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.Theoriginal AT system board measured 30.5×33 cm.Initially, printed-circuit-board manufacturers reduced the size of their chip-setbased system boards to match that of the original PC and PC-XTsystem boards (22×33 cm).This allowed the new 286 boards to beinstalled in the smaller XT-style cases.This particular size of sys-tem board is described as a baby AT system board.As VLSI technology improves, IC manufacturers continue to inte-grate higher levels of circuitry into their chips.All the functions ofthe 4-chip chip set system board of Figure 6.1 are duplicated usingthe 2-chip chip set depicted in Figure 6.4.The high level of circuitconcentration in this chip set allows the size of the system boardto be reduced even further.It is approximately half the length of astandard baby AT system board; therefore, this size system board isreferred to as a half-size system board.A+ Certification Training Guide378Figure 6.4A half-size sys-tem board.HALF SIZE 80286SYSTEM BOARDBABY SIZE 80286SYSTEM BOARDThe first chip-set based system boards came with special configura-tion software that had to be loaded from disk.Now, however, itmust consider the ROM BIOS as an integral part of the chip setbecause it is designed to support the register structure of the chipset.Therefore, replacing a ROM BIOS chip on a system board isnot as simple as placing another ROM BIOS in the socket.Thereplacement BIOS must be correct for the chip set being used.By combining larger blocks of circuitry into fewer ICs, a price re-duction spiral is created.Fewer ICs on the board leads to reducedmanufacturing costs to produce the board.The material cost of theboard is decreased due to its smaller physical size.The componentcost is decreased because it is cheaper to buy a few VLSI chips thanseveral SSI or MSI devices.Finally, the assembly cost is less becauseonly a few items must be mounted on the board.Reduced board costs create lower computer prices, which in turncreates greater consumer demand for the computers.Increaseddemand for the computers, and therefore the chip sets, acts tofurther push down the prices of all the computer components.Chip-set based system boards and I/O cards tend to change oftenas IC manufacturers continue to integrate higher levels of circuitryinto their devices.The newest system board designation is the ATXform factor developed by Intel for Pentium-based systems.Thisspecification is an evolution of the baby AT form factor thatmoves the standard I/O functions to the system board.The ATXChapter 6 System Boards379specification basically rotates the baby AT form factor by 90 de-grees, relocates the power supply connection, and moves the micro-processor and memory modules away from the expansion slots.The power supply orientation allows a single fan to be used tocool the system.This provides reduced cost, reduced system noise,and improved reliability.The relocated microprocessor and mem-ory modules allow full-length cards to be used in the expansionslots while providing easy upgrading of the microprocessor, RAM,and I/O cards.Figure 6.5 depicts a Pentium-based, ATX system board that direct-ly supports the FDD, HDD, serial, and parallel ports.The board is12 inches (305 mm) wide and 9.6 inches (244 mm) long.A re-vised, mini-ATX specification allows for 11.2×8.2-inch systemboards.The hole patterns for the ATX and mini-ATX boards re-quire a case that can accommodate the new boards.AlthoughATX shares most of its mounting hole pattern with the baby ATspecification, it does not match exactly.The fully implemented ATX format also contains specificationsfor the power supply and I/O connector placements.In particu-lar, the ATX specification for the power supply connection callsfor a single 20-pin power cable between the system board and thepower supply unit rather than the typical P8/P9 cabling.The newcable adds a +3.3 Vdc supply to the traditional +/- 12 Vdc and+/- 5 Vdc supplies.In most monitors, the frequency relationshipsbetween these signals are fixed [ Pobierz caÅ‚ość w formacie PDF ]