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System Boards
A system boards may also be called a planar board, motherboard or main board. There are various types of system boards that differ depending on the type of case that they fit in and the type of processor that they host. The form factor of the motherboard describes its general shape, what sorts of cases and power supplies it can use and its physical layout. A company can make 2 motherboards that have basically the same functionality but that use a different form factor and the only real differences will be the physical layout of the board and the position of the components. Common form factors include AT, Baby AT, ATX, Mini ATX, LPX, Mini LPX and NLX. The table below contains more information:
Style |
Where Found |
Match to Case and
Power Supply |
Full AT |
Very Old PCs |
Full AT, Full Tower |
Baby AT |
Older PCs |
All but Slimline, ATX |
ATX |
Newer PCs |
ATX |
Mini ATX |
Newer PCs |
ATX |
LPX |
Older Retail PCs |
Slimline |
Mini LPX |
Older Retail PCs |
Slimline |
NLX |
Newer Retail PCs |
Slimline |
NOTE: Laptop motherboards tend to be proprietary to the model for which they are designed.
Below is a graphic that shows some of the common features of motherboards. Note that these will vary from board to board depending on the form factor.
ATX System Board
System Boards typically have several components that are replaceable/upgradeable as follows:
- Processor - Upgrading the processor is a fairly simple process. Make sure that the new processor is supported by the motherboard.
- Memory(RAM) - Before upgrading memory, check the motherboard manual for specifications on supported memory types and speeds.
- CMOS Battery - These batteries are designed to last 2 to 5 years. Failure of this battery can result in an error code, however, the most noticeable symptom is the computer's lack of ability to keep proper time.
- BIOS ICs - Newer BIOS chips are "Flash" upgradeable using software. Older BIOSes require replacement of the BIOS ROM chip. Before upgrading, make sure that your processor is compatible with the BIOS and motherboard chipset.
- Cache Memory - On some system boards, the cache memory can be upgraded. It may be as simple as adding an IC to an open slot. Other times, you may have to remove the existing one to upgrade.
Motherboards also contain configurable jumpers and possibly even DIP switches(typically on older models). Jumpers use BERG pins and a small connector that slides onto the pins to designate "on". BERG connectors are also used to connect the front panel LEDs and switches to the board.
The back of the motherboard contains ports used for connecting various peripherals. Peripherals are composed of input and output devices including the mouse, keyboard, monitor, speakers, printer, etc. So what is the difference between an input device and an output device? It is just as the name says. The mouse and keyboard are input devices since they are used to provide the computer with information. Output devices provide YOU with information such as speakers, printers and the monitor. Older PC-XT and AT board typically had a 5 pin DIN keyboard connection. The newer ATX style uses the smaller 6 pin mini DIN connection.
Expansion Busses:
| Bus |
Format |
Notes |
| PC-bus |
8 Bit |
Used in PC and PC-AT models |
| ISA |
16 bit |
Runs at 8 or 8.33mhz |
| VESA |
32 bit |
Designed to address video limitations |
| EISA |
32 bit |
Supports Plug-and-Play and Bus mastering |
| MCA |
32 bit |
Supports PnP and Bus mastering |
| PCI |
32/64 bit |
Supports PnP, Burst Mode, Bus Mastering. Utilizes the host bridge to communicate with other types of expansion slots. The 32-bit version is the most common. |
| PCI-X |
32/64 bit |
Provides much faster speeds and is backward compatible with traditional PCI. The 64-bit version is more common than the 32-bit. |
| AGP |
32/64 bit |
Variation of PCI designed to handle 3D graphics better from video cards. |
Most modern motherboards contain AGP, PCI and ISA slots.
On newer and faster buses, a great deal of information is flowing through the channel every second. Normally, the processor is required to control the transfer of this information. Bus mastering involves having capable devices take control of the bus and do the work themselves instead of utilizing the CPU.
Plug-and-Play(PnP) - Compatible BIOSes can autodetect devices and assign resources to them. Non PnP compatible devices are configured first followed by PnP devices.
The IRQ(interrupt request) value is an assigned location where the computer can expect a particular device to interrupt it when the device sends the computer signals about its operation. Below is a list of common IRQ settings.
| IRQ |
Device |
| IRQ 0 | System Timer |
| IRQ 1 | Keyboard |
| IRQ 2/9 | Video Card |
| IRQ 3 | Open unless needed for Com 2 or 4 |
| IRQ 4 | Com 1, Com 3 |
| IRQ 5 | Open unless needed for LPT2 or sound card |
| IRQ 6 | Floppy Disk Controller |
| IRQ 7 | LPT1(parallel port) |
| IRQ 8 | Real time clock |
| IRQ 9/2 | linked to IRQ 2 |
| IRQ 10 | Open |
| IRQ 11 | USB |
| IRQ 12 | PS/2 Mouse |
| IRQ 13 | Math Co-processor |
| IRQ 14 | Hard Disk Controller |
| IRQ 15 | Open |
Input/output(I/O) addresses are resources used by virtually every device in a computer and represent locations in memory that are designated for use by various devices to exchange information between themselves and the rest of the PC. The following is a list of common I/O settings.
1FO-1F8 - Hard Drive Controller, 16-bit ISA
220 - Soundcard
278-27F - LPT2
2F8-2FF - COM2
320-32F - Hard Drive Controller, 8-bit ISA
378-37F - LPT1
3D0-3DF - Video Adapter
3F0-3F7 - Floppy Controller
3F8-3FF - COM1
Universal Serial Bus(USB) - A high-speed I/O bus that supports the daisy chaining of devices(up to 127). USB hubs are used to provide connections for multiple devices. USB supports the addition and removal of devices while they are on(hot-swapping). Devices are either full speed or low speed. Full speed device cabling can be up to 16 feet 5 inches(5 meters) in length. Low speed cabling is limited to 9 feet 10 inches(3 meters). USB supports Isochronous transfers that can stream data such as voice or video.
Further Reference:
http://www.techtutorials.com/Hardware/Motherboards/
http://www.pcguide.com/ref/mbsys/index.htm
USB Explained
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