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Jumper (computing)

Detail from an early model 386 board
showing jumpers and shunts

In electronics and particularly computing, a jumper is two or more connecting points that can be conveniently shorted together to setup or adjust a printed circuit board, such as a computer's motherboard.

Jumpers 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 shunt is slipped over the pins to complete the circuit. (In everyday usage, shunts are very commonly but incorrectly called "jumpers".)

Jumper shunts are almost always metal, and are usually encased in a non-conductive block of plastic for convenience, and to avoid the risk that an unshielded shunt will accidentally short out something critical (particularly if it is dropped onto a live circuit).

When a jumper shunt goes over two (or more) jumper pins, current is carried across, and the electronic equipment is thus instructed to activate certain settings acordingly. For example, computer CPU speed and voltage settings are 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".

As a general rule, the early generations of any given technology have many jumpers, often laid out in a way that is badly documented and difficult to set correctly. As time goes by, the 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 final production models typically had just a handful, or sometimes only one. Typically each jumper is assigned and labelled with a number, this number being referred to in an instruction list printed on the motherboard or just 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.

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), and 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 unscrew the case and probably use a pair of pliers.