of inflexibility, however; upgrading to a new sound card may be more difficult because of the reduced number of expansion slots on motherboards with integrated audio.
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