Hot swapping and
hot plugging are terms used to separately describe the functions of replacing system components without shutting down the system. Hot swapping describes changing components without significant interruption to the system, while hot plugging describes changing or adding components which interact with the operating system. Both terms describe the ability to remove and replace components of a machine, usually a
computerA computer is a machine that manipulates data according to a set of instructions.Although mechanical examples of computers have existed through much of recorded human history, the first electronic computers were developed in the mid-20th century . These were the size of a large room, consuming as...
, while it is operating. For hot swapping once the appropriate software is installed on the computer, a user can plug and unplug the component without
rebootingIn computing, booting is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the initial set of operations that the computer performs when power is switched on...
. A well-known example of this functionality is the
Universal Serial BusUSB is a way of setting up communication between a computer and peripheral devices. USB is intended to replace many varieties of serial and parallel ports. USB can connect computer peripherals such as mice, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal...
(USB) that allows users to add or remove peripheral components such as a mouse, keyboard, or printer.
Reasons for hot-swapping
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, to avoid the delay and nuisance of shutting down and then restarting complex equipment, or because it is essential that the equipment be permanently available.
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.
Equipment may be designed with
redundancyIn engineering, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
so that in the event of the failure of a component, other parts of the system carry out its functions while the faulty component is removed and a replacement connected. For example, computer
RAIDRAID is an acronym first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 to describe a redundant array of inexpensive disks, a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less...
disk arrays allow a faulty disk to be hot-swapped for a new one; the new one is configured to become part of the array automatically or by user command. A machine may have dual
power suppliesPower supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU...
, each adequate to power the machine; a faulty one may be hot-swapped.
System considerations
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.
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
redundancyIn engineering, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
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.
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-232In telecommunications, RS-232 is a standard for serial binary data signals connecting between a
DTE and a
DCE . It is commonly used in computer serial ports...
and lower-end
SCSISmall Computer System Interface, or SCSI , is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces...
devices. This is sometimes called cold plugging. However, if the system can detect and respond to addition or removal of hardware, it is referred to as
true hot plugging. Examples include USB, FireWire and higher-end
SCSISmall Computer System Interface, or SCSI , is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces...
devices.
Connectors
Most modern hot-swap methods use a specialized connector with staggered pins, so that certain pins are certain to be connected before others. 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 boardA printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring board...
s are made with staggered edge-fingers for direct hot-plugging into a backplane connector.
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; in some cases three pin lengths are used.
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 over that dynamic range. 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.
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.
Power electronics
The DC power supplies to a hot-swap component are usually
pre-chargedPre-charged refers to:* Pre-charge of the powerline voltages in a high voltage DC application;* Pre-charged batteries...
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:
- Long ground pins make contact; basic electrical safety and ESD protection becomes available.
- Long (or medium) pre-charge pins make contact; decoupling capacitors start to charge up.
- Real time delay of tens of milliseconds.
- Short power/signal pins make contact.
- Connector becomes fully seated; power-on reset signal asserted within component
- Soft-start circuit starts to apply power to the component.
- Real time delay of tens of milliseconds.
- Soft-start circuit completes sequence; power-on reset circuit deasserted
- Component begins normal operation.
Hot-swap power circuits can now be purchased commercially in specially designed
ASICsAn application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...
called hot-swap power managers (HSPMs).
Radio transmitters
Modern day radio transmitters (and some TV transmitters as well) use high power RF transistor power modules instead of tubes. Hot swapping power modules is not a new technology, as many of the radio transmitters manufactured in the 1930s were capable of having power tubes swapped out while the transmitter was running -- but this feature was not universally adopted due to the introduction of more reliable high power tubes.
In the mid-1990s, several radio transmitter manufactures in the US started offering swappable high power RF transistor modules.
- There was no industry standard for the design of the swappable power modules at the time.
- Early module designs had only limited patent protection.
- By the early 2000s, many transmitter models were available that used many different kinds of power modules.
The reintroduction of power modules has been good for the radio transmitter industry, as it has fostered innovation. Modular transmitters have proven to be more reliable than tube transmitters, when the transmitter is properly chosen for the conditions at the transmitting site.
Power limitations
- lowest power modular transmitter: generally 1.0 kW, using 600 W modules.
- highest power modular transmitter: 1.0 MW (for LW
- Entertainment :*LightWave 3D Software*Life's Work, a 1990s American situation comedy*Lethal Weapon, 1987 film*LimeWire*Long wave*Longwire antenna*Lovewrecked- Services :*Luftwaffe*Lutheran Worship*Lw, former symbol of lawrencium, now Lr....
, MWMedium Wave is that part of the medium frequency radio band used mainly for AM broadcasting. For Europe the MW band allocated for broadcasting is from 531 kHz - 1611 kHz...
).
- highest power modular transmitter: 45 kW (FM, TV).
Companies that produce transmitters using power modules
- Harris Broadcast (US)
- Telefunken (Germany)
- Thales (Western Europe)
- RIZ (Croatia)
Signal electronics
Circuitry attached to signal pins in a hot-swap component should include some protection against electrostatic discharge (
ESDESD may mean:*Electrostatic discharge, a sudden and momentary electric current that flows between two objects*Electronic services delivery, government services provided through the Internet or other electronic means...
). 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 SCSIParallel SCSI is one of the interface implementations in the SCSI family. In addition to being a data bus, SPI is a parallel electrical bus: There is one set of electrical connections stretching from one end of the SCSI bus to the other. A SCSI device attaches to the bus but does not interrupt it...
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
quiesceQuiesce is used to describe pausing or altering the state of running processes on a computer particularly those that might modify information stored on disk during a backup in order to guarantee a consistent and usable backup....
the operation of all components during the hot-swap operation.
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 Pike,
LispLisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older. Like Fortran, Lisp has changed a great deal...
, Erlang, Smalltalk, and
JavaJava is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
; C# and VB.NET only support hot swapping when run under a debugger.
Hotswapping allows organizations with long running Web servers to
fix bugs without taking down the server program.
It also facilitates the development of
Bioinformatics algorithms
where large amounts of data such as entire genomes are
being processed.
Interactive programmingInteractive programming is the procedure of writing parts of a program while it is already active. This focuses on the program text as the main interface for a running process, rather than an interactive application, where the program is designed in development cycles and used thereafter...
is a paradigm that makes extensive use of hot swapping so that the programming activity becomes part of the program flow itself.
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