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Jumpers

Jumpers are pins on a motherboard or other device, that are used to provide configuration information to the hardware. A single jumper consists of a pair of pins, with a small rectangular shunt that can be placed over both pins to short them together. The hardware is programmed to act one way when the jumper is shorted, and another way when it is left open. The jumpers are normally numbered JP1, JP2 etc.. For some functions, a group of jumpers is used.

Every motherboard differs in its jumper numbering, positioning, and most importantly, what the settings for each jumper mean. This is why having the motherboard manual is so crucial for anyone who wants to work on their PC. This procedure provides specific instructions and caveats for configuring your motherboard.

One of the newest innovations on the market is the "jumperless" motherboard, such as the IT5H made by Abit. With these boards you make many of the hardware settings, such as CPU type and speed, and even CPU voltage, using BIOS settings, while some others are retained as regular jumpers (typically, CMOS clear and cache size). This arrangement lets you change the clock speed of the CPU as easily as you would change any other BIOS setting, and makes upgrading the processor a snap. It also makes it easier for motherboard manufacturers to add support for new processor types as they are put onto the market.

Many people think that "jumperless" is the new wave of motherboard design, and certainly the ability to change more settings without opening up the box is a great advantage. Others like the control of being able to physically set a jumper (recalling with frustration some of the problems with new technologies like Plug and Play that remove this control to software with sometimes mixed results). Frankly, considering that most people set their motherboard jumpers only once (or at most, once a year or so when they upgrade) I consider the whole jumperless motherboard thing more of a fad than a real evolution in design. For those that like to overclock, jumperless designs have obvious advantages.

Here is a list and brief explanation of the settings that you are most likely to find in your manual:

• Processor/CPU Voltage: Almost all newer boards have one or more jumpers to set the voltage for the processor. Obviously, this needs to be set correctly or you risk destroying the chip. In addition, newer processors use two voltages: an "external" I/O voltage and an "internal" core voltage. For your motherboard to support these newer processors, it must have jumpers to set both of these voltage types.
  Sometimes there can be a mismatch between the voltage that the processor requires and the voltage that the motherboard supplies. This often happens because the motherboard is designed to support a new processor based on the spec, but the voltage is changed for design reasons later on. You should contact the motherboard manufacturer in this case. Usually you can still use the processor if the motherboard can supply a voltage close to the number required--say, within 0.1V--because processors have a range of voltages they can use, not just a single number. See here for more on processor voltages.
• Processor Speed / Bus Speed / Multiplier: Every 486 class or later motherboard (excluding jumperless designs) has a way to specify the speed of the processor. There are two main methods for selecting processor speed, and your manual will show you which your motherboard uses. Some motherboards provide a list of the various speeds supported, and a diagram of how to set the jumpers to match that speed. Others instead require you to set two separate jumpers: one controls the memory bus speed, and the other sets the processor "multiplier" (how many times the memory bus speed the processor runs).
• Processor Type: You may find this instead of, or in addition to, the two settings listed above. In an effort to save the user the hassle of figuring out voltages and speeds, some motherboards combine the jumpers into one jumper set or group. Then, they provide a long list with each processor type and speed that the board supports, and tell you how to set each of a group of jumpers so that it works appropriately. If you are setting up a Pentium 133, you'll find it on the list and be told how to set jumpers 1, 3, 7, 12, and 18, for example.
  The only problem with this arrangement is that unless they also tell you what each of the individual jumpers is actually controlling, it can be difficult to set up a newer processor that may be supported just fine by the board but not be listed in their documentation. You have less information about how the board actually works. You can sometimes find help in this case on the manufacturer's web site.
• Cache Size and Type: Some boards can have different amounts of cache, and some can have cache either built in or on a COASt module. There is often a jumper or two to set which is used and how much is on the board. Other boards only come with one arrangement of cache or can auto-detect when you insert a COASt module, so they will lack these jumpers.
• Memory Size/Type: Almost all newer PCs auto-detect the amount and type of RAM in the system, but many older 486 (and earlier) models require jumpers to be changed when changing the amount of memory (this is a pain in the butt, which is why it was done away with).
• Flash BIOS Enable: Many boards require you to set a jumper to a special position in order to enable the flash BIOS update feature. This jumper is usually set to the "normal" position except when doing a BIOS update.
• CMOS Clear: One of the most famous of annoying PC problems is the "lost BIOS setup password" problem, that locks the user out of the BIOS setup program. You'd be amazed how often this happens. Some newer PCs have built into the motherboard the ability to clear the BIOS by temporarily setting a jumper and then replacing it (in essence this disconnects the CMOS memory from the battery so it discharges and resets). This jumper is also usually set to the "normal" position.
• Battery Source: Some boards let you switch the battery from an internal to an external source, and a jumper is used to control the setting.
• Disable Jumpers: Some motherboards use special jumpers to enable you to disable parts of the circuitry. This is entirely board-dependent.

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