Jumper (computing)

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Top: jumper block on IDE hard drive with jumper; bottom: assorted jumpers

In electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit. Jumpers are typically used to set up or adjust printed circuit boards, such as the motherboards of computers.

[edit] Description

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 and often more. In general, each contact in a jumper block terminates in a small metal pin. 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 conductive; 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".

Jumper blocks and jumpers are also often used on motherboards to clear the CMOS information, resetting the BIOS configuration settings. This allows the computer to boot if a recent BIOS setting made it unable to boot, or if the CMOS boot password was forgotten.

[edit] Move to reduce jumpers

Early generations of any given computer hardware technology usually have many jumper blocks, often laid out in a way that is poorly documented and difficult to set correctly. Often, designers find ways to streamline and simplify the jumper layout. For example, a typical early model Intel 386 motherboard might have 30 or 40 jumper pairs, while the last production models typically had just a handful, or sometimes only one. Typically, each jumper block is assigned and labelled with a number, which is documented in an instructional list printed on the motherboard or in the manual.

The recent trend has been to try to eliminate jumpers entirely from hardware devices by the use of auto-configuration or software-controlled configuration. Configurations may be stored in NVRAM, loaded by a host processor, or negotiated at system initialization time. In some cases, hot pluggable devices may be able to renegotiate their configuration. 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. Nowadays, likely the most common usage of jumpers is to set the operating mode for ATA drives (master, slave, or cable select).

More traditional systems using boards with physical jumpers, on the other hand, tend not to be mis-set by end users (as, in general, non-technical people are less willing to physically alter hardware settings than they are to experiment with settings from the keyboard). They also have the advantage that they usually only ever need to be set once: while firmware settings can be easily lost or corrupted by a careless user, a virus, or a power failure, the only way to alter a correct jumper setting is to physically change it.