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Concept in computing

Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said to be hot swappable or hot pluggable; likewise, components which do not are cold swappable or cold pluggable. Most desktop computer hardware, such as CPUs and memory, are only cold pluggable. However, it is common for mid to high end servers and mainframes to feature hot swappable capability for hardware components, such as CPU, memory, PCIe, SATA and SAS drives. An example of hot swapping is the express ability to pull a Universal Serial Bus (USB) peripheral device, such as a thumb drive, external hard disk drive (HDD), mouse, keyboard, or printer out of a computer's USB slot or peripheral hub without ejecting it first. Most smartphones and tablets with tray loading holders can interchange SIM cards without powering down the system. Dedicated digital cameras and camcorders usually have readily accessible memory card and battery compartments for quick changing with only minimal interruption of operation. Batteries can be cycled through by recharging reserve batteries externally while unused. Many cameras and camcorders feature an internal memory to allow capturing when no memory card is inserted.

Hot swapping is used whenever it is desirable to change the configuration or repair a working system without interrupting its operation. It may simply be for convenience of avoiding the delay and nuisance of shutting down and then restarting complex equipment or because it is essential for equipment, such as a server, to be continuously active. Hot swapping may be used to add or remove peripherals or components, to allow a device to synchronize data with a computer, and to replace faulty modules without interrupting equipment operation. A machine may have dual power supplies, each adequate to power the machine; a faulty one may be hot swapped. Important cards such as disk controllers or host adapters may be designed with redundant paths in order for these to be replaceable in case of failure without necessitating interruption of associated computer system operation.

Machines that support hot swapping need to be able to modify their operation for the changed configuration, either automatically on detecting the change, or by user intervention. All electrical and mechanical connections associated with hot swapping must be designed so that neither the equipment nor the user can be harmed while hot swapping. Other components in the system must be designed so that the removal of a hot swappable component does not interrupt operation.

Protective covering plates, shields, or bezels may be used on either the removable components or the main device itself to prevent operator contact with live powered circuitry, to provide antistatic protection for components being added or removed, or to prevent the removable components from accidentally touching and shorting out the powered components in the operating device. Additional guide slots, pins, notches, or holes may be used to aid in proper insertion of a component between other live components, while mechanical engagement latches, handles, or levers may be used to assist in proper insertion and removal of devices that either require large amounts of force to connect or disconnect, or to assist in the proper mating and holding together of power and communications connectors.

There are two slightly differing meanings of the term hot swapping. It may refer only to the ability to add or remove hardware without powering down the system, while the system software may have to be notified by the user of the event in order to cope with it. Examples include RS 232 and lower end SCSI devices. Examples include USB, FireWire and higher end SCSI devices. Some implementations require a component shutdown procedure prior to removal. This simplifies the design, but such devices are not robust in the case of component failure. If a component is removed while it is being used, the operations to that device fail and the user is responsible for retrying if necessary, although this is not usually considered to be a problem. More complex implementations may recommend but do not require that the component be shut down, with sufficient redundancy in the system to allow operation to continue if a component is removed without being shut down. In these systems hot swap is normally used for regular maintenance to the computer, or to replace a broken component.

