05 0789729741 ch04 7/15/03 4:03 PM Page 195 CHAPTER 4 Motherboards and Buses 05 0789729741 ch04 7/15/03 4:03 PM Page 196 196 Chapter 4 Motherboards and Buses Motherboard Form Factors Without a doubt, the most important component in a PC system is the main board or motherboard. Some companies refer to the motherboard as a system board or planar. The terms motherboard, main board, system board, and planar are interchangeable, although I prefer the motherboard designation. This chapter examines the various types of motherboards available and those components typically contained on the motherboard and motherboard interface connectors. Several common form factors are used for PC motherboards. The form factor refers to the physical dimensions (size and shape) as well as certain connector, screw hole, and other positions that dictate into which type of case the board will fit. Some are true standards (meaning that all boards with that form factor are interchangeable), whereas others are not standardized enough to allow for inter- changeability. Unfortunately, these nonstandard form factors preclude any easy upgrade or inexpen- sive replacement, which generally means they should be avoided. The more commonly known PC motherboard form factors include the following: Obsolete Form Factors Modern Form Factors All Others ■ Baby-AT ■ ATX ■ Fully proprietary designs ■ Full-size AT ■ micro-ATX (certain Compaq, Packard Bell, Hewlett-Packard, ■ ■ LPX (semiproprietary) Flex-ATX notebook/portable sys- ■ WTX (no longer in production) ■ Mini-ITX (flex-ATX tems, and so on) ■ ITX (flex-ATX variation, never variation) produced) ■ NLX Motherboards have evolved over the years from the original Baby-AT form factor boards used in the original IBM PC and XT to the current ATX and NLX boards used in most full-size desktop and tower systems. ATX has a number of variants, including micro-ATX (which is a smaller version of the ATX form factor used in the smaller systems) and flex-ATX (an even smaller version for the lowest-cost home PCs). A new form factor called mini-ITX is also available; it’s really just a minimum-size version of flex-ATX designed for very small systems. NLX is designed for corporate desktop–type systems; WTX was designed for workstations and medium-duty servers, but never became popular. Table 4.1 shows the modern industry-standard form factors and their recommended uses. Table 4.1 Current Industry-Standard Motherboard Form Factors Form Factor Use ATX Standard desktop, mini-tower, and full-tower systems; most common form factor today; most flexible design for power users, enthusiasts, low-end servers/workstations, and higher-end home systems; ATX boards support up to seven expansion slots. Mini-ATX A slightly smaller version of ATX that fits into the same case as ATX. Many so-called ATX motherboards are actually mini-ATX motherboards; mini-ATX boards support up to six expansion slots. micro-ATX A smaller version of ATX, used in Mid-range desktop or mini-tower systems. Fits micro-ATX or ATX chassis. Flex-ATX Smallest version of ATX, used in expensive or low-end small desktop or mini-tower systems; entertainment or appliance systems. Fits in flex-ATX, micro-ATX, or ATX chassis. Mini-ITX Minimum-size flex-ATX version, used in set-top boxes and compact/small form factor comput- ers; highly integrated with one PCI expansion slot. Fits in mini-ITX, flex-ATX, micro-ATX, or ATX chassis. NLX Corporate desktop or mini-tower systems; fast and easy serviceability. 05 0789729741 ch04 7/15/03 4:03 PM Page 197 Motherboard Form Factors Chapter 4 197 Although the Baby-AT, Full-size AT, and LPX boards were once popular, they have all but been replaced by more modern and interchangeable form factors. The modern form factors are true stan- dards, which guarantees improved interchangeability within each type. This means that ATX boards can interchange with other ATX boards, NLX with other NLX, and so on. The additional features found on these boards as compared to the obsolete form factors, combined with true interchangeabil- ity, has made the migration to these newer form factors quick and easy. Today I recommend purchas- ing only systems with one of the modern industry-standard form factors. Each of these form factors, however, is discussed in more detail in the following sections. Anything that does not fit into one of the industry-standard form factors is considered proprietary. Unless there are special circumstances, I do not recommend purchasing systems with proprietary board designs. They will be virtually impossible to upgrade and very expensive to repair later because the motherboard, case, and often power supply will not be interchangeable with other