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. A+ Certification Training Guide92The system board is the center of the system.It contains the por-tions of the system that define its computing power and speed.System boards are also referred to as motherboards, or planar boards.Any number of options adapter cards may be installed to handle awide array of PC peripheral equipment.Typical adapter cardsinstalled in a system include a video adapter card and some sort ofInput/Output (I/O) adapter card.Peripheral devices such as printersand mice normally connect to options adapter cards through therear of the system unit.These cards plug into Expansion Slot Con-nectors on the back of the system board.In most desktop cases, thekeyboard also plugs into the back panel.CasesObjectivePCs have been built in a number of different case designs.Eachdesign offers characteristics that adapt the system for differentenvironments.These key characteristics for case design includemounting methods for the printed circuit boards, ventilationcharacteristics, total drive capacity, footprint (the amount of hori-zontal space they occupy), and portability.DesktopsSome of the most familiar PC case styles are the desktop designsillustrated in Figure 2.6.These units are designed to sit horizon-tally on a desk top (hence the name).The original IBM PC, PC-XT, and PC-AT designs use this case style.The PC and XT casestyles measured 21"w×17"d×5-1/2"h, while the AT case grew to23"w×17"d×5-1/2"h.A narrower desktop style, referred to as the baby AT case, was devel-oped to take up less desk space than the XT and AT styles.Thereduced footprint was accomplished by using a half-height powersupply unit and limiting the number of disk drives.The disk-drivecage did not reach down to the floor of the system unit, enablingthe system board to slide under the power supply and disk-drivecage.The widths of baby AT cases varied from manufacturer tomanufacturer. Chapter 2 PC Hardware93Figure 2.6Desktop casedesigns.AT CASE STYLE BABY AT CASE STYLEOOLOW-PROFILE CASE STYLEPC AND XT CASE STYLEA special variety of desktop cases, referred to as Low-profile Desk-tops, reduce the vertical height of the unit.A short bus-extendercard, called a back plane, mounts in an expansion slot and enablesoption adapter cards to be mounted in the unit horizontally.Thisenables the case to be shorter.The horizontal mounting of theI/O cards in a low-profile case tends to create heat build-up prob-lems.The heat rising off of the system board flows around theI/O cards, adding to the heat they are already generating.A stan-dard back plane card is depicted in Figure 2.7.Figure 2.7A back planecard.Low-profile power supplies and disk drives are also required toachieve the reduced height.Only the very earliest low-profileunits tried to incorporate a 5-1/4" floppy drive into the design.The vast majority of these units have a single 3-1/2" floppy driveonly.IBM s original PS/2 units incorporated these smaller compo-nents and shorter microchannel adapter cards to achieve a rela-tively low profile without mounting the cards horizontally. A+ Certification Training Guide94In desktop cases, the system board is generally located in the floorof the unit, toward the left-rear corner.The power supply is locat-ed in the right-rear corner.Raised reinforcement rails in the floorof the system unit contain threaded holes and slip-in slots towhich the system board is anchored.Small plastic feet are insertedinto the system board and set down in the slots.The system boardis secured when its feet are slid into the narrow portion of theslot.One or two brass standoffs are inserted into the threaded holesbefore the system board is installed.After the system board hasbeen anchored in place, a small machine screw is insertedthrough the system board opening and into the brass standoff.This arrangement provides electrical grounding between the sys-tem board and the case and helps to reduce electromagnetic fieldinterference (EFI) emitted from the board.Every electrical conductor radiates a field of electromagnetic en-ergy when an electrical current is passed through it [ Pobierz caÅ‚ość w formacie PDF ]
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. A+ Certification Training Guide92The system board is the center of the system.It contains the por-tions of the system that define its computing power and speed.System boards are also referred to as motherboards, or planar boards.Any number of options adapter cards may be installed to handle awide array of PC peripheral equipment.Typical adapter cardsinstalled in a system include a video adapter card and some sort ofInput/Output (I/O) adapter card.Peripheral devices such as printersand mice normally connect to options adapter cards through therear of the system unit.These cards plug into Expansion Slot Con-nectors on the back of the system board.In most desktop cases, thekeyboard also plugs into the back panel.CasesObjectivePCs have been built in a number of different case designs.Eachdesign offers characteristics that adapt the system for differentenvironments.These key characteristics for case design includemounting methods for the printed circuit boards, ventilationcharacteristics, total drive capacity, footprint (the amount of hori-zontal space they occupy), and portability.DesktopsSome of the most familiar PC case styles are the desktop designsillustrated in Figure 2.6.These units are designed to sit horizon-tally on a desk top (hence the name).The original IBM PC, PC-XT, and PC-AT designs use this case style.The PC and XT casestyles measured 21"w×17"d×5-1/2"h, while the AT case grew to23"w×17"d×5-1/2"h.A narrower desktop style, referred to as the baby AT case, was devel-oped to take up less desk space than the XT and AT styles.Thereduced footprint was accomplished by using a half-height powersupply unit and limiting the number of disk drives.The disk-drivecage did not reach down to the floor of the system unit, enablingthe system board to slide under the power supply and disk-drivecage.The widths of baby AT cases varied from manufacturer tomanufacturer. Chapter 2 PC Hardware93Figure 2.6Desktop casedesigns.AT CASE STYLE BABY AT CASE STYLEOOLOW-PROFILE CASE STYLEPC AND XT CASE STYLEA special variety of desktop cases, referred to as Low-profile Desk-tops, reduce the vertical height of the unit.A short bus-extendercard, called a back plane, mounts in an expansion slot and enablesoption adapter cards to be mounted in the unit horizontally.Thisenables the case to be shorter.The horizontal mounting of theI/O cards in a low-profile case tends to create heat build-up prob-lems.The heat rising off of the system board flows around theI/O cards, adding to the heat they are already generating.A stan-dard back plane card is depicted in Figure 2.7.Figure 2.7A back planecard.Low-profile power supplies and disk drives are also required toachieve the reduced height.Only the very earliest low-profileunits tried to incorporate a 5-1/4" floppy drive into the design.The vast majority of these units have a single 3-1/2" floppy driveonly.IBM s original PS/2 units incorporated these smaller compo-nents and shorter microchannel adapter cards to achieve a rela-tively low profile without mounting the cards horizontally. A+ Certification Training Guide94In desktop cases, the system board is generally located in the floorof the unit, toward the left-rear corner.The power supply is locat-ed in the right-rear corner.Raised reinforcement rails in the floorof the system unit contain threaded holes and slip-in slots towhich the system board is anchored.Small plastic feet are insertedinto the system board and set down in the slots.The system boardis secured when its feet are slid into the narrow portion of theslot.One or two brass standoffs are inserted into the threaded holesbefore the system board is installed.After the system board hasbeen anchored in place, a small machine screw is insertedthrough the system board opening and into the brass standoff.This arrangement provides electrical grounding between the sys-tem board and the case and helps to reduce electromagnetic fieldinterference (EFI) emitted from the board.Every electrical conductor radiates a field of electromagnetic en-ergy when an electrical current is passed through it [ Pobierz caÅ‚ość w formacie PDF ]