Most modern hot swap methods use a specialized connector with staggered pins, so that certain pins are certain to be connected before others. Most staggered pin designs have ground pins longer than the others, ensuring that no sensitive circuitry is connected before there is a reliable system ground. The other pins may all be the same length, but in some cases three pin lengths are used so that the incoming device is grounded first, data lines connected second, and power applied third, in rapid succession as the device is inserted. Pins of the same nominal length do not necessarily make contact at exactly the same time due to mechanical tolerances, and angling of the connector when inserted. At one time staggered pins were thought to be an expensive solution, but many contemporary connector families now come with staggered pins as standard; for example, they are used on all modern serial SCSI disk drives. Specialized hot plug power connector pins are now commercially available with repeatable DC current interruption ratings of up to 16 A. Printed circuit boards are made with staggered edge fingers for direct hot plugging into a backplane connector. Although the speed of plugging cannot be controlled precisely, practical considerations will provide limits that can be used to determine worst case conditions. For a typical staggered pin design where the length difference is 0.5 mm, the elapsed time between long and short pin contact is between 25 ms and 250 ms. It is quite practical to design hot swap circuits that can operate at that speed. As long as the hot swap connector is sufficiently rigid, one of the four corner pins will always be the first to engage. For a typical two row connector arrangement this provides four first to make corner pins that are usually used for grounds. Other pins near the corners can be used for functions that would also benefit from this effect, for example sensing when the connector is fully seated. This diagram illustrates good practice where the grounds are in the corners and the power pins are near the center. Two sense pins are located in opposite corners so that fully seated detection is confirmed only when both of them are in contact with the slot. The remaining pins are used for all the other data signals.

Circuitry attached to signal pins in a hot swap component should include some protection against electrostatic discharge (ESD). This usually takes the form of clamp diodes to ground and to the DC power supply voltage. ESD effects can be reduced by careful design of the mechanical package around the hot swap component, perhaps by coating it with a thin film of conductive material. Particular care must be taken when designing systems with bussed signals which are wired to more than one hot swap component. When a hot swap component is inserted its input and output signal pins will represent a temporary short circuit to ground. This can cause unwanted ground level pulses on the signals which can disturb the operation of other hot swap components in the system. This was a problem for early parallel SCSI disk drives. One common design solution is to protect bussed signal pins with series diodes or resistors. CMOS buffer devices are now available with specialized inputs and outputs that minimize disturbance of bussed signals during the hot swap operation. If all else fails, another solution is to quiesce the operation of all components during the hot swap operation.

Although most contemporary video game systems can interchange games and multimedia (e. g. Blu rays) without powering down the system, older generations of systems varied in their support of hot swapping capabilities. For example, whereas the Sony PlayStation and PlayStation 2 could eject a game disc with the system powered on, the Nintendo Game Boy Advance and the Nintendo 64 would freeze up and could potentially become corrupt if the game cartridge was removed with the power on. Manufacturers specifically warned against such practices in the owner's manual or on the game cartridge. It was supposedly for this reason that Stop 'N' Swop was taken out of the Banjo Kazooie series. With the Sega Genesis/Mega Drive system, it was sometimes possible to apply cheats (such as a player having infinite lives) and other temporary software alterations to games by hot swapping cartridges, even though the cartridges were not designed to be hot swappable.

Hot swappable keyboards enable changing the switches without having to disassemble the keyboard. On standard mechanical switch keyboards, the switch is directly soldered to the PCB. Hot swappable keyboards instead have a socket in its place that allows the switch to be freely replaced without re soldering. Due to hot swappable keyboards being less common, they often require being custom built or bought from custom keyboard manufacturers. and Elixir. Microsoft Visual Studio supports a kind of hot swapping called Edit and Continue, which is supported by C#, VB. NET and C/C++ when running under a debugger. Hot swapping is the central method in live coding, where programming is an integral part of the runtime process. In general, all programming languages used in live coding, such as SuperCollider, TidalCycles, or Extempore support hot swapping. Some web based frameworks, such as Django, support detecting module changes and reloading them on the fly. However, although the same as hotswapping for most intents and purposes, this is technically just a cache purge, triggered by a new file. This does not apply to markup and programming languages such as HTML and PHP respectively, in the general case, as these files are normally reinterpreted on each use by default. There are a few CMSes and other PHP based frameworks (such as Drupal) that employ caching, however. In these cases, similar abilities and exceptions apply. Hot swapping also facilitates developing systems where large amounts of data are being processed, as in entire genomes in bioinformatics algorithms.

The term "HOT PLUG" was registered as a trademark in the United States in November 1992 to Core International, Inc., and cancelled in May 1999.