Hot swapping is the ability to remove and replace components of a machine, usually a computer, while it is operating. Once the appropriate software is installed on the computer, one can plug and unplug the hot-swap component without rebooting. An example of this is the Universal Serial Bus (USB), that allows a user to add or remove peripheral components such as a mouse, keyboard, or printer. It usually requires more sophisticated software and hardware than plug-and-play. In general, a things components are its parts; the things that compose it. ... An illustration of a modern personal computer. ... Computer software (or simply software) refers to one or more computer programs and data held in the storage of a computer for some purpose. ... It has been suggested that System partition and boot partition be merged into this article or section. ... Type A USB connector Dual images of the two Type B USB connectors, mini and full size, side and front view, compared with a U.S. 5¢ piece (nickel) in both images for scale. ... Plug and Play is a term used in the computer field to describe a computers ability to have new devices, normally peripherals, added to it without having to restart the computer. ...

History

Historically, only expensive systems have this ability as it is very difficult to engineer correctly. However, the technology has begun to appear in the mid to low end computer systems in recent years.

Protocols that now support hot swapping include PCMCIA, USB, FireWire, Fibre Channel, and SATA. Protocols that do not support hot swapping include IDE. Hot-swap components can include anything from USB flash drives to server hard drives, PCI-X or PCI Express expansion cards (PCI-X usually only in servers), PCMCIA cards (PC Cards) used in laptops, the Personal Media Drive available on some Hewlett-Packard machines and even some Power supplies. The PCMCIA is the Personal Computer Memory Card International Association, an industry trade association that creates standards for notebook computer peripheral devices. ... Type A USB connector Dual images of the two Type B USB connectors, mini and full size, side and front view, compared with a U.S. 5¢ piece (nickel) in both images for scale. ... FireWire (also known as i. ... Fibre Channel is a gigabit speed network technology primarily used for Storage Networking. ... A SATA power connector. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA) is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... For other meanings of PCI, see PCI (disambiguation). ... PCI Express (formerly known as 3GIO for 3rd Generation I/O, not to be mistaken with PCI-X) is an implementation of the PCI computer bus that uses existing PCI programming concepts and communications standards, but bases it on a much faster serial communications system. ... The Hewlett-Packard Company (NYSE: HPQ), commonly known as HP, is a very large, global company headquartered in Palo Alto, California, United States. ... A power supply unit (sometimes abbreviated power supply or PSU) is a device or system that supplies electrical or other types of energy to an output load or group of loads. ...

System considerations

Machines that support hot swap need some ability to detect that a component has been removed. In addition all electrical and mechanical connections need to be designed such that neither the component nor the user can be harmed by removing it. Lastly, other components in the system must be designed such that the removal of a different component does not harm operation. Often there is some automatic recovery process.

Some implementations require a component shut-down procedure prior to removal. This simplifies the design, but such devices are not robust in the case of component failure. Modern small computers often have USB and/or FireWire (also known as IEEE 1394 and iLink) buses that support this simple hot swap. 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. As this action is normally done by end users this is not considered a problem. These hot swap operations are generally used either to move a peripheral component from one computer to another, or to allow a device to synchronize data with a computer. Note: USB may also mean upper sideband in radio. ... FireWire (also known as i. ... For an account of the words periphery and peripheral as they are used in biology, sociology, politics, computer hardware, and other fields, see the periphery disambiguation page. ...

More complex implementations may recommend that the component be shut down, but there is sufficient redundancy in the system such that if a component is removed without being shut down, operation continues. In these systems hot swap is normally used for regular maintenance to the computer, or to replace a broken component. Redundancy, in general terms, refers to the quality or state of being redundant, that is: exceeding what is necessary or normal, containing an excess. ...

