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In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or open, or bypass part of, an electrical circuit. Jumpers are typically used to set up or configure printed circuit boards, such as the motherboards of computers.
Jumper pins (points to be connected by the jumper) are arranged in groups called jumper blocks, each group having at least one pair of contact points. An appropriately sized conductive sleeve called a jumper, or more technically, a jumper shunt, is slipped over the pins to complete the circuit.
Jumpers must be electrically conducting; they are usually encased in a non-conductive block of plastic for convenience. This also avoids the risk that an unshielded jumper will accidentally short out something critical (particularly if it is dropped on a live circuit).
When a jumper is placed over two or more jumper pins, an electrical connection is made between them, and the equipment is thus instructed to activate certain settings accordingly. For example, with older PC systems, CPU speed and voltage settings were often made by setting jumpers. Informally, technicians often call setting jumpers "strapping". To adjust the SCSI ID jumpers on a hard drive, for example, is to "strap it up".
Jumperless designs have the advantage that they are usually fast and easy to set up, often require little technical knowledge, and can be adjusted without having physical access to the circuit board. With PCs, the most common use of jumpers is in setting the operating mode for ATA drives (master, slave, or cable select), though this use is declining with the rise of SATA drives. Jumpers have been used since the beginning of printed circuit boards.
BUT, even that jumperless/PNP (Plug and PRAY :P) and are there for quite some long, even today there is still some use for jumpers, and even that use is only for something advanced like, still there is an important use: CMOS/BIOS reset ... CMOS reset is a solution for bad BIOS setting resulting in "freeze of the system" usually that happens after trying to overclock but also may happen when you upgrade your CPU with an higher FSB (Front Side Bus) multiplier but working at a lower (again) FSB frequency than the older CPU. Usually, CMOS reset is a 3 contacts thing with a jumper in normal operation attached to two of the contacts, and when you need to reset CMOS you move to the other two contacts (contact with number 2 is always in middle so, you like move it from normal 2-3 to 1-2), and after keeping it there for few seconds you move it back on normal position and at first start (OFC, you move the jumper with computer off), all CMOS/BIOS settings are reset to default.