models. I call proprietary form factor systems “disposable” PCs because that’s what you must normally do with them when they are too slow or need repair out of warranty. Caution “Disposable” PCs might be more common than ever. Some estimate that as much as 60% of all PCs sold today are dis- posable models, not so much because of the motherboards used, but because of the tiny power supplies and cramped micro-tower cases that are favored on most retail-market PCs today. Although low-cost PCs using small chassis and power supplies are theoretically more upgradeable than past disposable type systems, you’ll still hit the wall over time if you need more than three expansion slots or want to use more than two or three internal drives. Because mini-tower systems are so cramped and limited, I consider them to be almost as disposable as the LPX systems they have largely replaced. You also need to watch out for systems that only appear to meet industry standards, such as certain Dell computer models built from 1996 to the present—especially the XPS line of systems. These computers often use rewired versions of the ATX power supply (or even some that are completely nonstandard in size and shape) and modified motherboard power con- nectors, which makes both components completely incompatible with standard motherboards and power supplies. In some of the systems, the power supply has a completely proprietary shape as well and the motherboards are not fully standard ATX either. If you want to upgrade the power supply, you must use a special Dell-compatible power supply. And if you want to upgrade the motherboard (assuming you can find one that fits), you must buy a standard power supply to match. The best alternative is to replace the motherboard, power supply, and possibly the case with industry-standard components simultaneously. For more details about how to determine whether your Dell computer uses nonstandard power connectors, see Chapter 21, “Power Supply and Chassis/Case.” If you want to have a truly upgradeable system, insist on systems that use ATX motherboards in a mid-tower or larger case with at least five drive bays. PC and XT The first popular PC motherboard was, of course, the original IBM PC released in August 1981. Figure 4.1 shows how this board looked. IBM followed the PC with the XT motherboard in March 1983, which had the same size and shape as the PC board but had eight slots instead of five. Both the IBM PC and XT motherboards were 9''×13'' in size. Also, the slots were spaced 0.8'' apart in the XT instead of 1'' apart as in the PC (see Figure 4.2). The XT also eliminated the little used cassette port in the back, which was supposed to be used to save BASIC programs on cassette tape instead of the much more expensive (at the time) floppy drive. Note The Technical Reference section of the DVD accompanying this book contains detailed information on the PC (5150) and XT (5160). All the information there is printable. 05 0789729741 ch04 7/15/03 4:03 PM Page 198 198 Chapter 4 Motherboards and Buses 8259 Interrupt controller Clock chip trimmer Keyboard I/O Cassette I/O 8-bit ISA bus slots 8-bit ISA bus slots Keyboard I/O { J9 J8 J1 J2 J3 J4 J5 J6 J7 J1 J2 J3 J4 J5 J6 J7 System-board System-board power power connections connections Intel 8087 math coprocessor Clock chip trimmer Intel 8088 processor 12Intel 8087 ROM BASIC 1 1 math Read-only coprocessor memory 8 8 Intel 8088 ROM BIOS processor 8259 System Interrupt Configuration controller DIP DIP switch switches block 2 8237 DMA controller 8237 DMA As 64K to 256K controller much as read/write 640K memory with read/write parity checking memory with parity { checking { P3 P4 P3 Speaker Cassette microphone DIP switch block 1 Pin 1 Pin 1 Speaker output output or auxiliary select Figure 4.1 IBM PC motherboard (circa 1981). Figure 4.2 IBM PC-XT motherboard (circa 1983). The minor differences in the slot positions and the deleted cassette connector on the back required a minor redesign of the case. In essence, the XT was a mildly enhanced PC, with a motherboard that was the same overall size and shape, used the same processor, and came in a case that was identical except for slot bracketry and the lack of a hole for the cassette port. Eventually, the XT motherboard design became very popular, and many other PC motherboard manufacturers of the day copied IBM’s XT design and produced similar boards. Full-Size AT The full-size AT motherboard form factor matches the original IBM AT motherboard design. This allows for a very large board of up to 12'' wide by 13.8'' deep. The full-size AT board first debuted in August 1984, when IBM introduced the Personal Computer AT (advanced technology).