There are two slightly differing meanings of the term hot swapping. In some contexts, it refers only to the ability to add or remove hardware without powering down the system, while in other contexts, it refers to the system's ability to autonomously detect when hardware is added or removed. In the former case, one can add or remove hardware without powering down, but the system software may have to be notified of the event in order to use the new hardware. Examples of the former include RS-232, FireWire and lower-end SCSI devices, while examples of the latter include USB, FireWire, PCI Express and higher-end SCSI devices. When comparing these two cases, it is common to only refer to the latter case as true hotplugging, and call the former case coldplugging. RS-232 (also referred to as EIA RS-232C or V.24) is a standard for serial binary data interchange between a DTE (Data terminal equipment) and a DCE (Data communication equipment). ... SCSI stands for Small Computer System Interface, and is a standard interface and command set for transferring data between devices on both internal and external computer buses. ... PCI Express (formerly known as 3GIO for 3rd Generation I/O, not to be mistaken with PCI-X) is an implementation of the PCI computer bus that uses existing PCI programming concepts and communications standards, but bases it on a much faster serial communications system. ... Coldplugging is often taken to mean the opposite of hotplugging. ...

Hot-swap connectors

Most modern hot-swap methods rely on a specialised connector with staggered pins. 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. Specialised hot-plug power connector pins are now commercially available with repeatable DC current interruption ratings of up to 16A. Printed circuit boards can be designed for direct hot-plugging onto a backplane, with staggered edge-fingers providing the same results as staggered connector pins. Close-up photo of one side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side. ...

Most staggered pin designs have the ground pins longer than all the other pins. This ensures that any sensitive circuitry is never connected before there is a reliable system ground. The rest of the connector might be made up of short pins, or in some cases there can be a total of three different pin-lengths.

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 0.5mm, the elapsed time between long and short pin contact is between 25ms and 250ms. It is quite practical to design hot-swap circuits that can operate over that dynamic range. Note that pins of the same nominal length do not make contact at exactly the same time due to mechanical tolerances.

As long as the hot-swap connector is sufficiently rigid, one of the four corners will always be first to make physical contact with the slot. 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 centre. 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.

Hot-swap power electronics

The DC power supplies to a hot-swap component are usually pre-charged by dedicated long pins that make contact before the main power pins. These pre-charge pins are protected by a circuit that limits the inrush current to an acceptable value that cannot damage the pins nor disturb the supply voltage to adjacent slots. The pre-charge circuit might be a simple series resistor, a Negative Temperature Coefficient (NTC) resistor, or a current-limiter circuit. Further protection can be provided by a "soft-start" circuit that provides a managed ramp-up of the internal DC supply voltages within the component.

A typical sequence for a hot-swap component being plugged into a slot could be as follows:

1. Long ground pins make contact - Basic electrical safety and ESD protection becomes available
2. Long (or medium) pre-charge pins make contact - Decoupling capacitors start to charge up
3. Realtime delay of tens of milliseconds
4. Short power/signal pins make contact
5. Connector becomes fully seated - Power-on reset signal asserted within component
6. Soft-start circuit starts to apply power to the component
7. Realtime delay of tens of milliseconds
8. Soft-start circuit completes sequence - Power-on reset circuit deasserted
9. Component begins normal operation

Hot-swap power circuits can now be purchased commercially in specially designed ASICs called Hot Swap Power Managers (HSPMs). An ASIC (Application-Specific Integrated Circuit) is an integrated circuit (IC) customised for a particular use, rather than intended for general-purpose use. ...

Hot-swap signal electronics

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. An electrostatic discharge (ESD) is a sudden flow of electric current through a material that is normally an insulator. ...

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 specialised inputs and outputs that minimise 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. Parallel SCSI is a computer term used to describe one of the physical implementations of SCSI communications. ...

Hot-swap software

Hot swapping can also refer to the ability to alter the running code of a program without having to interrupt its execution, although only a few languages support it. Those that do include Lisp, Erlang, and Smalltalk. Lisp is a family of computer programming languages with a long history and a distinctive fully-parenthesized syntax. ... Erlang is a general-purpose concurrent programming language and runtime system. ... Smalltalk is a dynamically typed object oriented programming language designed at Xerox PARC by Alan Kay, Dan Ingalls, Ted Kaehler, Adele Goldberg, and others during the 1970s. ...

See also

• Coldplug

Coldplugging is often taken to mean the opposite of hotplugging. ...

External links

• Hotplug support in Linux-based systems
• Datasheet for high current hot-swap connector pin
• Datasheet for HSPM hot-swap ASIC
• Datasheet for hot-swap CMOS logic family